Chapter XXI
Temperature of the earth—Temperature of the Air—Radiation—Foci of Maximum Cold—Thermal Equator—Its Temperature, mean and absolute— Isothermal Lines—Continental and Insular Climates—Extreme Climates— Stability of Climate—Decrease of Heat in Altitude—Line of Perpetual Snow—Density of the Atmosphere—The Barometer—Measurement of Heights— Variations in Density and their Causes—Horary Variations—Independent Effect of the dry and aqueous Atmospheres—Mean height of Barometer in different Latitudes—Depression in the Antarctic Ocean and in Eastern Siberia—Barometric Storms—Polar and Equatorial Currents of Air— Trade-Winds—Monsoons—Land and Sea Breezes—Gyration of the Winds in the Extra-Tropical Zones—Winds in Middle European Latitudes—Hurricanes—The Laws of their Motion—Their Effect on the Barometer—How to steer clear of them—The Storm-Wave—Storm-Currents—Arched Squalls—Tornadoes— Whirlwinds—Water Spouts.
THE atmosphere completely envelops the earth to the height of about 20 miles; it bulges at the equator, and is flattened at the poles, in consequence of the diurnal rotation. It is a mixture of water in an invisible state and of air; but the air is not homogeneous; 100 parts of it consist of 79 parts of hydrogen or azotic gas, and 21 of oxygen, the source of combustion and animal heat. Besides these, there is a little ammoniacal vapour, and a small quantity of carbonic acid gas, which is sufficient to supply all the vegetation on the earth with wood and leaves. No doubt exhalations of various kinds ascend into the air, such as those which produce miasmata, but they are in quantities too minute to be detected by chemical analysis, so that the atmosphere is found to be of the same composition at all heights above the sea hitherto attained.[126]
The temperature of the earth’s surface, and the phenomena of the atmosphere, depend upon the revolution and rotation of the earth, which successively expose all the parts of the earth, and the air which surrounds it, to a perpetual variation of the gravitating forces of the two great luminaries, and to annual and diurnal vicissitudes of solar heat. Atmospheric phenomena are consequently periodical and connected with one another, and their harmony, and the regularity of the laws which govern them, become the more evident in proportion as the mean values of their vicissitudes are determined from simultaneous observations made over widely-extended tracts of the globe. The fickleness of the wind and weather is proverbial, but, as the same quantity of heat is annually received from the sun, and annually radiated into space, it follows that all climates on the earth are stable, and that their changes, like the perturbations of the planets, are limited, and accomplished in fixed cycles, whose periods are still in many instances unknown. It is possible, however, that the earth and air may be affected by secular variations of temperature during the progress of the solar system through space, or from periodical changes in the sun’s light and heat, similar to those which take place in many of the fixed stars. The secular variation in the moon’s mean distance will no doubt alter the amount of her attractive force, though probably by a quantity inappreciable in the aërial tides; at all events, variations arising from such circumstances could only become perceptible after many ages.
From experiments made by M. Peltier it appears that, if the absolute quantity of heat annually received by the earth were equally dispersed over its surface, it would, in the course of a year, melt a stratum of ice 46 feet deep covering the whole globe. It is evident that, if so great a quantity of heat had been continually accumulated in the earth, instead of being radiated into space, it would have been transmitted through the surface to the poles, where it would have melted the ice, and the torrid zone, if not the whole globe, would by this time have been uninhabitable. In fact, every surface absorbs and radiates heat at the same time, and the power of radiation is always equal to the power of absorption, for, under the same circumstances, bodies which become soon warm also cool rapidly, and the earth, as a whole, is under the same law as the bodies at its surface.
Although part of the heat received from the sun in summer is radiated back again, by far the greater part sinks into the earth’s surface, and tempers the severity of the winter’s cold while passing through the atmosphere into the etherial regions.
The power of the solar rays depends on the manner in which they fall, as may be seen from the difference of climates. The earth is about 3,000,000 of miles nearer to the sun in winter than in summer, but the rays strike the northern hemisphere more obliquely in winter than in the other half of the year.
Diurnal variations of heat are perceptible only to a small distance below the surface of the ground, because the earth is a bad conductor: the annual influence of the sun penetrates much farther. At the equator, where the heat is greatest, it descends deeper than elsewhere with a diminishing intensity, but there, and everywhere throughout the globe, there is a stratum, at a depth varying from 40 to 100 feet below the surface of the ground, where the temperature never varies, and is nearly the same with the mean temperature of the country over it. This zone, unaffected by the sun’s heat from above, or by the internal heat from below, serves as an origin whence the effects of solar heat are estimated on one hand, and the internal temperature of the globe on the other. Below it the heat of the earth increases, as already mentioned, at the rate of one degree of Fahrenheit’s thermometer for every 50 or 60 feet of perpendicular depth; were it to continue increasing at that rate, every substance would be in a state of fusion at the depth of 21 miles; hitherto, however, the experiments in mines and Artesian wells, whence the earth’s temperature below the constant stratum is ascertained, have not been extended below 1700 feet.
M. de Beaumont has estimated by the theory of Fourier, from the observations of M. Arago, that the quantity of central heat which reaches the surface of the earth is capable, in the course of a year, of melting a shell of ice covering the globe a quarter of an inch thick.[127]
The superficial temperature of the earth is great at the equator, it decreases gradually towards the poles, and is an exact mean between the two at the 45th parallel of latitude; but a multitude of causes disturb this law even between the tropics. It is affected chiefly by the unequal distribution of land and water, by the height above the sea, by the nature of the soil, and by vegetation, so that a line drawn on a map through all the places where the mean temperature of the earth is the same would be very far from coinciding with the parallels of latitude, but would approximate more to them near the equator. Between the tropics the temperature of the earth’s surface is greater in the interior of continents than on the sea-coasts and islands, and in the interior of Africa it is greater than in any other part of the globe.
Temperature depends upon the property all bodies possess, more or less, of perpetually absorbing and emitting or radiating heat. When the interchange is equal, the temperature of a substance remains the same; but when the radiation exceeds the absorption, it becomes colder, and vice versâ. The temperature of the air is certainly raised by the passage of the solar heat through it, because it absorbs one-third of it before reaching the earth, but it is chiefly warmed by heat transmitted and radiated from the earth. The radiation is abundant when the sky is clear and blue, but clouds intercept it; so that a thermometer rises in cloudy weather, and sinks when the air becomes clear and calm; even a slight mist diminishes radiation from the earth, because it returns as much heat as it receives. The temperature of the air is subject to such irregularities from these circumstances, and from the difference in the radiating powers of the bodies at the surface of the globe, that it is necessary to find, by experiment, the mean or average warmth of the day, month, and year, at a great variety of places, in order to have a standard by which the temperature in different parallels of latitude may be compared.
The mean diurnal temperature of the air, at any place, is equal to half the sum of the greatest and least heights of the thermometer during 24 hours, and, as the height of the thermometer is twice in the course of that time equal to the mean temperature of the place of observation, it might seem easy to obtain its value; yet that is not the case, for a small error in observation produces a very great error in such minute quantities, so that accuracy can only be attained from the average of a great number of observations, by which the errors, sometimes in excess and sometimes in defect, neutralize or balance each other. The mean value of quantities is a powerful aid to the imperfections of our nature in arriving at truth in physical inquiries, and in none more than in atmospheric phenomena: almost all the certain knowledge man has acquired with regard to the density and temperature of the air, winds, rain, &c., has been acquired by that method.
The mean temperature of any one month at the same place differs from one year to another, but the mean temperature of the whole year remains nearly the same, especially when the average of 10 or 15 years is taken; for although the temperature in any one place may be subject to very great variations, yet it never deviates more than a few degrees from its mean state.[128]
The motion of the sun in the ecliptic occasions perpetual variations in the length of the day, and in the direction of his rays with regard to the earth; yet, as the cause is periodic, the mean annual temperature from the sun’s motion alone must be constant in each parallel of latitude. For it is evident that the accumulation of heat in the long days in summer, which is but little diminished by radiation during the short nights, is balanced by the small quantity of heat received during the short days of winter and its radiation in the long frosty and clear nights. Were the globe everywhere on a level with the surface of the sea, and of uniform substance, so as to absorb and radiate heat equally, the mean heat of the sun would be regularly distributed over its surface in zones of equal annual temperature parallel to the equator, and would decrease regularly to each pole. The distribution of heat, however, in the same parallel is very irregular in all latitudes, except between the tropics, from the inequalities in the level and nature of the surface of the earth, so that lines drawn on a map through all places having the same mean annual temperature are nearly parallel to the equator only between the tropics; in all other latitudes they deviate greatly from it, and from one another.[129] Radiation is the principal cause of temperature; hence, the heat of the air is most powerfully modified by the ocean, which occupies three times as much of the surface of the globe as the land, and is more uniform in its surface, and also in its radiating power. On the land the difference in the radiating force of the mountains and table-lands from that of the plains—of deserts from grounds covered with rich vegetation—of wet land from dry, are the most general causes of variation; the local causes of irregularity are beyond enumeration.
There are two points in the northern hemisphere, both in the 80th parallel of latitude, where the cold is more intense than in any other part of the globe with which we are acquainted. One north of Canada in 100° W. long. has a temperature of -3°·5 of Fahrenheit; while, at the Siberian point, in 95° E. long., the temperature of the air is +1°; consequently it is four and a half degrees warmer than that north of Canada—a difference that has an influence even to the equator, where the mean temperature of the air is different in different longitudes.
The line of the maximum temperature of the atmosphere, or the atmospheric thermal equator, which cuts the terrestrial equator in the meridians of Otaheite and Singapore, passes through the Pacific in its southern course, and through the Atlantic in its northern, has a mean temperature of 83°·84 of Fahrenheit. But by the comparison of many observations the mean equatorial temperature of the air is 82°·94 in Asia, 85°·10 in Africa, and 80°·96 in America: thus, it appears that tropical Africa is the hottest region on earth. Moreover, the atmosphere in the tropical zone of the Pacific, when free from currents, is two degrees and a quarter warmer than the corresponding zone in the Atlantic, which is 82°·40.
On account of the great extent of ocean, the isothermal lines in the southern hemisphere coincide more nearly with the parallels of latitude than in the northern. In the Antarctic Ocean the only flexure is occasioned by the cold of the south polar current, which flows along the western coast of the American continent. In the northern hemisphere the predominance of land and its frequent alternations with water, the prevalence of particular winds, irregularities of the surface, and the difference in the temperature of the points of maximum cold, cause the isothermal lines to deviate more from the parallels of latitude. They make two deep bends northward, one in the Northern Atlantic and another in the northeast of America, and at last they separate into two parts, and encircle the points of maximum cold.
Professor Dove has discovered that, in consequence of the excess of land in the northern hemisphere, and the difference in the effect produced by the sun’s heat according as it falls on a solid or liquid surface, there is an annual variation in the aggregate mean temperature at the surface of the earth, whose maximum takes place during the sun’s northern declination, and its minimum during its southern.[130]
Places having the same mean annual temperature, often differ materially in climate: in some, the winters are mild and the summers cool, whereas in others the extremes of heat and cold prevail: England is an example of the first; Quebec, St. Petersburg, and the Arctic regions, are instances of the second. The solar heat penetrates more abundantly and deeper into the sea than into the land; in winter it preserves a considerable portion of that which it receives in summer, and from its saltness does not freeze so soon as fresh water; hence, the ocean is not liable to the same changes of temperature as the land, and by imparting its heat to the winds it diminishes the severity of the climate on the coasts and in islands, which are never subject to such extremes of heat and cold as are experienced in the interior of continents. The difference between the influence of sea and land is strikingly exemplified in the high latitudes of the two hemispheres. In consequence of the unbounded extent of the ocean in the south, the air is so mild and moist that a rich vegetation covers the ground, while in the corresponding latitudes in the north the country is barren from the excess of land towards the Polar Ocean, which renders the air dry and cold. A superabundance of land in the equatorial regions, on the contrary, raises the temperature, while the sea tempers it.
Professor Dove has shown, from a comparison of observations, that northern and central Asia have what may be termed a true continental climate, both in summer and in winter—that is to say, a hot summer and cold winter; that Europe has a true insular or sea climate in both seasons, the summers being cool and the winters mild; and that in North America the climate is inclined to be continental in winter, and insular in summer. The extremes of temperature in the year are greater in central Asia than in North America, and greater in North America than in Europe, and that difference increases everywhere with the latitude. In Guiana, within the tropics, the difference between the hottest and coldest months in the year is 2°·2 of Fahrenheit, in the temperate zone it is about 60°, and at Yakutsk in Siberia 114°·4. Even in places which have the same latitude as in northern Asia, compared with others in Europe or North America, the diversity is very great. At Quebec the summers are as warm as those in Paris, and grapes sometimes ripen in the open air, yet the winters are as severe as those in St. Petersburg. In short, lines drawn on a map through places having the same mean summer or winter temperature are neither parallel to one another, to the isothermal or geothermal lines, and they differ still more from the parallels of latitude.[131]
Observations tend to prove that all the climates on the earth are stable, and that their vicissitudes are only oscillations of greater or less extent, which vanish in the mean annual temperature of a sufficient number of years. There may be a succession of cold summers and mild winters, but in some other country the contrary takes place; the distribution of heat may vary from a variety of circumstances, but the absolute quantity gained and lost by the whole earth in the course of a year is invariably the same.
Since the air receives its warmth chiefly from the earth, its temperature diminishes with the height so rapidly, that at a very small elevation the cold becomes excessive, as the perpetual snow on the mountain-tops clearly shows. The decrease of heat is at the rate of a degree of Fahrenheit’s thermometer for every 334 feet.
The atmosphere, being a heavy and elastic fluid, decreases in density upwards, according to a determinate law, so rapidly, that three-fourths of the whole air it contains are within four miles of the earth, and all the phenomena perceptible to us—as clouds, rain, snow, and thunder—occur within that limit. The air even on the tops of mountains is so rare as to diminish the intensity of sound, to affect respiration, and to occasion a loss of muscular strength in man and animals.[132]
Since the space in the top of the tube of a barometer is a vacuum, the column of mercury is suspended in the tube by the pressure of the atmosphere on the surface of the mercury in the cistern: hence, every variation in the density or height of the atmosphere occasions a corresponding rise or fall in the barometric column. The actual mean pressure of the atmosphere at the level of the sea is 15 pounds on the square inch; hence, the pressure on the whole earth is enormous.
The decrease in the density of the air affords a very accurate method of finding the height of mountains above the level of the sea, which would be very simple, were it not for changes of temperature which alter the density and interfere with the regularity of the law of its decrease. But as the heat of the air diminishes with the height above the earth at the rate of one degree of Fahrenheit’s thermometer for every 334 feet, tables are constructed, by the aid of which heights may be determined with great accuracy. In consequence of diminished pressure also, water boils at a lower temperature on mountain-tops than at the level of the sea, which affords another method of ascertaining heights.[133]
By the annual and diurnal revolutions of the earth, each column of air is alternately exposed to the heat and cold of summer and winter, of day and night, and also to variations in the attraction of the sun and moon, which disturb its equilibrium, and produce tides similar to those in the ocean. Those produced by the moon ebb and flow twice during a lunation, and diurnal variations in the barometer, to a very small amount, are also due to the moon’s attraction.[134] The annual undulations occasioned by the sun have their greatest altitudes at the equinoxes, and their least at the solstices, and the diurnal variations in the height of the barometer, which accomplish their rise and fall twice in 24 hours, are chiefly due to the effects of temperature on the dry air and moisture of the atmosphere, which, according to Mr. Dove’s discoveries, produce independent pressures upon the mercurial column.
A quantity of vapour is continually raised by the heat of the sun from the surface of the globe, which mixes in an invisible state with the dry air or gaseous part of the atmosphere. It is most abundant in the torrid zone, and, like the heat on which it depends, varies with the latitude, the season of the year, the time of the day, the elevation above the sea, and also with the nature of the soil, the land, and the water. There is no chemical combination between the aërial and aqueous atmospheres, they are merely mixed; and the diurnal variations arise from the superposition of two distinct diurnal oscillations, each going through its complete period in 24 hours; one taking place in the aërial atmosphere from the alternate heating and cooling of the air, which produce a flux and reflux over the point of observation; the other arising from the aqueous atmosphere, owing to the alternate production and destruction of vapour by the heat of the day and the cold of the night. The diurnal variations of the vapour have their maximum at or near the hottest hour of the day, and their minimum at or near the coldest, which is exactly the converse of the diurnal variations of the dry air. On the whole, there are two maxima and two minima heights of the barometer in the course of 24 hours from the combinations of these, but in the interior of continents far from water, where the air is very dry, there ought to be one maximum and one minimum during that period, according to this theory.
Between the tropics the barometer attains its greatest height at nine or half-past nine in the morning; it then sinks till four in the afternoon, after which it again rises and attains a second maximum at ten or half-past ten in the evening; it then begins to fall till it reaches a second time its lowest point at four in the morning. The difference in the height is 0·117 of an inch, which gradually decreases north and south. Baron Humboldt mentions that the diurnal variations of the barometric pressure are so regular between the tropics, that the hour of the day may be inferred from the height of the mercury to within fifteen or sixteen minutes, and that it is undisturbed by storm, tempest, rain, or earthquake, both on the coasts and at altitudes 13,000 feet above them. The mean height of the barometer between the tropics at the level of the sea is 30 inches with very little fluctuation, but, owing to the ascending currents of air from the heat of the earth, it is less under the equator than in the temperate zones. It attains a maximum in western Europe between the parallels of 40° and 45°; in the North Atlantic the maximum is about the 30th parallel, and in the southern part of that ocean it is near the tropic of Capricorn; the amplitude of the oscillations decreases from the tropics to about the 70th parallel, where the diurnal variations cease. They are affected by the seasons, being greatest in summer and least in winter. It appears, also, that the fluctuations are the reverse on mountain-tops from what they are on the plains, and probably at a certain height they would cease altogether.[135] It is a singular fact, discovered by our navigators, that the mean height of the barometer is an inch lower throughout the Antarctic Ocean and at Cape Horn than it is at the Cape of Good Hope or Valparaiso: that difference in the pressure of the atmosphere is probably connected with the perpetual gales off the extremity of South America. M. Erman observed a similar depression near the Sea of Okhotsk in eastern Siberia.
Besides the small horary undulations, there are vast waves moving over the oceans and continents in separate and independent systems, being confined to local yet very extensive districts, probably occasioned by long-continued rains or dry weather over wide tracts of country. By numerous barometrical observations made simultaneously in both hemispheres, the courses of several have been traced, some of which take 24, others 36 hours, to accomplish their rise and fall. One especially of these vast barometric waves, many hundreds of miles in breadth, has been traced over the greater part of Europe, and not its breadth only, but also the direction of its front, and its velocity, have been clearly ascertained. The course of another wave has been made out from the Cape of Good Hope, through many intermediate stations, to the observatory at Toronto in Canada. Since every undulation has its perfect effect independently of the others, each one is marked by a change in the barometer, and this is beautifully illustrated by curved lines on paper, constructed from a series of observations. The general form of the curve shows the course of the principal wave, while small undulations in its outline mark the maxima and minima of the minor oscillations. Although, like all other waves, these in the atmosphere are but waving forms, in which there is no transfer of air, yet winds arise from them like tide-streams in the ocean, and Sir John Herschel is of opinion that the crossing of two of these vast aërial waves, coming in different directions, may generate, at the point of intersection, those tremendous revolving storms, or hurricanes, which spread desolation far and wide.
The air expands and becomes lighter with heat, contracts and becomes heavier with cold, and, as there are 82 degrees of difference between the equatorial and polar temperature, the light warm air at the equator is constantly ascending to the upper regions of the atmosphere, and flowing north and south to the poles, from whence the cold heavy air rushes along the surface of the earth to supply its place between the tropics, for the same tendency to restore equilibrium exists in air as in other fluids. These two superficial currents, which have no rotatory motion when they leave the poles, are deflected from their meridional paths by friction from the continually increasing velocity of the earth’s rotation, as they come nearer and nearer to the tropics; and, as they revolve slower than the corresponding parts of the earth at which they arrive, the bodies on its surface strike against them with the excess of their velocity, so that the wind appears, to a person who thinks himself at rest, to blow in a direction contrary to that of the earth’s rotation. For that reason the current from the north pole becomes a north-east wind before arriving at the tropic of Cancer, and that from the south pole becomes a south-east wind before it comes to the tropic of Capricorn, their limit being the 28th parallel of latitude on each side of the equator. In fact, the difference of temperature puts the air in motion, and the direction of the resulting wind, at every place, depends upon the difference between the rotatory motion of the wind and the rotatory motion of the earth—the whole theory of the winds depends upon these circumstances.
Near the equator the trade-winds, north and south of it, so completely neutralize each other, that far at sea a candle burns without flickering [i. e. when it is flat calm]. This zone of calms and light breezes, known as the Variables, which has a breadth of about five degrees and a half, is subject to heavy rains and violent thunder-storms. On account of the arrangement of land and water, it does not coincide with the equator, but its centre runs along the sixth parallel of north latitude; however, it changes in position and extent with the declination of the sun, but never crosses the line.
Though the trade-winds extend to the 28th degree on each side of the equator, their limits vary considerably in different parts of the ocean, moving two or three degrees to the north or south, according to the position of the sun; and in the Atlantic the north-east trade-wind is less steady than the south-east.[136] These perennial winds are known by recent observations to be less uniform in the Pacific than in the Atlantic; they only blow permanently over that portion between the Galapagos Archipelago, off the coast of America, and the Marquesas. In the Indian Ocean the south-east trade-wind blows from a few degrees east of Madagascar to the coast of Australia, between 10° and 28° S. lat. The trade-winds are only constant far from land, because continents and islands intercept them, and change their course. On that account the numerous groups of islands westward from the Marquesas change the trade-winds into the periodical monsoons, which are steady currents of air in the Arabian Gulf, the Indian Ocean, and China Sea, arising from diminished atmospheric pressure at each tropic alternately, from the heat of the sun, thereby producing a regular alternation of north and south winds, which, combining with the rotation of the earth on its axis, become a north-east wind in the northern hemisphere, and a south-east in the southern. The former blows from April to October, the latter from October to April; the change is accompanied by heavy rain and violent storms of thunder and lightning. The ascent of the warm air between the tropics occasions a depression of the barometer amounting to the tenth of an inch, which is a measure of the force producing the trade-winds. In both hemispheres there is a regular variation in the mean height of the barometer within the zone in which these great aërial currents flow; it is higher at their polar limits, and decreases with extreme uniformity towards their equatorial boundaries, the difference in both hemispheres being 0·25 of an inch.
The unequal temperature of the land and sea causes sea-breezes which blow towards the land during the day, and land-breezes which blow sea-ward in the night; they are not perceptible in the mornings and evenings, because the temperature of the land and water is then nearly the same.
The trade-winds and monsoons are permanent, depending on the apparent motion of the sun; but it is evident from theory that there must be partial winds in all parts of the earth, occasioned by the local circumstances that affect the temperature of the air. Consequently, the atmosphere is divided into districts, both over the sea and land, in which the winds have nearly the same vicissitudes from year to year. The regularity is greatest towards the tropics, where the causes of disturbance are fewer. In the higher latitudes it is more difficult to discover any regularity, on account of the greater proportion of land, the difference in its radiating power, and the greater extremes of heat and cold. But even there a degree of uniformity prevails in the succession of the winds; for example, in all places where north and south winds blow alternately, a vane veers through every point of the compass in the transition, and in some places the wind makes several of these gyrations in the course of the year.[137] The south-westerly winds, so prevalent in the Atlantic Ocean between the 30th and 60th degrees of north latitude, are produced by the upper current being drawn down to supply the superficial current which goes towards the equator, and, as it has a greater rotatory motion than the earth in these latitudes, it produces a south-westerly wind. On this account the average voyage from Liverpool to New York in a sailing vessel is 40 days, while it is only 23 days from New York to Liverpool. For the same reason the average direction of the wind in England, France, Germany, Denmark, Sweden, and North America, is some point between south and west. North-westerly winds prevail in the corresponding latitudes of the southern hemisphere from the same cause. In fact, whenever the air has a greater velocity of rotation than the surface of the earth, a wind more or less westerly is produced; and when it has less velocity of rotation than the earth, a wind having an easterly tendency results. Thus, there is a perpetual change between the different masses of the atmosphere, the warm air tempering the cold of the higher latitudes, and the cold air mitigating the heat of the lower; it will be shown afterwards that the aërial currents are the bearers of principles on which the life of the animal and vegetable world depends.
Hurricanes are those storms of wind in which the portion of the atmosphere that forms them revolves in a horizontal circuit round a vertical or somewhat inclined axis of rotation, while the axis itself, and consequently the whole storm, is carried forwards along the surface of the globe, so that the direction in which the storm is advancing is quite different from the direction in which the rotatory current may be blowing at any point; the progressive motion may continue for days, while the wind accomplishes many gyrations through all the points of the compass in the same time. In the Atlantic the principal region of hurricanes is to the east of the West Indian islands, and in the Pacific it lies east of the island of Madagascar; consequently the former is in the northern hemisphere, the latter in the southern; but in every case the storm moves in an elliptical or parabolic curve. The West Indian hurricanes generally have their origin eastward of the Lesser Antillas or Caribbean islands, and the vertex of their path near the tropic of Cancer, or about the exterior limit of the north-east trade-wind. As the motion of the storm before it reaches the tropic is in a straight line from S.E. to N.W., and after it has passed the tropic from S.W., to N.E., the bend of the curve is turned towards Florida and the Carolinas. In the South Pacific Ocean the body of the storms moves in an exactly opposite direction. The hurricanes which originate south of the equator, and whose initial path is from N.E. to S.W., turn at the tropic of Capricorn, and then tend from N.W. to S.E., so that the bend of the curve is turned towards Madagascar.
The extent and velocity of the Atlantic hurricanes are great; the most rapid move at the rate of 43 miles an hour, the slowest 16. The hurricane which took place on the 12th of August, 1830, was traced from the eastward of the Caribbean islands to the banks of Newfoundland, a distance of more than 3000 miles, which it passed over in six days. Although that of the 1st of September, 1821, was not so extensive, its velocity was greater, as it moved at the rate of 30 miles an hour. Small storms are generally more rapid than those of great magnitude. Sometimes they appear to be stationary, sometimes they stop and again proceed on their course, like water-spouts. Hurricanes are occasionally contemporaneous, and so near to one another as to travel in almost parallel tracks. This happened in the China seas in October, 1840, when the two storms met at an angle of 47°, and it was supposed that the ship Golconda foundered in that spot with 300 people on board. A hurricane has been split or divided by a mountain into two separate storms, each of which continued its new course, and the gyrations were made with increased violence. This occurred in the gale of the 25th of December, 1821, in the Mediterranean, when the Spanish mountains and the maritime Alps became new centres of motion.
By the friction of the earth the axis of the storm bends a little forward, and the whirling motion begins in the higher regions of the atmosphere before it is felt on the earth: this causes a continual intermixture of the lower and warmer strata of air with those that are higher and colder, producing torrents of rain, and sometimes violent electric explosions.
The rotation as well as the course of the storm is in a different direction in the two hemispheres, though always alike in the same. In the northern hemisphere the gyration is contrary to the movement of the hands of a watch, that is to say, the wind revolves from east, through the north, to west, south, and east again; while in the southern hemisphere the rotation about the axis of the storm is in the contrary direction. Hurricanes happen south of the equator between December and April; in the West Indies between June and October. Rotatory storms frequently occur in the Indian Ocean, and the typhoons of the China seas are real hurricanes of great violence. Both conform to the laws of such winds in the northern hemisphere. The Atlantic storms probably reach Spain, Portugal, and the coast of Ireland. Two circular storms have passed over Great Britain, and small ones often occur between the Chops of the Channel and Madeira.
The revolving motion accounts for the sudden and violent changes observed during hurricanes. In consequence of the rotation of the air, the wind blows in opposite directions on each side of the axis of the storm, and the violence of the blast increases from the circumference towards the centre of gyration, but in the centre itself the air is in repose: hence, when the body of the storm passes over a place, the wind begins to blow moderately, and increases to a hurricane as the centre of the whirlwind approaches; then in a moment a dead and awful calm succeeds, suddenly followed by a renewal of the storm in all its violence, but now blowing in a direction diametrically opposite to what it had before: this happened in the island of St. Thomas on the 2d of August, 1837, where the hurricane increased in violence till half-past seven in the morning, when perfect stillness took place for 40 minutes, after which the storm recommenced in a contrary direction. The breadth of a hurricane is greatly augmented when its path changes its direction in crossing the tropic. In the Atlantic, the vortex of one of these tempests has covered an area from 600 to 1000 miles in diameter. The breadth of the lull in the centre varies from 5 to 30 miles: the height is from 1 to 5 miles at most; so that a person might see the strife of the elements from the top of a mountain, such as Teneriffe or Mowna Roa, in a perfect calm, for the upper clouds are frequently seen to be at rest during the hideous turmoil in the lower regions.
The sudden fall of the mercury in the barometer in latitudes habitually visited by hurricanes is a certain indication of a coming tempest. In consequence of the centrifugal force of these rotatory storms, the air becomes rarified, and, as the atmosphere is disturbed to some distance beyond the actual circle of gyration or the limits of the storm, the barometer often sinks some hours before its arrival: it continues sinking the first half of the hurricane, and again rises during the passage of the latter half, though it does not attain its greatest height till the storm is over. The diminution of atmospheric pressure is greater, and extends over a wider area, in the temperate zones than in the torrid, on account of the sudden expansion of the circle of rotation where the gale crosses the tropic.
As the fall of the barometer gives warning of the approach of a hurricane, so the laws of the storm’s motion afford to the seaman knowledge to avoid it. In the northern temperate zone, if the gale begins from the S.E. and veers by S. to W., the ship should steer to the S.E.; but if the gale begins from the N.E. and changes through N. to N.W., the vessel ought to go to the N.W. In the northern part of the torrid zone, if the storm begin from the N.E. and veer through E. to S.E., the ship should steer to the N.E.; but if it begin from the N.W. and veer by W. to S.W., the ship should steer to the S.W., because she is on the south-western side of the storm. Since the laws of storms are reversed in the southern hemisphere, the rules for steering vessels are necessarily reversed also.[138]
A heavy swell or storm-wave is peculiarly characteristic of these tempests. In the centre of the hurricane the pressure of the atmosphere is so much diminished by rotation, that the mercury in the barometer falls from one to two, and even two and a-half inches. On that account, the pressure of the ocean beyond the range of the wind raises the water in the centre of the vortex about two feet above its usual level, and proportionally to the degree of diminished pressure over the whole area of the storm. This mass of water, or storm-wave, is driven bodily along with, or before, the tempest, and rolls in upon the land like a huge wall of water. It is similar to the earthquake wave, and is by no means the heaping up of the water after a long gale. Ships have been swept by it out of docks and rivers, and it has sometimes carried vessels over reefs and banks so as to land them high and dry; this happened to two ships on the coast of the Eastern Andaman islands, in 1844. Coringa, on the Coromandel coast, is particularly subject to inundations from that cause. In 1789, the town and 20,000 inhabitants were destroyed by a succession of these great waves during a hurricane, and as many perished there in 1839.
Besides storm-waves, storm-currents are raised, which revolve with the rotation of the wind, and are of the greatest force near the centre of the vortex.
The rise of the sea by the pressure of the surrounding ocean, and the irresistible fury of the wind, makes a tremendous commotion in the centre of the storm, where the sea rises, not in waves, but in pyramidal masses: the noise during its passage resembles the deafening roar of the most tremendous thunder; and in the typhoons in the China seas it is like numberless voices raised to the utmost pitch of screaming. In general, there is very little thunder and lightning; sometimes a vivid flash occurs during the passage of the centre, or at the beginning of the storm; yet in Barbadoes the whole atmosphere has been enveloped in an electric cloud.
A thick lurid appearance, with dense masses of cloud in the horizon, ominous and terrible, are the harbingers of the coming tempest. The sun and clouds frequently assume a fiery redness, the whole sky takes a wild and threatening aspect, and the wind rises and falls with a moaning sound, like that heard in old houses on a winter’s night: it is akin to the “calling of the sea,” a melancholy noise which, in a dead calm, presages a storm on some parts of the English coast.
Those intensely violent gales, of short duration, called arched squalls, because they rise from an arch of clouds on the horizon, are not rotatory; they occur in the Straits of Malacca, attended by fierce thunder and lightning and a lurid phosphorescent gleam. The north-western gales in the Bay of Bengal, the tornadoes on the African coast, and the pampéros of the Rio de la Plata, are of the same nature. On an average, a strong gale moves at the rate of 40 miles an hour, a storm at about 56, and hurricanes at 90.
Whirlwinds are frequent in tropical countries, especially in deserts; sometimes several are seen at one time in the Arabian deserts, of all sizes, from a few feet to some hundred yards in diameter. They occur in all kinds of weather, by night as well as by day, and come without the smallest notice, rooting up trees, overwhelming caravans, and throwing down houses; and as they produce water-spouts when they reach the sea, they dismantle and even sink ships. The water-spouts so frequently seen on the ocean originate in adjacent strata of air of different temperatures, running in opposite directions in the upper regions of the atmosphere. They condense the vapour, and give it a whirling motion, so that it descends tapering to the sea below, and causes the surface of the water to ascend in a pointed spiral till it joins that from above, and then it looks like two inverted cones, being thinner in the middle than either above or below. When a water-spout has a progressive motion, the upper and under part must move in the same direction, and with equal velocity, otherwise it breaks, which frequently happens.
Chapter XXII
Evaporation—Distribution of Vapour—Dew—Hoar-Frost—Fog—Region of Clouds— Forms of Clouds—Rain—Distribution of Rain—Quantity—Number of rainy Days in different Latitudes—Rainless Districts—Snow Crystals—Line of perpetual Snow—Limit of Winter Snow on the Plains—Sleet—Hail— Minuteness of the ultimate Particles of Matter—Their Densities and Forms—Their Action on Light—Colour of Bodies—Colour of the Atmosphere— Its Absorption and Reflection of Light—Mirage—Fog Images—Coronæ and Halos—The Rainbow—Iris in Dewdrops—The Polarization of the Atmosphere— Atmospheric Electricity—Its Variations—Electricity of Fogs and Rain— Inductive Action of the Earth—Lightning—Thunder—Distribution of Thunder-Storms—Back Stroke—St. Elmo’s Fire—Phosphorescence—Aurora— Magnetism—Terrestrial Magnetism—The Dip—Magnetic Poles and Equator— Magnetic Intensity—Dynamic Equator—Declination—Magnetic Meridian—Lines of equal Variation—Horary Variations—Line of Alternate Horary Phenomena—Magnetic Storms—Coincidence of the Lines of equal Magnetic Intensity with Mountain Chains—Diamagnetism.
MOISTURE is evaporated in an invisible form from every part of the land and water, and at all temperatures, even from snow. Mr. Darwin mentions that the snow once entirely disappeared from the volcano of Aconcagua, in Chile, which is 23,300 feet high, from evaporation under a cloudless sky and an excessively dry air. The vapour rises and mixes with the atmosphere; and as its pressure and density diminish with the height above the surface of the earth, in consequence of gravitation, there is absolutely less moisture in the higher than in the lower regions of the air.
Seven-tenths of the atmosphere rests on the ocean; therefore the sea has the greatest influence in modifying climates and supplying the air with moisture. The evaporation is greatest between the tropics, from the excess of heat and the preponderance of the ocean, and its average quantity decreases from thence to the poles. Over the open sea, in all latitudes, the air is saturated with moisture; and in that over the coasts the quantity is very great, but it diminishes from the coasts to the interior of the continents. In the interior of the United States of North America, in the deserts of Asia, and in the interior of New Holland, the air is continually dry. There is scarcely any evaporation in the deserts of Africa, and the extreme heat, increased by the reverberation of the sand, opposes aqueous precipitations, so this land is doomed to perpetual sterility. The air over the steppes of Siberia is likewise nearly deprived of moisture. The greatest degree of dryness on record is that observed by M. Erman between the valleys of the Irtish and Obi, after a continued south-west wind and a temperature of 74° 7ʹ of Fahrenheit.
Throughout all the countries in the northern hemisphere where observations have been made on the variations of atmospheric moisture, it appears that the air contains less vapour in January than in any other month of the year, yet at that time there is the greatest dampness; while in July the air is driest, and yet, on account of the heat, evaporation is the greatest: the reason is, that the heat in July dissolves the moisture and increases its elasticity or tension so much that it becomes insensible, whereas the cold of winter condenses it and renders it apparent.
The quantity of atmospheric moisture varies also with the hours of the day and night. In early morning the evaporation accumulates near the surface of the ground from the resistance of the air above it, but as the sun rises above the horizon the warm air descends and carries the vapour with it; so that the quantity near the ground is diminished till evening, when, on account of the lowness of the temperature, the ascending currents cease, and the air becomes loaded with vapour, and deposits its excess in the shape of dew or hoar-frost. For in the night the earth radiates part of the heat it received during the day through the atmosphere into space, and the temperature of the bodies on its surface sinks below that of the air; and by abstracting part of the heat which holds the humidity of the air in solution a deposition takes place. If the radiation be great, the dew is frozen and becomes hoar-frost, which is the ice of dew. Cloudy weather is unfavourable for the formation of dew by preventing the free radiation of heat, and actual contact is necessary for its formation, as it is never suspended in the air like fog. Dew falls in calm serene nights, but not on all substances indifferently; it wets them in proportion to their power of radiation, leaving those dry that radiate feebly or not at all. Dew is most abundant on coasts; in the interior of continents there is very little, except near lakes or rivers. When dew is congealed into hoar-frost it forms beautiful crystals, and the cold which produces it is very hurtful to vegetation, but a slight covering preserves plants from its effects.
When the atmosphere is so saturated with the vapour of water that it is precipitated in the air itself, a fog is the result, which consists of small globular particles of water. When dew is formed the earth is colder than the air in contact with it; but the case is exactly the contrary when fogs take place, the moist soil being warmer than the air. In countries where the soil is moist and warm, and the air damp and cold, thick and frequent fogs arise, as in England, where the coasts are washed by a sea of elevated temperature, and the excess of the heat of the Gulf-stream above the cold moist air is the cause of the perpetual fogs in Newfoundland.
Superior to all these phenomena, and at a considerable height above the earth, the air is very dry, because, under ordinary circumstances, the vapour ascends in a highly elastic and invisible state till it reaches a stratum of air of lower temperature, and then it is condensed into clouds. The region of clouds is a zone at a height varying from one to four miles above the surface of the earth, which is saturated with moisture. From friction and other causes the currents of air in the lower parts of that zone run horizontally on each other; and as they generally differ in moisture, temperature, and velocity, the colder condense the invisible vapour in the warmer, and make it apparent in the form of a cloud, which differs in no respect from a fog, except that one floats high in the air, while the other rests on the ground.
At moderate heights clouds consist of vapour, but at great elevations, where the cold is severe, they are an assemblage of minute crystals of ice. They assume three primary characters, from whence four subordinate forms are derived. The cirrus, or cat’s-tail of sailors, is the highest; it sometimes resembles a white brush, at other times it consists of horizontal bands of slender silvery filaments. To these all Kämtz’s measurements assign a height of 19,500 feet, which is confirmed by their appearance being the same when seen from the tops of mountains or from the plains; consequently, they must consist of minute particles of ice or flakes of snow floating in the higher regions of the zone of clouds. The cirri for the most part arrange themselves in parallel bands which converge to opposite points in the horizon by the effects of perspective, and as they travel in their longitudinal direction they appear to be stationary. In the middle and higher latitudes of the northern hemisphere they tend from south-west to north-east, and at the equator from south to north. It is supposed that their parallel form arises from their being conductors between two foci of electricity, but, whatever the cause of this arrangement may be, it is very extensive. Among these clouds, which occasionally appear like fleecy cotton or wool, halos and parhelia are formed, which often precede a change of weather, announcing rain in summer, in winter frost and snow.
Cumuli, or summer-clouds, are rounded forms resting on a straight band in the horizon, and resemble mountains covered with snow. They are formed by ascending currents drawing the vapours into the higher regions of the atmosphere; sometimes they rise and cover the whole sky, and in the evening they frequently become more numerous and of deeper tint, presaging storm or rain.
The stratus is the third of the primary characters of clouds: it is a horizontal band, which forms at sunset and vanishes at sunrise. The subordinate varieties of clouds are combinations of these three principal classes.[139] The winds, the great agents in all atmospheric changes, carry the vapour to a distance, where it is often condensed on the tops of mountains into clouds which seem to be stationary, but which in reality are only maintained by a constant condensation of fresh vapour, which is carried off, as soon as formed, by the wind, and becomes invisible on entering warmer air.
When two masses of air of different temperature meet, the colder, by abstracting the heat which holds the moisture in solution, causes the particles to coalesce and form drops of water, which fall in the shape of rain by their gravitation. And when two strata of different temperature moving rapidly in contrary directions come into contact, a heavy fall of rain takes place; and as the quantity of aqueous vapour is most abundant in tropical regions, the drops are larger and the rain heavier than elsewhere.
Since heat is the cause of evaporation, rain is very unequally distributed, and with it decreases from the equator to the poles. From the island of Otaheite [Tahiti], in the Pacific, to Uleaborg, in Finland, the annual quantity of rain that falls decreases from 150 inches to 13. It is, however, more abundant in the New World than in the Old; 115 inches fall annually in tropical America, while in the Old World the annual fall is only 76 inches; so also in the temperate zone of the United States the annual quantity is 37 inches, while in the Old Continent it is but 31-3/4 inches.
Between the tropics the rains follow the sun: when he is north of the equator the rains prevail in the northern tropic; and when he is south of that line, in the southern: hence, one half of the year is extremely wet and the other half extremely dry; the change taking place near the equinoxes. Nevertheless, in countries situate between the 5th and 10th parallels of latitude, north and south, there are two rainy seasons, and two dry; one occurs when the sun passes the zenith in his progress to the nearest tropic, and the other at his return, but in the latter the rains are less violent and of shorter duration. Although the quantity of water which falls between the tropics in a month is greater than that of a whole year in Europe, yet the number of rainy days increases with the latitude, so that there are fewest where the quantity is greatest. Neither does it fall continually during the rainy season between the tropics, for the sky is generally clear at sunrise, it becomes cloudy at ten in the morning, at noon the rain begins to fall, and, after pouring for four or five hours, the clouds vanish at sunset, and not a drop falls in the night, so that a day of uninterrupted rain is very rare.
At sea, within the region of the trade-winds, it seldom rains, but in the narrow zone between them known as the Variables, in both the great oceans, it rains almost continually, attended by violent thunder and lightning.
Throughout the whole region where the monsoons prevail, it is not the sun, directly, but the winds, that regulate the periodical rains. That region extends from the eastern coasts of Africa and Madagascar across the Indian Ocean to the northern districts of Australia, and from the tropic of Capricorn to the face of the Himalaya, the interior of China, and even to Corea, inclusive. In these countries the western coasts are watered during the south-west monsoon, which prevails from April to October; and the eastern coasts are watered during the north-east monsoon, which blows from October to April. For example, the south-west wind condenses the vapour on the summit of the Ghauts, and violent rains fall daily on the coast of Malabar, while on the Coromandel coast the sky is serene. Exactly the contrary takes place during the north-east monsoon; it rains on the coast of Coromandel, while there is fair weather on the Malabar coast, and the table-land of the Deccan partakes of both. In the southern hemisphere the rainy season corresponds with the south-west monsoon, and the dry with the south-eastern.
Between the tropics it rains rarely during the night, and for months together not a drop falls; while in the temperate zone it often rains in the night, and rain falls at all seasons, though more abundantly in some than in others. It seldom rains in summer throughout the north of Africa, Madeira, the southern parts of Spain and Portugal, Sicily, southern Italy, all Greece, and the north-western part of Asia; but it falls copiously during the other seasons, especially in winter; consequently, that extensive region is called the province of winter rains.
The province of autumnal rains includes all Europe south of the Carpathians, western France, the delta of the Rhine, northern and western Scandinavia, and the British isles; throughout these countries more rain falls in autumn than in the other three seasons.
The province of summer rains comprises the eastern parts of France, the Netherlands (with the exception of the delta of the Rhine), the north of Switzerland, all Germany north of the Alps, the Carpathian mountains, Denmark, southern Scandinavia, all central Europe, and the countries beyond the Ural Mountains to the interior of Siberia, where showers are very rare in winter. In some places it rains almost perpetually, as in the island of Sitka, on the north-eastern coast of North America, where the year has sometimes passed with only 40 days of fair weather.
In the southern hemisphere, in Chile and the south-western part of America, winter is the rainy season, while on the eastern side of the Cordilleras the rains occur in summer. In Tierra del Fuego and the extreme point of the continent the two provinces meet, the periodical precipitation disappears, and it snows and rains throughout the year in torrents. At Cape Horn the quantity of rain which fell in 41 days measured nearly 154 inches. This excessive fall of rain occurs along the whole western shores of Patagonia, from the Straits of Magellan to Cape Tres Montes—a circumstance favoured by the high and rugged coasts, and the incessant westerly winds, which carry the vapour exhaled from the ocean to be precipitated here in the form of rain.
South Africa and Australia resemble each other in their rainy seasons, which in both countries take place in the winter months.
The annual amount of rain at the equator is 95 inches, which falls in 78 or 80 days, giving an average of 1·14 inch daily; while at St. Petersburg the annual amount is 17 inches, which falls in 169 days, the average being little more than the tenth of an inch daily.
The quantity of rain decreases in ascending from the plains to table-lands, especially if these be edged by mountains, because they precipitate the vapour before it arrives at the high plains. On the contrary, the quantity increases in ascending from plains to the tops or slopes of rugged mountains, on account of partial currents of air which condense the moisture into clouds.
The quantity of rain decreases on receding from the coasts into the interior of continents, because more vapour rises from the sea than from the land. The vapour from the Gulf-stream produces a greater quantity of rain and fog in the southern counties of England and Ireland than that which falls in the other parts of the islands.
The number of rainy days depends upon the direction of the wind. In Europe, if the wind always blew from the north-east, it would never rain, because it blows over a great extent of continent; whereas it would never cease raining were the wind always to blow from the south-west, because it would come loaded with vapour from the Atlantic. Hence, the greatest quantity of rain falls on the west coasts of Great Britain and Ireland, the coast of Scandinavia, the eastern Alps, and the centre of Portugal; in the two last it depends partly on the height and serrated form of the mountains. In western Europe it rains on twice as many days as in the eastern part; in Ireland there are three times as many rainy days as in Italy or Spain. In fact, on the western side of Ireland it rains on 208 days out of the 365. In England, France, and the north of Germany, there are from 152 to 155 rainy days in the year; the number decreases towards the interior of the continent, so that in Siberia it only rains on 60 days in the year.
There are enormous tracts of land on which rain never falls, and others where it rains at long intervals and in small quantities. The most extensive rainless district stretches from the borders of Morocco eastward through the desert of Africa, the low coasts of Arabia, Persia, and the desert province of Meekran, in Beloochistan, occupying a space of 80 degrees of longitude and 17 of latitude. The desert of Gobi, on the table-land of Tibet, and part of Mongolia, form another rainless province in the great continent; while, in the New World, the rainless districts are—the table-land of Mexico, part of Guatemala and California, and the western declivity of the Andes of Peru, towards the Pacific; in all occupying a surface equal to 5,500,000 square miles. The whole of the moisture is intercepted by the Andes of Peru; so that rain only occurs on the coast once or twice in a century—to the great terror of the inhabitants when it does fall. [The absence of rain is here compensated for by copious dews and mists, termed “llovisnas.”] South Africa, and Australia beyond the tropics, suffer from droughts, which are periodical in Australia; they recur in the countries of the eastern coasts in a period of 12 years, and continue 3 years. The Pampas of South America are also subject to droughts, though they do not appear to be periodical, nor do they continue more than a season.[140]
When the temperature of the air is near the freezing-point or below it, snow falls instead of rain; but the colder the air the less moisture does it contain, consequently the less snow falls, which is the reason of the comparatively small quantity on the high plains of the Himalaya and Andes. Snow sometimes assumes the form of grains; but is generally in regular crystals of great beauty, varying in form according to the degree of cold. Captain Scoresby, whose voyages in the Polar Seas afforded him constant opportunities of studying them, of which he so diligently availed himself, mentions five principal kinds of snow crystals, each of which had many varieties, in all amounting to 96. M. Kämtz, however, is of opinion that there are several hundred.
Snow never falls between the tropics except on the tops of very high mountains. The mean elevation of the line of perpetual snow above the level of the sea in these hot regions is about 15,207 feet, from whence it decreases on both sides, and at last grazes the surface of the earth at the arctic and antarctic circles, subject however to various flexures. In the Andes, near Quito, the lowest level has an elevation of 15,795 feet, which is higher than the top of Mont Blanc; from thence it varies very irregularly, both to the north and south. In 18° of N. lat. it descends to 14,772 feet on the mountains of Mexico, while on the south it rises to 18,000 feet in some parts of the western Cordillera of the Bolivian Andes, owing to the extensive radiation from the subjacent plains and valleys. The line is at an altitude of 17,000 feet on the western Cordillera, whence it sinks to 13,800 feet at Copiapo, to 12,780 near Valparaiso; it is only 8300 in the southern end of the Chilian Andes, and 3390 in the Straits of Magellan. In lat. 31° N. the snow-line is at an elevation of 12,981 feet on the southern side of the Himalaya, and at 16,620 feet on the northern side, while Captain Gerard gives from 18,000 to 19,000 as its altitude on the mountains in the middle of the plain of Tartary. On Mont Blanc the line is at the height of 8500 feet, so that mountain is snow-clad for 7000 feet below its summit. In the Pyrenees it is 8184 feet, and at the island of Mageroe it is at 2160 feet above the Polar Ocean.
In the southern hemisphere, snow never falls on the low lands at the level of the sea north of the 48th parallel of latitude, on account of the predominance of water, whereas in the northern hemisphere it falls on the plains much nearer the equator, on account of the excess of land, but its limit is a curved line, on account of the alternations of land and water. In the western part of the great continent, the southern limit of the fall of snow on the low lands nearly coincides with the 30th parallel of north latitude, so that it includes all Europe. In the American continent it follows nearly the same line, extending through the southern parts of the United States. In China, snow falls at the level of the sea as far south as Canton; on the north-western coast of America, on the contrary, it does not fall at that level till about the 48th degree of N. lat.—these are the two extremes. Although Europe lies within the region of snow, the quantity that falls is very different in different places, increasing greatly from south to north. On an average, it snows only one day and a-half at Rome in the year, while at Petersburg there are 171 snowy days, but in that city the quantity of rain is to that of snow as 1000 to 384.
Sleet, which is formed of small particles of rounded hail, falls in squally weather in spring and autumn. True hail, when large, is pear-shaped, and consists of a nucleus of frozen snow coated with ice, and sometimes with alternate layers of snow and ice. Hailstones have often fallen as large as pigeons’ and even hens’ eggs. The masses and blocks of ice of great size, which have not unfrequently fallen, appear to have been formed of hailstones of large size frozen together. It appears to be formed in the high cold regions of the atmosphere, by the sudden condensation of vapour during the contention of opposing winds, and is intimately connected with electricity, since its fall is generally accompanied with thunder and lightning. Hail-showers are of short duration, exceedingly partial, and extend over a country in long narrow bands; one which took place on the 13th of July, 1788, began in the morning in the south of France, and reached Holland in a few hours, destroying a narrow line of country in its passage.
Local circumstances, no doubt, have a great influence on its formation; it occurs more frequently in countries at a little distance from mountains than in those close to them or farther off, and at all hours, but most frequently at the hotter time of the day. In the interior of Europe one half of the hail-storms take place in summer. Hail is very rare on the tropical plains, and often altogether unknown, though it frequently falls at heights of 1700 or 1800 feet above them. If the air is very cold throughout the greater part of the stratum through which hail falls, it is probably increased in size during its descent; and, on the contrary, large drops of rain which precede a thunder-storm are supposed to be hail melted in its passage through low warm air.
LIGHT.
We know nothing of the size of the ultimate particles of matter, except that they must be inconceivably small, since organized beings possessing life and exercising all its functions have been discovered so minute that a million of them would occupy less space than a grain of sand.
The air is only visible when in mass; the smallest globule of steam tells no more of its atoms than the ocean; the minutest grain of sand magnified appears like the fragment of a rock—no mechanical division can arrive at the indivisible. Although the ultimate atoms are beyond the power of vision, chemical compounds show that the divisibility of matter has a limit, and that the particles have different densities; moreover, the cleavage of crystalline substances gives reason to believe that they have different forms.[141] Thus, the reasoning power of man has come to the aid of his imperfect sense of vision, so that what were before imaginary things are now real beings with definite weights, and uniting by fixed laws. Though nothing had been known of their size, their effects were evident in the perceptions of sweet and sour, salt and bitter, and in the endless varieties of aroma in the food we eat and the liquors we drink. Moreover, their different densities are evident, as they arise by their buoyancy in the perfume of the rose, or sink by their weight in the heavy odour of mignonette. Every substance on earth is merely a temporary compound of the ultimate atoms, sooner or later to be resolved into its pristine elements, which are again to be combined in other forms, and according to other laws; so that literally there is nothing new under the sun, for there is no evidence of new matter being added to the earth, nor of that which exists being annihilated. Fire, which seems utterly to destroy, only resolves bodies into their elementary parts, to become what they were before, the support of animal or vegetable life, or to form new mineral compounds. It is to the action of these particles on the light of the sun that nature owes all its colours.
When a sunbeam passes through a glass prism[142] an oblong image of the sun is formed, consisting of colours in the following order—red, orange, yellow, green, blue, indigo, and violet. Sir John Herschel discovered lavender rays beyond the violet, and dark red rays exterior to the red, which are not so easily brought into evidence as the rest.
Even the most transparent substances absorb light; air, water, the purest crystal, stop some of the rays as they pass through them. A portion of the light is also reflected from the surface of all bodies; were it otherwise, they would be invisible. We should be unconscious of the presence and form of material substances beyond our reach except by the reflected rays,—
“The mist of light from whence they take their form Hides what they are.”
As the same light does not come to all eyes, each person sees his own rainbow, the same flower by different rays. White substances reflect all the light, black substances absorb all but that which renders them visible, while coloured bodies decompose the light, absorb some of the colours, and reflect or transmit the rest. Thus, a violet absorbs all but the violet rays, which it reflects; a red flower only reflects the red and absorbs the rest; a yellow substance absorbs all but the yellow. In the same manner transparent substances, whether solid or fluid, absorb some colours and transmit others: thus, an emerald absorbs all but the green, a ruby all but the red; whereas a diamond does not decompose the light, but transmits every ray alike. Very few, however, of the colours, whether transmitted or reflected, are pure, but the substance takes its hue from the colour that predominates.
The atmosphere absorbs all the colours of the sun’s light except the blue, which is its true colour. In countries where the air is pure, the azure of the sky is deep; it is still more so at great elevations, where the density of the air is less; and its colour is most beautiful as it gradually softens the outlines of the mountains into extreme distance, or blends the sea with the sky. The air reflects and scatters part of the white solar beams, whence the brightness and cheerfulness of day; that property, together with the refractive power of the aqueous vapour, gives the roseate hue to the early morning, and the gold and scarlet tints to the closing day. Were it not for the reflective power of the air, the sun and moon would be like sharply-defined balls of fire in the profoundly black vault of the heavens, and dark night would instantly follow sunset. When the sun is 18 degrees below the horizon, the air, at the height of 30 miles, is still dense enough to reflect his rays, and divide the day from the night by the solar shades of twilight.
A considerable portion of the sun’s light is absorbed by the atmosphere: the loss increases with the density and obliquity of incidence and the density of the air. It is diminished 1300 times by the thickness of the air in the horizon, which enables us to look at the sun when setting without being dazzled.
Mirage, or the delusive appearance of water, so frequent in deserts, is owing to the reflection of light between two strata of air of different densities, occasioned by the radiation of heat from the arid soil. It is very common on the extensive plains in Asia and Africa, and especially in Upper Egypt; villages on small eminences above the plain appear as if they were built on islands in the middle of a lake when the dry sandy ground is heated by the mid-day sun. Sometimes objects appear double, and occasionally several images appear above one another, some direct and some inverted; this is particularly the case in high latitudes, where the Icy Sea cools the stratum of air resting on it.[143]
In the polar regions, or on the tops of mountains, when the sun is in the horizon the shadow of a person is sometimes thrown on an opposite cloud or mist, the head being surrounded by concentric coloured rings or circles, the number varying from one to five; Captain Scoresby saw four of these rings, on one occasion, round the shadow of his head, as he stood between the sun and a thick low fog: the first ring consisted of concentric bands of white, yellow, red, and purple; the second consisted of concentric bands of blue, green, yellow, red, and purple; the third of green, white, yellowish white, red, and purple; and in the fourth were greenish white, deeper on the edges. These appearances, called glories, or fog-images, and the coronæ or small concentric coloured circles which surround the sun or moon when partly obscured by thin white clouds, are owing to the refraction of the light in the aqueous particles of the cloud or fog. The colours in the concentric bands of the coronæ, however, differ from the foregoing; that nearest the sun is of deep blue, white, and red; the circle exterior to that consists of purple, blue, green, pale yellow, and red; but the series is very rarely complete.
Halos, which surround the sun in large circles, or a complicated combination of circles, are, on the contrary, supposed to be produced by the light falling on minute crystals of ice suspended in the atmosphere; they are particularly brilliant and frequent in high latitudes. It is scarcely possible to give an idea of these beautiful and singular objects. Sometimes a large coloured circle surrounds the sun or passes through his centre, which is occasionally touched or cut by segments of others. One seen at St. Petersburg on the 29th of June, 1790, consisted of four coloured circles of different sizes intersecting each other, which were either cut or touched by segments of eight others, and at the points of intersection mock suns or parhelia appeared. The sky is very hazy on these occasions. Mock suns, without circles and halos, are by no means uncommon round both sun and moon, but seldom of that complicated kind. They are situate between the observer and the sun, whereas the rainbow is always in that part of the sky opposite the sun, because it is produced by refraction and reflection of the sun’s rays in the drops of rain; and when the light is intense and the rain abundant, there are two concentric bows, the prismatic colour of the innermost of which are the most vivid, the violet being within and the red outside: sometimes the inner edge exhibits a repetition of colours in fine fringes, in which red and green predominate. The colours are reversed in the exterior bow, the violet being outside and the red on the inner edge. Besides these two principal and most common bows, supernumerary rainbows occasionally appear within the interior bow, generally green and violet, though there are sometimes more or less perfect repetitions of all the colours. In squally weather a rainbow is sometimes seen on a blue sky when rain is falling, but it is generally on clouds; it is constantly seen when the sun shines on the fine drops of fountains and cascades. As the light of the moon is feeble, lunar rainbows are rare, and, for the most part, colourless. In the early morning, when the sun throws his slanting beams across the fields, a miniature bow, with all its vivid colours, may be seen in each dewdrop as it hangs on the points of the bending grass.
Light is said to be polarized when, after having been once refracted or reflected, it is rendered incapable of being again refracted or reflected at certain angles. For example, if a crystal of brown tourmaline be cut longitudinally into thin slices, and polished, the light of a candle may be seen through a slice as if it were glass. But if one of these slices be held perpendicularly between the eye and the candle, and a second slice be turned round between the eye and the other plate of tourmaline, the image of the candle will vanish and come into view at every quarter-revolution of the plate, varying through all degrees of brightness down to total or almost total evanescence, and then increasing again by the same degrees as it had decreased. Thus, the light, in passing through the first plate of tourmaline, is said to be polarized because it has been rendered incapable of passing through the second piece of tourmaline in certain positions.
A ray of light acquires the same property if it be reflected from a pane of plate-glass at an angle of 57 degrees; it is by that rendered incapable of being reflected by another pane of plate-glass in certain definite positions, for the image of the light vanishes and reappears alternately at every quarter-revolution of the second pane.
If a thin plate of mica be interposed when the image of the candle has vanished, the darkness will instantly disappear, and a succession of the most gorgeous colours will come into view, varying with every inclination of the mica, from the richest reds to the most vivid greens, blues, and purples. The most splendid colours arranged in symmetrical forms are exhibited by thin plates of an infinite variety of substances besides mica. They display some of the most beautiful objects in nature, and show differences otherwise inappreciable in the arrangement of the molecules of crystalline bodies.[144]
M. Arago discovered that the light of the sun is polarized by the reflection of the atmosphere, but not equally so on every part of the sky; the polarization is least in the vicinity of the sun, and greatest at 90° from him, for there his light is reflected at an angle of 45°, which is the polarizing angle for air.[145] There are three points in the sky where the light is not polarized: one of these neutral points, discovered by M. Arago, is 18° 30ʹ above the point diametrically opposite to the sun when he is in the horizon; the second neutral point, discovered by M. Babinet, is 18° 30ʹ above the sun when he is rising or setting; and the third, discovered by Sir David Brewster, is 15° or 16° below the sun. These points vary with the height of the sun, and the two latter rise and coincide in his centre when he is in the zenith.[146]
Now, the portion of polarized light sent to the eye from any part of a clear sky is in a plane passing through that point, the eye of the observer, and the centre of the sun. If that point be the north pole of the heavens, it is clear that, as the sun moves in his diurnal course, the plane will move with him as an hour circle, and may be used as a dial to determine the hour of the day. Professor Wheatstone, by whom that beautiful application of the polarization of the atmosphere has been made, has constructed a clock, of very simple form, which shows the time of day with great accuracy, and which has many advantages over a sun-dial.
ELECTRICITY.
Electricity pervades the earth, the air, and all substances, without giving any visible sign of its existence when in a latent state, but, when elicited, it exhibits forces capable of producing the most sudden, violent, and irresistible effects. It is roused from its dormant state by every disturbance in the chemical, mechanical, or calorific condition of matter, and then experience shows that bodies in one electric state repel, and in another they attract each other. Probably their mutual attraction and repulsion arise from the redundancy and defect of electricity; in the first case they are said to be positively, in the latter negatively electric.[147] When they have different kinds of electricity they attract each other, and, when not opposed, the electricity coalesces with great rapidity, producing the flash, explosion, and shock, and that with the more violence the greater the tension or pressure of the electricity on the surrounding air which resists its escape. Equilibrium is then restored, and the electricity remains latent till called forth by a new exciting cause. The electrical state of substances is easily disturbed, for, without contact, positive electricity tends to produce negative electricity in a body near it, and vice versâ: the latter is then said to be electric by induction.
The electricity of the atmosphere arises from evaporation, and the chemical changes that are in perpetual progress on the globe; no electricity, however, is developed by the evaporation of pure water, but it arises abundantly from water containing matter susceptible of chemical action during the evaporation; consequently, the ocean is one of the greatest sources of atmospheric electricity; combustion is another, and a large portion arises from vegetation. The air, when pure, is almost always positively electric; but as the chemical changes on the earth sometimes produce positive and sometimes negative electricity, it is subject to great local variations; a passing cloud or a puff of wind produces a change, and a distant storm renders it negative for the time, but the earth is always in a negative state. The quantity of electricity varies with the hours of the day and the seasons; it is more powerful in the day than in the night, in winter than in summer, and it diminishes from the equator to the poles. It thunders daily in many places, in others never, as on the east coast of Peru and in the Arctic regions, except where there are violent volcanic explosions, which always generate electricity, as in Iceland. Wherever there are no trees or high objects to conduct it to the ground, the quantity of positive electricity increases with the height above the surface of the earth. Violent thunder-storms take place on the tops of the Andes and Himalaya mountains, at heights of 26,650 feet above the plains.
Electricity becomes very strong when dew is deposited, and in some cases it is strongly developed in fogs. Mr. Cross found it so powerful on one occasion, that it was dangerous to approach the apparatus for measuring its intensity. A continued succession of explosions lasted nearly five hours, and the stream of fire between the receiving-ball and the atmospheric conductor was too vivid to look at. M. Peltier has found that the common fogs arising from the mere condensation of the moisture in the air are neutral, but that others, which are produced by exhalations from the earth, are sometimes positive, sometimes negative; the subject, however, requires further investigation.
Though in long-continued mild rains there are no traces of electricity, yet, when rain or snow falls from the higher regions of the atmosphere, it is more or less developed, sometimes positive, sometimes negative, depending a good deal on the direction of the wind. The atmosphere being positively electric, negative rain is supposed to arise from the evaporation of the drops in passing through dry air; the vapour carries off the positive electricity and leaves the drop in a negative state—a circumstance which seems to be confirmed by the electricity of cascades, near which there always is more or less negative electricity; the positive flows into the earth, while the other remains united to the drops of the cascade.
The inductive action of the earth upon the clouds, and of the different strata of clouds on each other, produces great variations in their electrical state. If rain falls from the lowermost of two strata of positively electrical clouds, the inductive action of the earth renders the under surface positive and the upper negative, and the rain is positive. By-and-bye the under surface of the cloud and the earth become neutral; and after a time the lower cloud becomes charged with negative electricity by the induction of the upper strata, and the rain is then negatively electric. Clouds are very differently charged; grey clouds have negative—red, white, and orange clouds positive electricity; and when clouds differently charged meet, an explosion takes place. When the sky is clear and the air calm and warm, a succession of small white fleecy clouds rising rapidly above the horizon, and flying swiftly in the very high regions of the atmosphere, is a certain presage of a thunder-storm.
Electricity of each kind is probably elicited by the friction of currents of air, or masses of clouds moving rapidly in different directions, as in thunder-storms, when small white clouds are seen flying rapidly over the black mass; yet the quick and irregular motion of clouds in storms is probably owing to the strong electrical attraction and repulsion among themselves, though both may be concerned in these hostile encounters. When two clouds differently charged by the sudden condensation of vapour, and driven by contending winds, approach within a certain distance, the thickness of the coating of electricity increases on the two adjacent sides, and, when the accumulation becomes so great as to overcome the coercive pressure of the atmosphere between them, a discharge takes place which occasions a flash of lightning. The actual quantity of electricity in any part of a cloud is very small. The intensity of the flash depends upon the extent of surface occupied by the electricity, which acquires its intensity by its instantaneous condensation.
The air, being a non-conductor, does not convey the electricity from the clouds to the earth, but it acquires from them an opposite electricity, and when the tension is very great the force of the electricity becomes irresistible, and an interchange takes place between the clouds and the earth, but the motion of the lightning is so rapid that it is difficult to ascertain when it goes from the clouds to the earth, or from the earth to the clouds, though there is no doubt it does both: explosions have burst from the ground, and people have been killed by them.
When the air is highly rarified by heat, its coercive power is diminished, so that the electricity escapes from the clouds in the form of diffuse lambent sheets of lightning without thunder or rain, frequently seen in warm summer evenings, sometimes even near the zenith, and quite different from that sheet-lightning at the horizon which is in general only the reflection of the forked lightning of a distant storm. When the quantity of electricity developed by the sudden condensation of vapour is very great, the lightning is always forked; its zigzag form is occasioned by the unequal conducting power of the air, by which it is sometimes divided into several branches. The author once saw a flash divide into four parallel streams—a very uncommon occurrence. Occasionally, in very great storms, the lightning sends off lateral branches. It often appears as a globe of fire moving so slowly that it is visible for several seconds, while the flashes of forked lightning do not last the millionth part of a second. Professor Wheatstone, who has measured the velocity of lightning by experiments of great ingenuity, found that it far surpasses the velocity of light, and would encircle the globe in the twinkling of an eye. This inconceivable velocity is beautifully exemplified in the electric telegraph, by which the most violent and terrific agent in nature is rendered obedient to man, and conveys his thoughts as rapidly as they are formed. The colour of lightning is generally a dazzling white or blue, though in highly rarified air it is rose-colour or violet.
The sudden compression of the air during the passage of lightning must convert a great quantity of latent into sensible heat, for heat in a latent or insensible state exists in all bodies independent of their temperature. Heat is absorbed and becomes insensible to the thermometer when solids become liquids, and when liquids are changed to vapour; and it again becomes sensible when vapour is condensed, and when liquids become solid. When water freezes, all the heat that kept it liquid is given out; and when ice melts, it absorbs heat from everything near it. The air is full of heat in a latent state, whatever its temperature may be, but it can be squeezed out by sudden compression so as to kindle tinder. Every aërial wave, every sound, every word spoken must set free an infinitesimal quantity of heat; so everything that tends to rarify the air must cause it to absorb a proportional quantity.
The rolling noise of thunder is probably owing to the difference between the velocity of lightning and that of sound. Thunder may be regarded as originating in every point of a flash of lightning at the same instant; and as sound takes a considerable time to travel, it will arrive first from the nearest point; and if the flash run in a direct line from a person, the noise will come later and later from the remote points of its path, in a continued roar. Should the direction of the flash be inclined, the succession of sounds will be more rapid and intense; and if the lightning describe a circular course above a person, the sound will arrive at the same instant from every point with a stunning crash.[148]
In passing to the earth, lightning follows the best conductors—metals by preference, then damp substances—which is the reason why men and animals are so often struck. If it meets with a bad conductor, it shivers it to pieces and scatters the fragments to a considerable distance. A powerful flash scatters gunpowder, while a feeble one ignites it; the hardest trees are split and torn to shreds; when a tree is struck, the heat of the flash converts the sap into steam, the expansive force of which shivers the tree. The surface of rocks is vitrified by it; and when it falls on a sandy soil, its course underground is marked by vitrified tubes many feet long.
Thunder-storms occur daily within the region of the Variables, which is also the region of storms: in countries under the influence of the monsoons they are tremendous at the changes of these periodical winds; where the trade-winds prevail they are hardly known, though electrical discharges are frequent at their limits. In Greece and Italy there are about 40 thunder-storms annually, which occur in spring and autumn, while north of the Alps they chiefly take place in summer. There are about 24 in the year on the coasts of the Atlantic and in Germany, but they are much more frequent among mountains than on plains. In the interior of the old continent they rarely occur in winter, and three-fourths of the number happen in summer. They are of such rare occurrence in high latitudes, that in a residence of 6 years in Greenland Sir Charles Geiseke only heard it thunder once.
Some storms arise from the contention of opposite currents in the air; others are occasioned by currents of warm air ascending from the earth, which are suddenly condensed as they enter the upper regions of the atmosphere, and, as this sometimes happens at the hottest hour of the day, these storms are periodical for many successive days, recurring always at the same hour. Sometimes they extend over a great expanse of country, and the lightning darts from all points of the compass. A person may be killed at the distance of 20 miles from the explosion by the back stroke. If the two extremities of a highly-charged cloud dip towards the earth, they will repel the electricity of the earth, if it be of the same kind with their own, and will attract the other kind; and if a discharge should take place at one end of the cloud, the equilibrium will instantly be restored by a flash from that part of the earth which is under the other, sufficiently strong to destroy life, and it is the most dangerous, though never so strong as the direct stroke.
When thunder-clouds are very low, there is frequently no lightning; the electricity produced by induction is so powerful that it escapes from pointed objects in the shape of flame without heat, known as St. Elmo’s fire. These flames are not unfrequently seen at the topmasts of ships and the extremities of their yard-arms. Bodies between the clouds and earth may be electrized by induction, and their electricity will be seen in the form of flame, as showers of phosphorescent snow.
Phosphorescence is ascribed to electricity; various substances emit light when decaying, as fish and wood. Although many marine animals are phosphorescent, yet the luminous appearance which the sea often assumes is not always to be attributed to them, but probably to the decaying animal matter it contains.
The aurora is decidedly an electrical phenomenon. It generally appears soon after sunset in the form of a luminous arch stretching more or less from east to west, the most elevated point being always in the magnetic meridian of the place of the observer: across the arch the coruscations are rapid, vivid, and of various colours, darting like lightning to the zenith, and at the same time flitting laterally with incessant velocity. The brightness of the rays varies in an instant: they sometimes surpass the splendour of stars of the first magnitude, and often exhibit colours of admirable transparency, blood-red at the base, emerald-green in the middle, and clear yellow towards their extremity. Sometimes one, and sometimes a quick succession of luminous currents run from one end of the arch or bow to the other, so that the rays rapidly increase in brightness; but it is impossible to say whether the coruscations themselves are actually affected by a horizontal motion of translation, or whether the more vivid light is conveyed from ray to ray. The rays occasionally dart far past the zenith, vanish, suddenly reappear, and, being joined by others from the arch, form a magnificent corona or immense dome of light. The segment of the sky below the arch is quite black, as if formed by dense clouds; yet M. Struve is said to have seen stars in it, consequently the blackness must be from contrast. The lower edge of the arch is evenly defined; its upper margin is fringed by the coruscations, their convergence towards the north, and that of the arch itself, being probably an effect of perspective.
Either the aurora must be high above the earth, or its coruscations must be very extensive, since the same display is visible at places wide asunder. It has frequently been seen in North America and all over the north of Europe at the same time, sometimes even as far south as Italy, yet Sir Edward Parry certainly saw a ray dart from it to the ground near him. M. Struve, Admiral Wrangel, and others who have had many opportunities of seeing the aurora in high latitudes, assign a very moderate elevation to it. The arch probably passes through the magnetic pole; hence, in the north of Greenland it lies south of the observer, and Sir Edward Parry saw it to the south in Melville Island, which is in 70° N. lat.; consequently it must appear in the zenith in some places. Dr. Faraday conjectures that the electric equilibrium of the earth is restored by the aurora conveying the electricity from the poles to the equator, for it appears in the high southern latitudes, as well as in the northern; and the Rev. G. Fisher has lately suggested, that, as the principal display of the aurora takes place at or near the margin of the polar ice, the electricity may be conveyed by the conducting power of the frozen particles which abound in the air in these latitudes, and which, being rendered fitfully luminous by the passage of the electricity, produce the arch and the ever-varying flashes of the aurora.
The aurora has a powerful influence on the magnetic needle, even in places where the display is not seen. Its vibrations seem to be slower or quicker according as the auroral light is quiescent or in motion, and the disturbances of the magnetic needle and the auroral displays were simultaneous at Toronto, in Canada, on 13 days out of 24, the remaining days having been clouded; and contemporaneous observations show that on these 13 days there were also magnetic disturbances at Prague and at Van Diemen’s Land, so that the “occurrence of aurora at Toronto on these occasions may be viewed as a local manifestation connected with magnetic effects, which, whatever may have been their origin, probably prevailed on the same day over the whole surface of the globe.”[149]
MAGNETISM.
Magnetism is one of those unseen imponderable existences which, like electricity and heat, are known only by their effects. It is certainly identical with electricity, for, although it never comes naturally into evidence, magnets can be made to exhibit all the phenomena of electrical machines.
Terrestrial magnetism, which pervades the whole earth, is extremely complicated; it varies both with regard to space and time, and, probably, depends upon the heat of the sun, upon his motion in the ecliptic, which produces changes of temperature, on galvanic currents circulating through the surface of the globe, and possibly on the earth’s rotatory motion.
The distribution of terrestrial magnetism is determined by the declination-needle, or mariner’s compass, and the dipping-needle; they consist of magnetized needles or bars of steel, so suspended that the declination-needle revolves in a horizontal direction, and the dipping-needle moves in a plane perpendicular to the horizon. The north end of the declination-needle or magnet points to the north, and the south end to the south, and it only remains at rest when in that position. The direction of the needle is the magnetic meridian of the place of observation.
The north end of the dipping-needle bends or dips below the horizon in the northern hemisphere, and the south end bends or dips beneath it in the southern hemisphere, and between the two there is a line which encircles the whole earth, where the dipping-needle remains horizontal. That line, which is the magnetic equator or line of no dip, crosses the terrestrial equator in several places, extending alternately on each side, but never deviating more than 12 degrees from it. The deviation is greater in that part of the Pacific where there are most islands, and it is greatest both to the south and north in traversing the continents of Africa and America; thus, it appears that the configuration of the land and water has an influence on terrestrial magnetism. North and south of the magnetic equator the needle dips more and more, till at last it becomes perpendicular to the horizon in two points, or rather linear spaces, known as the north and south magnetic poles, which are quite distinct from the poles of the earth’s rotation. One, whose position was determined by Captain Ross, is in 70° N. lat. and 97° W. long., while that in the southern hemisphere, determined by Sir James Ross, in the interior of Victoria Island, is in 70° S. lat. and 162° E. long. Lines of equal dip are such as may be drawn on a globe through all those places where the dipping-needle makes the same angle with the horizon. The angle of the dip is not always the same: according to Colonel Sabine, who is the highest authority on this subject, it has been decreasing in the northern hemisphere, for the last fifty years, at the rate of three minutes annually: it is also subject to variations of short periods, and it seems to be affected by shocks of earthquakes, even when very distant.
The intensity of the magnetic force is as variable and even more complicated than the other magnetic phenomena: it is measured by the number of vibrations made by the declination-needle in a given time. It is very different in different parts of the earth, but there are four points in which the intensity is greater than anywhere else. Two of these are in the northern and two in the southern hemisphere; they neither coincide with the poles of the earth’s rotation nor with the magnetic poles, nor are they all of equal intensity.
One of these foci of maximum magnetic intensity is situate in North America, south-west from Hudson’s Bay; another is in northern Siberia in 120° E. long. In the southern hemisphere, one of the points of maximum magnetic intensity is in the South Atlantic in 20° S. lat. and 324° E. long., and the other is situate in 60° S. lat. and 131° 20ʹ E. long.[150] In consequence of the unequal intensity of the force in these 4 foci, the decrease in magnetic power from them towards the equator is extremely irregular, so that the dynamic equator, which is a line supposed to be drawn through all the points on the earth where the intensity is the least, encircles the globe in a waving line, which neither coincides with the geographical nor magnetic equator; it forms the division between the magnetic intensities in the two hemispheres. Lines drawn on a globe through all the points where the magnetic intensity is the same are so complicated that it is scarcely possible to convey an idea of them in words. They form a series of ovals round each of the foci of maximum force, then a figure of 8 in each hemisphere having a focus and its ovals in each loop, then they open into tortuous lines which encompass the globe, but which become less so as they approach the dynamic equator. The complication is increased by the foci in the two hemispheres being unsymmetrically placed with regard to one another, as well as by the difference in their intensities.
The declination or horizontal needle only remains at rest when in a magnetic meridian, that is, when it points to the north and south magnetic poles. The magnetic meridians coincide with the geographical meridians in some places, and in these the magnet points to the true north and south, that is, to the poles of the earth’s rotation. But if it be carried successively to different longitudes, it will deviate sometimes to the east, sometimes to the west of the true north. Imaginary lines on the globe, passing through all places where the magnet points to the poles of the earth’s rotation, are lines of no variation; and lines passing through all places where the magnet deviates by an equal quantity from the geographical meridians are lines of equal variation; they are also very irregular, and form two closed systems or loops,—that is, they surround two points, one in northern Siberia and another in the Pacific, nearly in the meridian of the Pitcairn Islands and the Marquesas.[151]
The whole magnetic system is perpetually undergoing secular and periodical changes, which are so irregular and complicated that half a century is sufficient to alter the form and position of all the lines that have been mentioned. The foci of magnetic intensity, and the whole system represented by the magnetic lines, are moving along the two hemispheres in opposite directions; those in the northern hemisphere are going from west to east, and those in the southern from east to west; and as the foci of maximum intensity move with different velocities, the forms, as well as the places, of the curves are slowly, yet continually, changing. The weaker magnetic focus in the northern hemisphere moved through 50 degrees of longitude in 250 years.
The declination is subject to periodic variations depending upon the position of the moon, and to annual variations arising from the motion of the sun in the ecliptic, as well as to horary variations corresponding to changes of temperature from the diurnal rotation of the earth.
Throughout the middle latitudes of the northern hemisphere the north end of the magnet has a mean motion from east to west from eight in the morning till half-past one, it then moves to the east till evening, after which it makes another excursion to the west, and returns again to its original position at eight in the morning. The extent of its variation is greater in the day than in the night, in summer than in winter. It decreases from the middle latitudes in Europe, where it is 13 or 14 minutes, to the equator, where it is only 3 or 4; but at the equator the variations are performed with extreme regularity. The horary motions of the south end of the magnet in the southern hemisphere are accomplished in an exactly opposite direction. Between these two magnetic hemispheres there is a line passing through an infinity of places, and very nearly coinciding with the line of minimum magnetic intensity, where the horary phenomena of both hemispheres are combined, each predominating alternately at opposite seasons. At St. Helena, which is one of the places in question, and nearly on the line of minimum intensity, the horary motion of the north end of the magnet corresponds in direction during one half of the year with the movement in the northern hemisphere, and in the other half of the year the direction at the same hours corresponds with that in the southern hemisphere, the passage from the one to the other being at the equinoxes, when the diurnal variations at the usual hours partake more or less of the characteristics of both on different days.[152]
It thus appears that there are six points on the earth peculiarly remarkable for magnetic phenomena, all of which are distinct from one another, and from the poles of the earth’s rotation—namely, two magnetic poles where the dipping-needle makes an angle of 90 degrees with the horizon. The magnetic equator corresponds with these in every point of which the angle of the dip is zero: it encircles the earth, and intersects the terrestrial equator, but does not coincide with it. The other four points are the foci of maximum magnetic intensity, and to them the dynamical equator or line of minimum magnetic intensity corresponds, also surrounding the earth in an irregular line, but which coincides with neither the terrestrial nor magnetic equator. Besides these, and either partly or nearly coinciding with the line of minimum intensity, is that line which is supposed to pass through all places where the horary variations of the magnet partake of the phenomena of each hemisphere alternately.
The earth’s magnetism is subject to vast unaccountable commotions or storms of immense extent, which occur at irregular intervals, and are of short duration. In 1818, a magnetic storm, shown by a violent agitation of the needle, took place at the same time over 47 degrees of longitude, extending through all the countries from Paris to Kasan; and on the 25th of September, 1841, one of these storms was simultaneously observed at Toronto in North America, at the Cape of Good Hope, Prague in Europe, at Macao in China, and there is reason to believe that it extended to Van Diemen’s Land. Similar storms have happened simultaneously in Sicily and at Upsala in Sweden; others of less extent and shorter periods more frequently occur, and are, like the greater storms, not to be attributed to any known cause.
M. Necker de Saussure has traced a marked coincidence between the prevailing direction of the stratified masses of the mountain chains and that of the curves of equal magnetic intensity. The coincidence is perfect in the Ural chain, for there the lines of force tend north and south; and they do not deviate much from the stratification in the great plains of European Russia. There is every reason to believe that a coincidence takes place in the Scandinavian mountains, for a line of equal magnetic intensity passes parallel to the Norwegian coast. In Scotland, a line almost coincides with the Grampians; and as it becomes less northerly before reaching Portugal and Spain, it is there also in singular coincidence with the sierras on the table-land; the Pyrenees, however, form an exception to the law. A magnetic line follows the break of the chain of the Alps with great precision. The intersection of two upheavals makes these mountains alter their direction from S.W. and N.E. to E. nearly, and near to that change the magnetic line takes a similar bend, and coincides with the Caucasus, Taurus, Hindoo-Coosh, Himalaya, and Chinese mountains, after which it again tends to the north, and follows the Yablonia chain to Behring’s Straits.
In Africa, the lines of equal magnetic force coincide with the Komri, and with the lofty sea-coast range which unites the mountains of Abyssinia with those at the Cape of Good Hope. Throughout North America the lines of equal force coincide with the Alleghanies, and on the coast of the Pacific they take the direction of the Rocky Mountains. In Mexico, the stratified rocks are parallel to the mountains of Anahuac, which is the same with the direction of the magnetic curves, and a similar coincidence takes place in the Parima ranges, and in the coast-chain of Venezuela. The Andes, and the lines of equal magnetic intensity, are completely discordant, for they cross one another; but lines of equal magnetic force stretch from the southern promontories of America and Asia to the mountains of Victoria Land.
There is strong presumptive evidence of the influence of the electric and magnetic currents on the formation and direction of the mountain masses and mineral veins, but their slow persevering action on the ultimate atoms of matter has been placed beyond doubt by the formation of rubies and other gems, as well as various other mineral substances, by voltaic electricity.
The existence of electric currents on the surface of the earth has been deduced from terrestrial magnetism, and from the connection between the diurnal variations of the magnet and the apparent motion of the sun; also from the electro-magnetic properties of metalliferous veins, and from atmospheric electricity, which is continually passing between the air and the earth.
Dr. Faraday’s brilliant discoveries have changed the received opinions with regard to the magnetic properties of matter. Although all bodies are magnetic, they show that it assumes a totally different form in different substances. For example, if a bar of iron be freely suspended between the poles of an electro-magnet, or very powerful horse-shoe magnet, it will be attracted by both poles, and will rest in the direction between them—that is, on the line of force. But if a bar of bismuth be suspended in the same manner, it will be repelled by both poles, and will assume a direction at right angles to that which the iron took, and thus the same force, whether electric or magnetic, produces opposite effects upon these two metals. Substances affected after the manner of iron are magnetic—those affected after the manner of bismuth are said to be diamagnetic. All substances come under one or other of these two classes: the diamagnetic are infinitely more abundant than the magnetic; almost all bodies on earth belong to that class. Many of the metals, acids, oils, sugar, starch, animal matter, flame, and all the gases, whether light or heavy, have the diamagnetic property less or more, but oxygen less than any other, and that is the reason why atmospheric air is the most feebly diamagnetic of all substances at its natural temperature; for when very hot it becomes more diamagnetic, and if extremely cold it takes a place among the magnetic class. Important results with regard to the magnetic state of the globe will undoubtedly be deduced from this new property of matter, and Dr. Faraday’s observations on that subject show that he is not without such anticipations.
“When we consider the magnetic condition of the earth as a whole, without reference to its possible relation to the sun, and reflect upon the enormous amount of diamagnetic matter which forms its crust; and when we remember that magnetic curves of a certain amount of force, universal in their presence, are passing through these matters, and keeping them constantly in a state of tension, and therefore of action, we cannot doubt that some great purpose, of utility to the system and to us its inhabitants, is fulfilled by it. If the sun have anything to do with the magnetism of the globe, then it is possible that part of this effect may be due to the action of the light that comes to us from that body; and in that view the air seems most strikingly placed round our sphere, investing it with a transparent diamagnetic, which, therefore, is permeable to his rays, and, at the same time, moving with great velocity across them. Such conditions seem to suggest the possibility of magnetism being thence generated.”
Chapter XXIII
Vegetation—Nourishment and Growth of Plants—Effects of the different Rays of the Solar Spectrum—Classes—Botanical Districts.
IN the present state of the globe, a third part only of its surface is occupied by land, and probably not more than a fourth part of that is inhabited by man, but animals and vegetables have a wider range. The greater part of the land is clothed with vegetation and inhabited by quadrupeds, the air is peopled with birds and insects, and the sea teems with living creatures and plants. These organized beings are not scattered promiscuously, but all classes of them have been originally placed in regions suited to their respective wants. Many animals and plants are indigenous only in determinate spots, while a thousand others might have supported them as well, and to many of which they have been transported by man.
Plants extract inorganic substances from the ground, which are indispensable to bring them to maturity, but the atmosphere supplies the vegetable creation with the principal part of its food.
The black or brown mould which is so abundant is the produce of decayed vegetables. When the autumnal leaves, the spoil of the summer, fall to the ground, and their vitality is gone, they enter into combination with the oxygen of the atmosphere, and convert it into an equal volume of carbonic acid gas, which, consequently, exists abundantly in every good soil, and is the most important part of the food of vegetables. This process is slow, and stops as soon as the air in the soil is exhausted; but the plough, by loosening the earth, and permitting the atmosphere to enter more freely and penetrate deeper into the ground, accelerates the decomposition of the vegetable matter, and consequently the formation of carbonic acid.
In loosening and refining the mould, the common earth-worm is the fellow-labourer with man; it eats earth, and, after extracting the nutritious part, ejects the refuse, which is the finest soil, and may be seen lying in heaps at the mouth of its burrow. So instrumental is this creature in preparing the ground, that it is said that there is not a particle of the finer vegetable mould that has not passed through the intestines of a worm: thus, the most feeble of living things is employed by Providence to accomplish the most important ends.
The food of the vegetable creation consists of carbon, hydrogen, nitrogen, and oxygen—all of which plants obtain entirely from the atmosphere in the form of carbonic acid gas, water, and ammonia. They imbibe these three substances, and, after having decomposed them, they give the oxygen to the air, and consolidate the carbon, water, and nitrogen into wood, leaves, flowers, and fruit.
The vitality of plants is a chemical process entirely due to the sun’s light; it is most active in clear sunshine, feeble in the shade, and nearly suspended in the night, when plants, like animals, have their rest.
The atmosphere contains only one two-thousandth part of carbonic acid gas, yet that small quantity yields enough of carbon to form the solid mass of all the magnificent forests and herbs that clothe the face of the earth, and the supply of that necessary ingredient in the composition of the atmosphere is maintained by the breath of animals, by volcanos, and by combustion. The green parts of plants constantly imbibe carbonic acid in the day; they decompose it, assimilate the carbon, and return the oxygen pure to the atmosphere. As the chemical action is feeble in the shade and in gloomy weather, only a part of the carbonic acid is decomposed, then both oxygen and carbonic acid are given out by the leaves; but during the darkness of the night a chemical action of a different character takes place, and almost all the carbonic acid is returned unchanged to the atmosphere, together with the moisture which is evaporated from the leaves both night and day. Thus, plants give out pure oxygen during the day, and carbonic acid and water during the night.
Since the vivifying action of the sun brings about all these changes, a superabundance of oxygen is exhaled by the tropical vegetation in a clear unclouded sky, where the sun’s rays are most energetic, and atmospheric moisture most abundant. In the middle and higher latitudes, on the contrary, under a more feeble sun and a gloomy sky, subject to rain, snow, and frequent atmospheric changes, carbonic acid is given out in greater quantity by the less vigorous vegetation. But here, as with regard to heat and moisture, equilibrium is restored by the winds; the tropical currents carry the excess of oxygen along the upper strata of the atmosphere to higher latitudes, to give breath and heat to men and animals; while the polar currents, rushing along the ground, convey the surplus carbonic acid to feed the tropical forests and jungles. Harmony exists between the animal and vegetable creations; animals consume the oxygen of the atmosphere, which is restored by the exhalation of plants, while plants consume the carbonic acid exhaled by men and animals; the existence of each is thus due to their reciprocal dependence. Few of the great cosmical phenomena have only one end to fulfil, they are the ministers of the manifold designs of Providence.
When a seed is thrown into the ground, the vital principal is developed by heat and moisture, and part of the substance of the seed is formed into roots, which suck up water mixed with carbonic acid from the soil, decompose it, and consolidate the carbon. In this stage of their growth, plants derive their whole sustenance from the ground. As soon, however, as the sugar and mucilage of the seed appear above the ground, in the form of leaves or shoots, they absorb and decompose the carbonic acid of the atmosphere, retain the carbon for their food, give out the oxygen in the day, and pure carbonic acid in the night. In proportion as plants grow, they derive more of their food from the air and less from the soil, till their fruit is ripened, and then the whole of their nourishment is derived from the atmosphere. Trees are fed from the air after their fruit is ripe till their leaves fall; annuals till they die. Air-plants, and several species of cactus and others, derive all their food from the atmosphere. It is wonderful that so small a quantity of carbonic acid as exists in the air should suffice to supply the whole vegetation of the world—and still more wonderful that a seed, minute enough to be wafted invisibly by a breath of air, should be the theatre of all the chemical changes that make it germinate.[153]
Plants absorb water from the ground by their roots; they decompose it, and the hydrogen combines in different proportions with their carbonic acid to form wood, sugar, starch, gum, vegetable, oils, and acids. As the green parts combine with the oxygen of the air, especially during night, when the functions of plants are torpid, it is assimilated on the return of daylight, and assists in forming oils, resins, and acids. The combination of the oxygen of the air with the leaves, and also with the blossom and fruit, during night, is quite unconnected with the vital process, as it is the same in dead plants. An acid exists in the juice of every plant, generally in combination with an alkali. It must be observed, however, that these different substances are produced at different stages in the growth; for example, starch is formed in the roots, wood, stalk, and seed, but it is converted into sugar as the fruit ripens, and the more starch the sweeter the fruit becomes. Most of these new compounds are formed between the flowering of the plant and the ripening of the fruit, and indeed they furnish the materials for the flowers, fruit, and seed.
Ammonia, the third organic constituent of plants, is the last residue from the decay and putrefaction of animal matter. It is volatilized, and rises into the atmosphere, where it exists as a gas, but in so small a quantity that it is with difficulty detected by chemical analysis; yet, as it is very soluble in water, enough is brought to the ground by rain to supply the vegetable world. Ammonia enters plants by their roots along with rain-water, and is resolved within them into its constituent elements, hydrogen and nitrogen. The hydrogen aids in forming the wood, acids, and other substances before mentioned; while the nitrogen enters into every part of the plant and forms new compounds; it exists in the blossom and fruit before it is ripe, and in the wood, as albumen; it also forms gluten, which is the nutritious part of wheat, barley, oats, and all other cerealia, as well as of esculent roots, as potatoes, beet-root, &c. Nitrogen exists abundantly in peas, beans, and pulse of every kind; it enters into the composition of most elementary vegetable substances; in short, a plant may grow without ammonia, but it cannot produce seed or fruit; the use of animal manure is to supply plants with this essential article of their food. Thus, the decomposition and consolidation of the elementary food of plants, the formation of the green parts, the exhalation of moisture by their leaves, its absorption by their roots, and all the other circumstances of vegetable life, are owing to the illuminating power of the sun. Heat can be supplied artificially in our northern climates, but it is impossible to replace the splendour of a southern sun. His illuminating influence is displayed in a remarkable degree by the cacalia ficoides; its leaves combine with the oxygen of the atmosphere during the night, and are as sour as sorrel in the morning; as the sun rises they gradually lose their oxygen, and are tasteless at noon; by the continued action of light they lose more and more, till towards evening they become bitter. The difference of a clear or cloudy sky has an immense effect on vegetation; the ripening of fruit depends upon the habitual serenity of the sky more than on summer temperature alone.
The blue rays of the solar spectrum have most effect on the germination of seed; the yellow rays, which are the most luminous, on the growing plant. That is on account of the chemical rays, now so well known by their action in Daguerreotype impressions. They are most abundant beyond the visible part of the solar spectrum, and diminish through the violet, blue, and green, to the yellow, where they cease. They penetrate the ground, and have a much greater influence on the germination of seeds than ordinary light or darkness. That invisible principle, together with light, is essential to the formation of the colouring matter of leaves; it is most active in spring, and is in very considerable excess compared with the quantity of light and heat; but as summer advances the reverse takes place; the calorific radiation, or those hot rays corresponding to the extreme red of the spectrum, which facilitate the flowering and forming of the fruit, become by far the most abundant; and a set of invisible rays, which exist near the point of maximum heat in the solar spectrum, are also most abundant in summer. Mr. Hunt found that the hot rays immediately beyond the visible red destroy the colour of palm-leaves; and for that reason the glass of the palm-house at Kew Gardens is tinged pale yellow-green by oxide of copper, which excludes the scorching rays in question, though it is permeable by the other rays of heat, those of light, and the chemical rays.[154]
In spring and summer the oxygen taken in by the green leaves in the night aids in the formation of oils, acids, and the other parts that contain it; but as soon as autumn comes, the vitality or chemical action of vegetables is weakened; and the oxygen, no longer given out in the day, though still taken in during the night, becomes a minister of destruction; it changes the colour of the leaves, and consumes them when they fall. Nitrogen, so essential during the life of plants, also resumes its chemical character when they die, and by its escape hastens their decay.
Although the food which constitutes the mass of plants is derived principally from water and the gases of the atmosphere, fixed substances are also requisite for their growth and perfection, and these they obtain from the earth by their roots. The inorganic matters are the alkalis, phosphates, silica, sulphur, iron, and others.
It has already been mentioned that vegetable acids are found in the juices of all the families of plants. They generally are in combination with one or other of the alkaline substances, as lime, soda, potash, and magnesia, which are as essential to the existence of plants as the carbonic acid by which these acids are formed: for example, vines have potash; plants used as dyes never give vivid colours without it; all leguminous plants require it, and only grow naturally on ground that contains it. None of the corn tribe can produce perfect seeds unless they have both potash and phosphate of magnesia; nor can they or any of the grasses thrive without silica, which gives the hard coating to straw, to the beard of wheat and barley, to grass, canes, and bamboos; it is even found in solid lumps in the hollows and joints of cane, known in India by the name of tabashir. To bring the cerealia to perfection, it is indispensable that in their growth they should be supplied with carbonic acid for the plant, silica to give it strength and firmness, and nitrogen for the grain.
Phosphoric acid, combined with an earth or alkali, is found in the ashes of all vegetables, and is essential to many. Pulse contain but little of it, and on that account are less nutritious than the cerealia. The family of the cruciferæ, as cabbages, turnips, mustard, &c., contain sulphur in addition to the substances common to the growth of all plants; each particular tribe has its own peculiarities, and requires a combination suited to it. On that account there is often a marked difference in the arborescent vegetation on the same mountain, depending on the nature of the rocks.
The ocean furnishes some of the matters found in plants; the prodigious quantity of sea-water constantly evaporated carries with it salt in a volatilized state, which, dispersed over the land by the wind, supplies the ground with salt and the other ingredients of sea-water. The inorganic matters which enter plants by their roots are carried by the sap to every part of the vegetable system. The roots imbibe all liquids presented to them indiscriminately, but they retain only the substances they require at the various stages of their growth, and throw out such parts as are useless, together with the effete or dead matter remaining after the nutriment has been extracted from it. Plants, like animals, may be poisoned, but the power they have of expelling deleterious substances by their roots generally restores them to health. The feculent matter injures the soil; besides, after a time the ground is drained of the inorganic matter requisite for any one kind of plant: hence the necessity for a change or rotation of crops.
A quantity of heat is set free and also becomes latent in the various transmutations that take place in the interior of plants; so that they, like the animal creation, have a tendency to a temperature of their own, independent of external circumstances.
The quantity of electricity requisite to resolve a grain weight of water into its elementary oxygen and hydrogen is equal to the quantity of atmospheric electricity which is active in a very powerful thunder-storm; hence, some idea may be formed of the intense energy exerted by the vegetable creation in the decomposition of the vast mass of water and other matters necessary for its sustenance. But there must be a compensation in the consolidation of the vegetable food, otherwise a tremendous quantity would be in perpetual activity. It is said to be given out from the points of their leaves, so, possibly, some part of the atmospheric electricity may be ascribed to this cause; but there is reason to believe that electricity, excited by the power of solar light, constitutes the chemical vitality of vegetation.
The colouring matter of flowers is various, if we may judge from the effect which the solar spectrum has upon their expressed juices. The colour is very brilliant on the tops of mountains and in the Arctic lands. Possibly the diminished weight of the air may have some effect, for it can scarcely be supposed that barometrical changes should be entirely without influence on vegetation.
The perfume of flowers and leaves is owing to a volatile oil, which is often carried by the air to a great distance: in hot climates it is most powerful in the morning and evening. The odour of the Humiria has been perceived at the distance of three miles from the coast of South America, a species of Tetracera sends its perfume as far from the island of Cuba, and the aroma of the Spice Islands is wafted out to sea. The variety of perfumes is infinite, and shows the innumerable combinations of which a few simple substances are capable, and the extreme minuteness of the particles of matter.
In northern and mean latitudes, winter is a time of complete rest to the vegetable world, and in tropical climates the vigour of vegetation is suspended during the dry, hot season, to be resumed at the return of the periodical rains. The periodical phenomena of the appearance of the first leaves, the flowering, ripening of the fruit, and the fall of the leaf, depend upon the annual and diurnal changes of temperature, moisture, electricity, and perhaps on magnetism, and succeed with such perfect harmony and regularity, that, were there a sufficient number of observations, lines might be drawn on a globe passing through all places where the leaves of certain plants appear simultaneously, and also for the other principal phases of vegetation. In places where the same plant flowers on the same day, the fruit may not ripen at the same period in both; it would therefore be interesting to know what relation lines passing through those would have to one another and to the isothermal lines; more especially with regard to the plants indispensable to man, since the periodicity of vegetation affects his whole social condition.[155]
Almost all plants sleep during the night; some show it in their leaves, others in their blossom. The Mimosa tribe not only close their leaves at night, but their foot-stalks droop; in a clover-field not a leaf opens until after sunrise. The common daisy is a familiar instance of a sleeping flower; it shuts up its blossom in the evening, and opens its white and crimson-tipped star, the “day’s eye,” to meet the early beams of the morning sun; and then also “winking mary-buds begin to ope their golden eyes.” The crocus, tulip, convolvulus, and many others, close their blossoms at different hours towards evening, some to open them again, others never. The ivy-leaved lettuce opens at eight in the morning, and closes for ever at four in the afternoon. Some plants seem to be wide awake all night, and to give out their perfume then only, or at nightfall. Many of the jessamines are most fragrant during the twilight: the Olea fragrans, the Daphne odorata, and the night-stock reserve their sweetness for the midnight hour, and the night-flowering Cereus turns night into day. It begins to expand its magnificent sweet-scented blossom in the twilight, it is full blown at midnight, and closes, never to open again, with the dawn of day;—these are “the bats and owls of the vegetable kingdom.”[156]
Many plants brought from warm to temperate climates have become habituated to their new situation, and flourish as if they were natives of the soil; such as have been accustomed to flower and rest at particular seasons change their habits by degrees, and adapt themselves to the seasons of the country that has adopted them. It is much more difficult to transfer alpine plants to the plains. Whether from a change of atmospheric pressure or mean temperature, all attempts to cultivate them at a lower level generally fail: it is much easier to accustom a plant of the plains to a higher situation.
Plants are propagated by seeds, offsets, cuttings, and buds; hence they, but more especially trees, have myriads of seats of life, a congeries of vital systems acting in concert, but independently of each other, every one of which might become a new plant. In this respect the fir and pine tribe are inferior to deciduous trees, which lose their leaves annually, because they are not easily propagated except by seeds. It has been remarked that all plants that are propagated by buds from a common parent stock have the same duration of life; this has been noticed particularly with regard to some species of apple-trees in England. It appears that all the garden varieties of fruit, whether from buds, layers, or cuttings, wear out after a time; and that seedlings have a great tendency to revert to the original wild character of the plant.
A certain series of transitions takes place throughout the lives of plants, each part being transformed and passing into another; a law that was first observed by the illustrious poet Göthe. For example, the embryo leaves pass into common leaves, these into bracteæ, the bracteæ into sepals, the sepals into petals, which are transformed into stamens and anthers, and these again pass into ovaries with their styles and stigmas, that are to become the fruit and ultimately the seed of a new plant.
Plants are naturally divided into three classes, differing materially in organization:—The Cryptogamia, whose flowers and seeds are either too minute to be easily visible, or are hidden in some part of the plant, as in fungi, mosses, ferns, and lichens, which are of the least perfect organization. Next to these are the monocotyledonous plants, as grasses and palms, in which the foot-stalks of the old leaves form the outside of the stem; plants of this class have but one-seed-lobe, which forms one little leaf in their embryo state. Their flowers and fruit are generally referable to some law in which the number 3 prevails, as, for example, the petals and other parts are three in number. The dicotyledonous plants form the third class, which is the most perfect in its organization, and by much the most numerous, including the trees of the forest and most of the flowering shrubs and herbs. They increase by coatings from without, as trees, where the growth of each year forms a concentric circle of wood round the pith or centre of the stem: the seeds of these plants have two lobes, which in their embryo state appear first in two little leaves above ground, like most of the European species. The parts of the flowers and fruit of this class generally have some relation to the number 5.
The three botanical classes are distributed in very different proportions in different zones: monocotyledonous plants, such as grasses and palms, are much more rare than the dicotyledonous class. Between the tropics there are four of the latter to one of the grass or palm tribes, in the temperate zones six to one, and in the polar regions only two to one, because mosses and lichens are most abundant in the high latitudes, where dicotyledonous plants are comparatively rare. In the temperate zones one-sixth of the plants are annuals, omitting the cryptogamia; in the torrid zone scarcely one plant in twenty is annual, and in the polar regions only one in thirty. The number of ligneous vegetables increases on approaching the equator, yet in North America there are 120 different species of forest-trees, whereas in the same latitudes in Europe there are only 34. The social plants, grasses, heaths, furze, broom, daisies, &c., which cover large tracts, are rare between the tropics, except on the mountains and table-lands and on the llanos of equatorial America.
Equinoctial America has a more extensive and richer vegetation than any other part of the world; Europe has not above half the number of indigenous species of plants; Asia, with its islands, has somewhat less than Europe; Australia, with its islands in the Pacific, still less; and there are fewer vegetable productions in Africa than in any part of the globe of the same extent.
Since the constitution of the atmosphere is very much the same everywhere, vegetation depends principally on the sun’s light, moisture, and the mean annual temperature, and it is also in some degree regulated by the heat of summer in the temperate zones, and also by exposure, for such plants as require warmth are found at a lower level on the north than on the south side of a mountain. Between the tropics, wherever rain does not fall, the soil is burnt up and is as unfruitful as that exposed to the utmost rigour of frost; but where moisture is combined with heat and light, the luxuriance of the vegetation is beyond description. The abundance and violence of the periodical rains combine with the intense light and heat to render the tropical forests and jungles almost impervious from the rankness of the vegetation. This exuberance gradually decreases with the distance from the equator; it also diminishes progressively as the height above the level of the sea increases, so that each height has a corresponding parallel of latitude where the climates and floras are similar, till the perpetual snow on the mountain-tops, and its counterpart in the polar regions, have a vegetation that scarcely rises above the surface of the ground. Hence, in ascending the Himalaya or Andes from the luxuriant plains of the Ganges or Amazons, changes take place in the vegetation analogous to what a traveller would meet with in a journey from the equator to the poles. This law of decrease, though perfectly regular over a wide extent, is perpetually interfered with by local climate and soil. From the combination of various causes, as the distribution of land and water, their different powers of absorption and radiation, together with the form, texture, and clothing of the land, and the prevailing winds, it is found that the isothermal lines, or imaginary lines drawn through places on the surface of the globe which have the same mean annual temperature, do not correspond with the parallels of latitude. Thus, in North America the climate is much colder than in the corresponding European latitudes. Quebec is in the latitude of Paris, and the country is covered with deep snow four or five months in the year, and it has occurred that a summer has passed there in which not more than 60 days have been free from frost.
In the southern hemisphere, beyond the 34th parallel, the summers are colder and the winters milder than in corresponding latitudes of the northern hemisphere. Neither does the temperature of mountains vary exactly with their height above the sea; other causes, as prevailing winds, difference of radiation, and geological structure, concur in producing irregularities which have a powerful effect on the vegetable world.
However, no similarity of existing circumstances can account for whole families of plants being confined to one particular country, or even to a very limited district, which, as far as we can judge, might have grown equally well on many others. Latitude, elevation, soil, and climate, are but secondary causes in the distribution of the vegetable kingdom, and are totally inadequate to explain why there are numerous distinct botanical districts in the continents and islands, each of which has its own vegetation, whose limits are most decided when they are separated by the ocean, mountain-chains, sandy deserts, salt-plains, or internal seas. Each of these districts is the focus of families and genera, some of which are found nowhere else, and some are common to others, but, with a very few remarkable exceptions, the species of plants in each are entirely different or representative.[157] This does not depend upon the difference in latitude, for the vegetation of the United States of North America is totally unlike that of Europe under the same isothermal lines, and even between the tropics the greatest dissimilarity often prevails under different degrees of longitude: consequently, the cause of this partial distribution of plants, and that of animals also, which is according to the same law, must be looked for in those early geological periods when the earth first began to be tenanted by the present races of organized beings.
As the land rose at different periods above the ocean, each part, as it emerged from the waves, had probably been clothed with vegetation, and peopled with animals, suited to its position with regard to the equator, and to the climate and condition of the globe then being. And as the conditions and climate were different at each succeeding geological epoch, so each portion of the land, as it rose, would be characterized by its own vegetation and animals, and thus at last there would be many centres of creation, as at this day, all differing more or less from one another, and hence, alpine floras must be of older date than those in the plains. The vegetation and faunas of those lands that differed most in age and place would be most dissimilar, while the plants and animals of such as were not far removed from one another in time and place would have correlative forms or family likenesses, yet each would form a distinct province. Thus, in opposite hemispheres, and everywhere at great distances, but under like circumstances, the species are representatives of one another, rarely identical: when, however, the conditions which suit certain species are continuous, identical species are found throughout, either by original creation or by migration. The older forms may have been modified to a certain extent by the succeeding conditions of the globe, but they never could have been changed, since immutability of species is a primordial law of nature. Neither external circumstances, time, nor human art, can change one species into another, though each to a certain extent is capable of accommodating itself to a change of external circumstances, so as to produce varieties even transmissible to their offspring.
The flora of Cashmere and the higher parts of the Himalaya mountains is similar to that of southern Europe, yet the species are representative, not identical. In the plains of Tartary, where from their elevation the degree of cold is not less than in the wastes of Siberia, the vegetation of one might be mistaken for that of the other; the gooseberry, currant, willow, rhubarb, and in some places the oak, hazel, cypress, poplar, and birch, grow in both, but they are of different species. The flora near the snow-line on the lofty mountains of Europe, and lower down, has also a perfect family likeness to that in high northern latitudes. In like manner many plants on the higher parts of the Chilian Andes are similar, and even identical, with those in Tierra del Fuego; nay, the Arctic flora has a certain resemblance to that of the Antarctic regions, and even occasional identity of species. These remarkable coincidences may be accounted for by the different places having been at an early geological period at the same level above the ocean, and that they continue to retain part of their original flora after their relative positions have been changed. The tops of the Chilian Andes were probably on a level with Tierra del Fuego when both were covered with the same vegetation, and in the same manner the lofty plains of Tartary may have acquired their vegetation when they were on the level of southern Siberia.
In the many vicissitudes the surface of the globe has undergone, continents formed at one period were broken up at another into islands and detached masses by inroads of the sea and other causes. Now, Professor E. Forbes has shown that some of the primary floras and faunas have spread widely from their original centres over large portions of the continents before the land was broken up into the form it now has, and thus accounts for the similarity and sometimes identity of the plants and animals of regions now separated by seas,—as, for example, islands, which generally partake of the vegetation and fauna of the continents adjacent to them. Taking for granted the original creation of specific centres of plants and animals, Professor E. Forbes has clearly proved that “the specific identity, to any extent, of the flora and fauna of one area, with those of another, depends on both areas forming, or having formed, part of the same specific centre, or on their having derived their animal and vegetable population by transmission, through migration, over continuous or closely contiguous land, aided, in the case of alpine floras, by transportation on floating masses of ice.”
By the preceding laws the limited provinces and dispersion of animal and vegetable life are explained, but the existence of single species in regions very far apart has not yet been accounted for.
Very few of the exogenous or dicotyledonous plants are common to two or more countries far apart: among the few, the Samolus Valerandi, a common English plant, is a native of Australia; the Potentilla tridentata, not found in Britain, except on one hill in Angusshire, is common to Arctic Europe and the mountains of North America; and in the Falkland Islands there are more than 30 flowering plants identical with those in Great Britain.
There are many more instances of wide diffusion among the monocotyledonous plants, especially grasses: the Phleum alpinum of Switzerland grows without the smallest variation at the Straits of Magellan, and Mr. Bunbury met with the European quaking-grass in the interior of the country at the Cape of Good Hope; but the cellular or cryptogamous class is most widely diffused—plants not susceptible of cultivation, of little use to man, and of all others the most difficult to transport. The Sticta aurata, found in Cornwall, is a native of the Cape of Good Hope, St. Helena, the West Indian islands, and Brazil; the Trichomanes brevisetum, long supposed to be peculiar to the British isles, is ascertained to grow in Madeira, South America, &c.; and our eminent botanist, Mr. Brown, found 38 British lichens and 28 British mosses in New Holland, yet in no two parts of the world is the vegetation more dissimilar; and almost all the lichens brought from the southern hemisphere by Sir James Ross, amounting to 200 species, are also inhabitants of the northern hemisphere, and mostly European.
In islands far from continents the number of plants is small, but of these, a large proportion occur nowhere else. In St. Helena, of 30 flower-bearing plants, 1 or 2 only are native elsewhere, but in 60 species of cryptogamous plants Dr. Hooker found only 12 peculiar to the island.
Some plants are more particularly confined to certain regions: the species of Cinchona which furnish the Peruvian bark grow along the eastern declivity of the Andes, as far as 18° S. lat.; the cedar of Lebanon is indigenous on that celebrated mountain only; and the Disa grandiflora is limited to a very small spot on the top of the Table-mountain at the Cape of Good Hope; but whether these are remnants whose kindred have perished by a change of physical circumstances, or centres only beginning to spread, it is impossible to say.
Plants are dispersed by currents: of 600 plants from the vicinity of the river Zaire on the coast of Africa, 13 are found also on the shores of Guiana and Brazil, evidently carried by the great equatorial current to countries congenial in soil and climate. The seeds of the Mimosa scandens, the Guilandina Bonduc, and the cashew-nut, are wafted from the West India islands to the coasts of Scotland and Ireland by the Gulf-stream, a climate and soil which do not suit them, therefore they do not grow. Of all the great orders, the species of Leguminosæ are most widely dispersed on coasts, because their seeds are not injured by the water. Winds also waft seeds to great distances; birds and quadrupeds, and above all man, are active agents in dispersing plants.
Chapter XXIV
Vegetation of the Great Continent—Of the Arctic Islands—And of the Arctic and North Temperate Regions of Europe and Asia.
THE southern limit of the polar flora, on the great continent, lies mostly within the Arctic Circle, but stretches along the tops of the Scandinavian mountains, and reappears in the high lands of Scotland, Cumberland, and Ireland, on the summits of the Pyrenees, Alps, and other mountains in southern Europe, as well as on the table-land of eastern Asia, and on the high ridges of the Himalaya.
The great European plain to the Ural Mountains, as well as the low lands of England and Ireland, were at one period covered by a sea full of floating ice and icebergs, which made the climate much colder than it now is. At the beginning of that period the Scandinavian range, the other continental mountains, and those in Britain and Ireland, were islands of no great elevation, and were then clothed with the Arctic flora, or a representative of it, which they still retain now that they form the tops of the mountain-chains, and at that time both plants and animals were conveyed from one country to another by the floating ice. It is even probable, from the relations of the fauna and flora, that Greenland, Iceland, and the very high European latitudes, are the residue of a great northern land which had sunk down at the close of the glacial period, for there were many vicissitudes of level during that epoch. At all events, it may be presumed that the elevation of the Arctic regions of both continents, if not contemporaneous, was probably not far removed in time. Similarity of circumstances had extended throughout the whole Arctic regions, since there is a remarkable similarity and occasional identity of species of plants and animals in the high latitudes of both continents, which is continued along the tops of their mountain-chains, even in the temperate zones; and there is reason to believe that the relations between the faunas and floras of Boreal America, Asia, and Europe, must have been established towards the close of the glacial period.
The flora of Iceland approaches that of Britain, yet only one in four of the British plants are known in Iceland. There are 870 species in Iceland, of which more than half are flower-bearing: this is a greater proportion than is found in Scotland, but there are only 32 of woody texture. This flora is scattered in groups according as the plants like a dry, marshy, volcanic, or marine soil. Many grow close to the hot-springs: some not far from the edge of the basin of the Great Geyser, where every other plant is petrified; and species of Confervæ flourish in a spring said to be almost hot enough to boil an egg. The grains cannot be cultivated on account of the severity of the climate, but the Icelanders make bread from metur, a species of wild corn, and also from the bulbous root of Polygonum viviparum; their greatest delicacy is the Angelica archangelica; Iceland moss, used in medicine, is an article of commerce. There are 583 species in the Feroe islands, of which 270 are flowering plants: many thrive there that cannot bear the cold of Iceland.
ARCTIC FLORA OF THE GREAT CONTINENTS.
In the most northern parts of the Arctic lands the year is divided into one long intensely cold night and one bright and fervid day, which quickly brings to maturity the scanty vegetation. Within the limit of perpetual congelation the Palmella nivalis (or red snow of Arctic voyagers), a very minute red or orange-coloured plant, finds nourishment in the snow itself, the first dawn of vegetable life; it is also found colouring large patches of snow in the Alps and Pyrenees.
Lichens are the first vegetables that appear at the limits of the snow-line, whether in high latitudes or mountain-tops, and they are the first vegetation that takes possession of volcanic lavas and new islands, where they prepare soil for plants of a higher order: they grow on rocks, stones, and trees, in fact on anything that affords them moisture. More than 2400 species are already known; no plants are more widely diffused, and none afford a more striking instance of the arbitrary location of species, as they are of so little direct use to man that they could not have been disseminated by his agency. The same kind prevail throughout the Arctic regions, and the species common to both hemispheres are very numerous. Some lichens produce brilliant red, orange, and brown dyes; and the tripe de roche, a species of Gyrophora, is a miserable substitute for food, as our intrepid countryman Sir John Franklin and his brave companions experienced in their perilous Arctic journey.
Mosses follow lichens on newly-formed soil, and they are found everywhere throughout the world in damp situations, but in greatest abundance in temperate climates: 800 species are known, of which a great part inhabit the Arctic regions, constituting a large portion of the vegetation.
In Asiatic Siberia, north of the 60th parallel of latitude, the ground is perpetually frozen at a very small depth below the surface: a temperature of 70° below zero of Fahrenheit is not uncommon, and, in some instances, the cold has been 120° below zero. Then it is fatal to animal life, especially if accompanied by wind. In some places trees grow and corn ripens even at 70° of north latitude; but in the most northern parts boundless swamps, varied by lakes both of salt and fresh water, cover wide portions of this desolate country, which is buried under snow nine or ten months in the year. As soon as the snow is melted by the returning sun, these extensive morasses are covered with coarse grass and rushes, while mosses and lichens mixed with dwarf willows clothe the plains; saline plants abound, and whole districts produce Diotis ceratoides.
In Nova Zembla and other places in the far north, the vegetation is so stunted that it barely covers the ground, but a much greater variety of minute plants of considerable beauty are crowded together there in a small space than in the alpine regions of Europe where the same genera grow. This arises from the weakness of the vegetation; for in the Swiss Alps the same plant frequently occupies a large space, excluding every other, as the dark-blue gentian, the violet-coloured pansy, the pink and yellow stone-crops. In the remote north, on the contrary, where vitality is comparatively feeble and the seeds do not ripen, thirty different species may be seen crowded together in a brilliant mass, no one having strength to overcome the rest. In such frozen climates plants may be said to live between the air and the earth, for they scarcely rise above the soil, and their roots creep along the surface, not having power to enter it. All the woody plants, as the Betula nana, the reticulated willow, Andromeda tetragona, with a few berry-bearing shrubs, trail along the ground, never rising more than an inch or two above it. The Salix lanata, the giant of these boreal forests, never grows more than five inches above the surface, while its stem, 10 or 12 feet long, lies hidden among the moss, owing shelter to its lowly neighbour.
The chief characteristic of the vegetation of the Arctic regions is the predominance of perennial and cryptogamous plants, and also of the sameness of its nature; but more to the south, where night begins to alternate with day, a difference of species appears in longitude as well as in latitude. A beautiful flora of vivid colours adorns these latitudes both in Europe and Asia during their brief but bright and ardent summer, consisting of potentillas, gentians, chickweeds, saxifrages, sedums, Ranunculi, spiræas, drabas, artemisias, claytonias, and many more. Such is the power of the sun, and the consequent rapidity of vegetation, that these plants spring up, blossom, ripen their seed, and die, in six weeks: in a lower latitude woody plants follow these, as berry-bearing shrubs, the glaucous Kalmia, the trailing Azalea and rhododendrons. The Siberian flora differs from that in the same European latitudes by the North American genera Phlox, Mitella, Claytonia, and the predominance of asters, Solidago, Spiræa, milk-vetches, wormwood, and the saline plants goosefoot and saltworts.
Social plants abound in many parts of the northern countries, as grass, heath, furze, and broom: the steppes are an example of this on a very extensive scale. Both in Europe and Asia they are subject to a rigorous winter, with deep snow and chilling blasts of wind; and as the soil generally consists of a coating of vegetable mould over clay, no plants with deep roots thrive upon them; hence, the steppes are destitute of trees, and even bushes are rare except in ravines: the grass is thin, but nourishing. Hyacinths and some other bulbs, mignonette, asparagus, liquorice, and wormwood, grow in the European steppes; the two last are peculiarly characteristic. The Nelumbium speciosum grows in one spot five miles from the town of Astracan, and nowhere else in the wide domains of Russia: the leaves of this beautiful aquatic plant are often two feet broad, and its rose-coloured blossoms are very fragrant. It is also native in India and Tibet, where it is held sacred, as it was formerly in Egypt, where it is said to be extinct: it is one of the many instances of a plant growing in countries far apart.
Each steppe in Siberia has its own peculiar plants; the Peplis and Camphorosma are peculiar to the steppe of the Irtish, and the Amaryllis tatarica abounds in the meadows of eastern Siberia, where the vegetation bears a great analogy to that of north-western America: several genera and species are common to both.
Half the plants found by Wormskiold in Kamtchatka are European, with the exception of eight or ten, which are American. Few European trees grow in Asiatic Siberia, notwithstanding the similarity of climate, and most of them disappear towards the rivers Tobol and Irtish.
In Lapland and in the high latitudes of Russia, large tracts are covered with birch-trees, but the pine and fir tribe are the principal inhabitants of the north. Prodigious forests of these are spread over the mountains of Norway and Sweden, and in European Russia 200,000,000 acres are clothed with these Coniferæ alone, or occasionally mixed with willows, poplars, and alders. Although soils of pure sand and lime are absolutely barren, yet they generally contain enough of alkali to supply the wants of the fir and pine tribes, which require ten times less than oaks and other deciduous trees.
The Siberian steppes are bounded on the south by great forests of pine, birch, and willow: poplars, elms, and Tartarian maple overhang the upper courses of the noble rivers which flow from the mountains to the Frozen Ocean, and on the banks of the Yenessei the Pinus Cembra, or Siberian pine, with edible fruit, grows 120 feet high. The Altaï are covered nearly to their summit with similar forests, but on their greatest heights the stunted larch crawls on the ground, and the flora is like that of northern Siberia: round the lake Baikal the Pinus Cembra grows nearly to the snow-line.
Forests of black birch are peculiar to Dahuria, where there are also apricot and apple trees, and rhododendrons, of which a species grows in thickets on the hills, with yellow blossoms. Here, and everywhere else throughout this country, are found all the species of Caragana, a genus entirely Siberian. Each terrace of the mountains, and each steppe on the plains, has its peculiar plants, as well as some common to all: perennial plants are more numerous than annuals.
If temperature and climate depended upon latitude alone, all Asia between the 50th and 30th parallels would have a mild climate; but that is far from being the case, on account of the structure of the continent, which consists of the highest table-lands and the lowest plains on the globe.
The table-land of Tibet, where it is not cultivated, has the character of great sterility, and the climate is as unpropitious as the soil: frost, snow, and sleet begin early in September, and continue with little interruption till May; snow, indeed, falls every month in the year. The air is always dry, because in winter moisture falls in the form of snow, and in summer it is quickly evaporated by the intense heat of the sun. The thermometer sometimes rises to 144° of Fahrenheit in the sun, and even in winter his direct rays have great power for an hour or two, so that a variation of 100° in the temperature of the air has occurred in twelve hours. Notwithstanding these disadvantages, there are sheltered spots which produce most of the European grain and fruits, though the natural vegetation bears the Siberian character, but the species are quite distinct. The most common indigenous plants are Tartarian furze and various prickly shrubs resembling it, gooseberries, currants, hyssop, dog-rose, dwarf sow-thistle, Equisetum, rhubarb, lucern, and asafœtida, on which the flocks feed. Prangos, an umbelliferous plant, with broad leaves and scented blossom, is peculiar to Ladak and other parts of Tibet. Mr. Moorcroft says it is so nutritious, that sheep fed on it become fat in twenty days. There are three species of wheat, three of barley, and two of buckwheat, natives of the lofty table-land, where the sarsinh is the only fruit known to be indigenous. Owing to the rudeness of the climate trees are not numerous, yet on the lower declivities of some mountains there are aspens, birch, yew, ash, Tartaric oak, various pines, and the Pavia, a species of horse-chestnut. Much of the table-land of Tartary is occupied by the Great Gobi and other deserts of sand, with grassy steppes near the mountains; but of the flora of these regions we know nothing.
FLORA OF BRITAIN AND OF MIDDLE AND SOUTHERN EUROPE.
The British islands afford an excellent illustration of distinct provinces of animals and plants, and also of their migration from other centres. Professor E. Forbes has determined five botanical districts, four of which are restricted to limited provinces, whilst the fifth, which comprehends the great mass of British plants, is, everywhere, either alone or mixed with the others. All of these, with a very few doubtful exceptions, have migrated before the British islands were separated from the continent. The first, which is of great antiquity, includes the flora of the mountain districts of the west and south-west of Ireland, and is similar to that in the south of Spain, but the more delicate plants had been killed by the change of climate after the separation of Ireland from the Asturias. The flora in the south of England and the south-east of Ireland is different from that in all other parts of the British islands; it is intimately related to the vegetation of the Channel Islands and the coast of France opposite to them, yet there are many plants in the Channel Islands which are not indigenous in Britain. In the south-west of England, where the chalk-plants prevail, the flora is like that on the adjacent coast of France.
The tops of the Scottish mountains are the focus of a separate flora, which is the same with that in the Scandinavian Alps, and is very numerous. Scotland, Wales, and a part of Ireland received this flora when they were groups of islands in the Glacial Sea. The rare Eriocaulon is found in the Hebrides, in Connemara, and in Northern America, and nowhere else. Some few individuals of this flora grow on the summits of the mountains in Cumberland and Wales. The fifth, of more recent origin than the alpine flora, including all the ordinary flowering plants, as the common daisy and primrose, hairy ladies’ smock, upright meadow crowfoot, and the lesser celandine, together with our common trees and shrubs, has migrated from Germany before England was separated from the continent of Europe by the British Channel. It can be distinctly traced in its progress across the island, but the migration was not completed till after Ireland was separated from England by the Irish Channel, and that is the reason why many of the ordinary English plants, animals, and reptiles, are not found in the sister island, for the migration of animals was simultaneous with that of plants, and took place between the last of the tertiary periods and the historical epoch, that of man’s creation: it was extended also over a great part of the continent.[158]
Deciduous trees are the chief characteristic of the temperate zone of the old continent, more especially of middle Europe; these thrive best in soil produced by the decay of the primary and ancient volcanic rocks, which furnish abundance of alkali. Oaks, elms, beech, ash, larch, maple, lime, alder, and sycamore, all of which lose their leaves in winter, are the prevailing vegetation, occasionally mixed with fir and pine.
The undergrowth consists of wild apple, cherry, yew, holly, hawthorn, broom, furze, wild rose, honeysuckle, clematis, &c. The most numerous and characteristic herbaceous plants are the umbelliferous class, as carrot and anise, the campanulas, the Cichoraceæ, a family to which lettuce, endive, dandelion, and sow-thistle belong. The cruciform tribe, as wallflower, stock, turnip, cabbage, cress, &c., are so numerous, that they form a distinguishing feature in the botany of middle Europe, to which 45 species of them belong. This family is almost confined to the northern hemisphere, for, of 800 known species, only 100 belong to the southern, the soil of which must contain less sulphur, which is indispensable for these plants.
In the Pyrenees, Alps, and other high lands in Europe, the gradation of botanical forms, from the summit to the foot of the mountains, is similar to that which takes place from the Arctic to the middle latitudes of Europe. The analogy, however, is true only when viewed generally, for many local circumstances of climate and vegetation interpose; and although the similarity of botanical forms is very great between certain zones of altitude and parallels of latitude, the species are, for the most part, different.
Evergreen trees and shrubs become more frequent in the southern countries of Europe, where about a fourth part of the ligneous vegetation never entirely lose their leaves. The flora consists chiefly of ilex, oak, cypress, hornbeam, sweet chestnut, laurel, laurustinus, the apple tribe, manna or the flowering ash, carob, jujube, juniper, terebinths, lentiscus and pistaccio which yield resin and mastic, arbutus, myrtle, jessamine (yellow and white), and various pines, as the Pinus maritima, and Pinus Pinea, or stone pine, which forms so picturesque a feature in the landscape of southern Europe. The most prevalent herbaceous plants are Caryophylleæ, as pinks, Stellaria, and arenarias, and also the labiate tripe, mint, thyme, rosemary, lavender, with many others, all remarkable for their aromatic properties, and their love of dry situations. Many of the choicest plants and flowers which adorn the gardens and grounds in northern Europe are indigenous in these warmer countries: the anemone, tulip, mignonette, narcissus, gladiolus, iris, asphodel, amaryllis, carnation, &c. In Spain, Portugal, Sicily, and the other European shores of the Mediterranean, tropical families begin to appear in the arums, plants yielding balsams, oleander, date and palmetto palms, and grasses of the group of Panicum or millet, Cyperaceæ or sedges, Aloe and Cactus. In this zone of transition there are six herbaceous for one woody plant.
FLORA OF TEMPERATE ASIA.
The vegetation of western Asia approaches nearly to that of India at one extremity, and Europe at the other; of 281 genera of plants which grow in Asia Minor and Persia, 109 are European. Syria and Asia Minor form a region of transition, like the other countries on the Mediterranean, where the plants of the temperate and tropical zones are united. We owe many of our best fruits and sweetest flowers to these regions. The cherry, almond, oleander, syringa, locust-tree, &c., come from Asia Minor; the walnut, peach, melon, cucumber, hyacinth, ranunculus, come from Persia; the date-palm, fig, olive, mulberry, and damask rose, come from Syria; the vine and apricot are Armenian, the latter grows also everywhere in middle and northern Asia. The tropical forms met with in more sheltered places are the sugar-cane, date and palmetto palms, mimosas, acacias, Asclepias gigantea, and arborescent Apocineæ. On the mountains south of the Black Sea, American types appear in rhododendrons and the Azalea pontica, and herbaceous plants are numerous and brilliant in these countries.
The table-land of Persia, though not so high as that of eastern Asia, resembles it in the quality of the soil, which is chiefly clayey, sandy, or saline, and the climate is very dry; hence, vegetation is poor, and consists of thorny bushes, acacias, mimosas, tamarisk, jujube, and asafœtida. Forests of oak cover the Lusistan mountains, but the date-palm is the only produce of the parched shores of the Arabian Gulf and of the oases on the Persian table-land. In the valleys, which are beautiful, there are clumps of Oriental plane and other trees, hawthorn, tree-roses, and many of the odoriferous shrubs of Arabia Felix.
Afghanistan produces the seedless pomegranate, acacias, date-palms, tamarisks, &c. The vegetation has much the same general character as that of Egypt. The valleys of the Hindoo Coosh are covered with clover, thyme, violets, and many odoriferous plants: the greater part of the trees in the mountains are of European genera, though all the species of plants, both woody and herbaceous, are peculiar. The small leguminous plant, from whose leaves and twigs the true indigo dye is extracted, grows spontaneously on the lower offsets of the Hindoo Coosh. This dye has been in use in India from the earliest times, but the plant which produces it was not known in England till towards the end of the 16th century. Since that time it has been cultivated in the West Indies and tropical America, though in that country there is a species indigenous.
Hot arid deserts bound India on the west, where the stunted and scorched vegetation consists of tamarisks, thorny acacia, deformed Euphorbiæ, and almost leafless thorny trees, shaggy with long hair, by which they imbibe moisture and carbon from the atmosphere. Indian forms appear near Delhi, in the genera Flacourtia and others, mixed with Syrian plants. East of this transition the vegetation becomes entirely Indian, except on the higher parts of the mountains, where European types prevail.
The Himalaya mountains form a distinct botanical district. Immediately below the snow-line the flora is almost the same with that on the high plains of Tartary, to which may be added rhododendrons and andromedas, and among the herbaceous plants primroses appear. Lower down, vast tracts are covered with prostrate bamboos, and European forms become universal, though the species are Indian, as gentians, plantagos, campanulas, and gale. There are extensive forests of Coniferæ, consisting chiefly of Pinus excelsa, Deodora, and Morinda, with many deciduous forest and fruit trees of European genera. A transition from this flora to a tropical vegetation takes place between the altitudes of 9000 and 5000 feet, because the rains of the monsoons begin to be felt in this region, which unites the plants of both. Here the scarlet and other rhododendrons grow luxuriantly; walnuts, and at least 25 species of oak, attain a great size, one of which, the Quercus semi-carpifolia, has a clean trunk from 80 to 100 feet high. Geraniums and labiate plants are mixed in sheltered spots with the tropical genera of Scitamineæ, or the ginger tribe; bignonias and balsams, and camellias, grow on the lower part of this region.
It is remarkable that Indian, European, American, and Chinese forms are united in this zone of transition, though the distinctness of species still obtains: the Triosteum, a genus of the honeysuckle tribe, is American; the Abelia, another genus of the same, together with the Camellia and Tricyrtis, are peculiarly Chinese; the daisy and wild thyme are European. A few of the trees and plants mentioned descend below the altitude of 5000 feet, but they soon disappear on the hot declivities of the mountain, where the Erythrina monosperma and Bombax heptaphyllum are the most common trees, together with the Millingtoniæ, a tribe of large timber-trees, met with everywhere between the Himalaya and 10° N. lat. The Shorea robusta, Dalbergia, and Cedrela, a genus allied to mahogany, are the most common trees in the forests of the lower regions of these mountains.
The temperate regions of eastern Asia, including Chinese Tartary, China, and Japan, have a vegetation totally different from that of any other part of the globe similarly situated, and show in a strong point of view the distinct character which vegetation assumes in different longitudes. In Mandshuria and the vast mountain-chains that slope from the eastern extremity of the high Tartarian table-land to the fertile plains in China, the forests and flora are generally of European genera, but Asiatic species; in these countries the buckthorn and honeysuckle tribes are so numerous as to give a peculiar character to the vegetation. Mixed with these and with roses are thickets of azaleas covered with blossoms of dazzling brightness and beauty.
The transition zone in this country lies between the 35th and 27th parallels of north latitude, in which the tropical flora is mixed with that of the northern provinces. The prevailing plants on the Chinese low grounds are Glycine, Hydrangea, the camphor laurel, Stillingia sebifera, or wax-tree, Clerodendron, Hibiscus Rosa-sinensis, Thuia orientalis, Olea fragrans, the sweet blossoms of which are mixed with the finer teas to give them flavour; Melia azedarach, or Indian pride, the paper mulberry, and others of the genus, and Camellia sasanqua, which covers hills in the province of Kiong-si. The tea-plant, and other species of Camellia, grow in many parts; the finest tea is the produce of a low range of hills from between the 33d and 25th parallels, an offset from the great chain of Peling. Thea viridis and bohea are possibly only varieties of the same plant; the green tea is strong and hardy, the black a small delicate plant. The quality of the tea depends upon the stage of growth at which it is gathered; early leaves make the best tea, those picked late in the season give a very coarse tea. Bohea grows in the province of Fu-kian, hyson in Song-lo. Pekoe or pak-ho, which means white down in Chinese, consists of the first downy sprouts or leaf-buds of three-years-old plants. A very costly tea of this kind, never brought to Europe, and known as the tea of the Wells of the Dragon, is used only by persons of the highest rank in China. The true Imperial tea, also, called Flos theæ, which is not, as was supposed, the flower-buds, but merely a very superior quality of tea, seldom reaches Europe; that sold under this name is really Chusan tea flavoured with blossoms of Olea fragrans.[159] The Chinese keep tea a year before they use it, because fresh tea has an intoxicating quality which produces disturbance of the nervous system like the effect of Erythroxylon Coca on the Peruvians. It is a remarkable circumstance that tea and coffee, belonging to different families, natives of different quarters of the globe, should possess the same principle, and it is not less remarkable that their application to the same use should have been so early discovered by man.
The tea-plant grows naturally in Japan and upper Assam; it is hardy, and possesses great power of adaptation to climate. It has lately been cultivated in Brazil, in Provence, and in Algiers, but at an expense which renders it unprofitable. Tea comes to Europe almost exclusively from China, but the plant thrives so well in the north-western provinces of India that the English will ultimately compete with the Chinese in producing it, especially for the consumption of Tibet. Tea was first brought to Europe by the Dutch in 1610; a small quantity came to England in 1666, and now the annual consumption of tea in Great Britain is about fifty millions of pounds.[160]
The climate of Japan is milder than its latitude would indicate, owing to the influence of the surrounding ocean. European forms prevail in the high lands, as they do generally throughout the mountains of Asia and the Indian Archipelago, with the difference of species, as Abies, Cembra, Strobus, and Larix. The Japanese flora is similar to the Chinese, and there are 30 American plants, besides others of Indian and tropical climates. These islands, nevertheless, have their own peculiar flora, distinct in its nature; as the Sophora, Kerria, Aucuba, Mespilus, and Pyrus Japonica, Rhus vernix, Illicium anisatum, or the anise-tree, Daphne odorata, the soap-tree, various species of the Calycanthus tribe, the custard-apple, the Khair mimosa, which yields the catechu, the litchi, the sweet orange, the Cycas revoluta, a plant resembling a dwarf palm, with various other fruits. Many tropical plants mingle with the vegetation of the cocoanut and fan palms.
Thus, the vegetation in Japan and China is widely different from that in the countries bordering the Mediterranean, though between the same parallels of latitude. In the tropical regions of Asia, where heat and moisture are excessive, the influence of latitude vanishes altogether, and the peculiarities of the vegetation in different longitudes become more evident.
Chapter XXV
Flora of Tropical Asia—Of the Indian Archipelago, India, and Arabia.
TROPICAL Asia is divided by nature into three distinct botanical regions: the Malayan peninsula, with the Indian Archipelago; India, south of the Himalaya, with the island of Ceylon; and the Arabian peninsula. The two first have strong points of resemblance, though their floras are peculiar.
FLORA OF THE INDO-CHINESE PENINSULA AND THE INDIAN ARCHIPELAGO.
Many of the vegetable productions of the peninsula beyond the Ganges are the same with those of India, mixed with the plants of the Indian Archipelago, so that this country is a region of transition, though it has a splendid vegetation of innumerable native productions, dyes of the most vivid hues, spices, medicinal plants, and many with the sweetest perfume. The soil in many places yields three crops in the year; the fruits of India, and most of those of China, come to perfection in the low lands. The arang forms an exception to the extreme beauty of the multitude of palms which adorn the Malayan peninsula; though it is eminently characteristic of that country, it is an ugly plant, covered with black fibres like horsehair, sufficiently strong to make cordage. It is cultivated for the sugar and wine made from its juice. Teak is plentiful; almost all that is used in Bengal comes from the Birman empire, though it is less durable than that of the Malabar coast. The Hopea odorata is so large that a canoe is made of a single trunk; the Gordonia integrifolia is held in such veneration that every Birman house has a beam of it.
There are seven species of native oak in the forests; the Mimosa catechu, which furnishes the terra japonica used in medicine; the trees which produce varnish and stick-lac; the Glyphyria nitida, a myrtle, the leaves of which are used as tea in Bencoolen, called by the natives the tree of long life. The coasts are wooded by the Heritiera robusta, a large tree which thrives within reach of the tide; bamboos with stems a foot and a half in diameter grow in dense thickets in the low lands. The Palmyra palm and the Borassus flabelliformis grow in extensive groves in the valley of the Irawaddy: it is a magnificent tree, often 100 feet high, remarkable for its gigantic leaves, one of which would shelter 12 men.
The anomalous trees the Zamias and Cycadeæ, somewhat like a palm with large pinnated leaves, but of a different family, are found here and in tropical India; those in America are of a different species. Orchideæ and tree-ferns are innumerable in the woody districts of the peninsula.
The vegetation of the Indian Archipelago is gorgeous beyond description; although in many instances it bears a strong analogy to that of the Malayan peninsula, tropical India, and Ceylon, still it is in an eminent degree peculiar. The height of the mountains causes variety in the temperature sufficient to admit of the growth of dammar pines, oaks, rhododendrons, magnolias, valerians, honeysuckles, bilberries, gentians, oleasters, and other European orders of woody and herbaceous plants; yet there is not one species in common.
Palm-trees are more abundant in these islands than in any other part of the world, especially in the Sunda group, the origin of many, a few of which are now widely spread over the eastern countries. Three species of Areca, attaining a height of from 40 to 50 and more feet, are cultivated in all the hot parts of India; and Caryota Urens, the fruit of which is acrid, yet it yields wine and sugar, are all native. The attempt is vain to specify the multitudes of these graceful trees which form so characteristic a feature in the vegetation of these tropical islands, where a rich moist soil with intense heat brings them to such perfection. It has been observed that monocotyledonous plants are generally more plentiful in islands than on continents, and also that they extend farther into the southern than into the northern hemisphere, which may be accounted for by the moist and mild climate of the former.
Jungle and dense pestilential woods entirely cover the smaller islands and the plains of the larger; the coasts are lined with thickets of mangroves, a matted vegetation of forest-trees, bamboos, and coarse grass, entwined with climbing and creeping plants, and overgrown by orchideous parasites in myriads; and the gutta-percha is also a native of these alluvial tracts. The forest-trees of the Indian Archipelago are almost unknown; teak and many of the continental trees grow there, but the greater number are peculiarly their own. The naturalist Rumphius had a cabinet inlaid with 400 kinds of wood, the produce of Amboyna and the Molucca islands.
Sumatra, Java, and the adjacent islands, are the region of the Dryobalanops camphora, in the stems of which solid lumps of a remarkable and costly kind of camphor are found. All the trees of that order, and of several others, are peculiar to these islands, and 78 species of trees and shrubs of the Melastomaceous tribe grow there and in continental India. There are thickets of the sword-leaved vaquois-tree and of the Pandanus or screw-pine, a plant resembling the anana, with a blossom like that of a bulrush, very odoriferous, and in some species edible.
This is the region of spices, which are very limited in their distribution: the Myristica moschata (the nutmeg and mace-plant) is confined to the Banda Islands, but it is said to have been discovered lately in New Guinea. The Amboyna and the Molucca groups are the focus of the Caryophyllus aromaticus, a myrtle, the buds of which are known as cloves. Various species of cinnamon and cassia, both of the laurel tribe, together with varieties of pepper, different from those in India and Ceylon, grow in this archipelago. All the pepper-plants require great heat: they are rare in Africa, but plentiful in America and the Indian Archipelago; the common black pepper is peculiar to the hottest parts of Asia, extending only a few degrees on each side of the equator. In 1842 more than 30,000,000 pounds weight of pepper were produced in Sumatra alone. Some of the most excellent fruits are indigenous here only, as the dourio, the ayer ayer, Loquat, the choapa of Molucca, peculiar kinds of orange, lemon, and citron, with others known only by name elsewhere. Those common to the continent of India are the jambrose, rose-apple, jack, various species of bread-fruit, mango, mangosteen, and the banana, which is luxuriant.
Here the nettle tribe assume the most pernicious character, and the upas-tree of Java, one of the most deadly vegetable poisons; and even the plants resembling our common nettle are so acrid that the sting of one in Java occasions not only pain but illness, which lasts for days. A nettle in the island of Timor, called by the natives the “Devil’s leaf,” is so poisonous that it produces long illness and even death. The chelik, a shrub growing in the dense forests, produces a poison even more deadly than the upas. Some of the fig genus, which belongs also to the natural order of nettles, have acrid juices. Trees of the cashew tribe have a milky sap: the fine japan lacquer is made from the juice of the Stagmaria verniciflua. Barringtonia and palms are very splendid here, the latter generally of peculiar species and limited in their distribution, as the Nipa. No country is richer in club-mosses and orchideous plants, which overrun the trees in thousands in the deep dark mountain-forests, choked by huge creeping plants, an undergrowth of gigantic grasses, through which not a ray of light penetrates.
Sir Stamford Raffles describes the vegetation of Java as “fearful.” In these forests the air is heavy, charged with dank and deadly vapours, never agitated by a breath of wind; the soil, of the deepest black vegetable mould, always moist and clammy, stimulated by the fervid heat of a tropical sun, produces trees whose stems are of a spongy texture from their rapid growth, loaded with parasites, particularly the orchideous tribe, of which no less than 300 species are peculiar to that island. Tree-ferns are in the proportion of one to twenty of the other plants, and form a large portion of the vegetation of Java and all these islands; and there are above 200 tropical species of club-mosses growing to the length of 3 feet, whereas in cold countries they creep on the ground.
The Rafflesias, of which there are four species, are the most singular productions of this archipelago. The most extraordinary one is common to Java and Sumatra, where it was discovered by Dr. Arnold, and therefore is called Rafflesia Arnoldi. It is a parasitical plant, with buds the size of an ordinary cabbage, and the flower, which smells of carrion, is of a brick-red colour, 3-1/2 feet in diameter: that found by Mr. Arnold weighed 15 pounds, and the cup in its centre could contain 12 pints of liquid.
According to Sir Stamford Raffles there are six distinct climates in Java, from the top of the mountains to the sea, each having an extensive indigenous vegetation. No other country can show an equal abundance and variety of native fruit and esculent vegetables. There are 100 varieties of rice, and of fragrant flowers, shrubs, and ornamental trees the number is infinite. Abundant as the Orchideæ are in Java, Ceylon, and the Birmese empire, these countries possess very few that are common to them all, so local is their distribution. Ferns are more plentiful in this archipelago than elsewhere: tree-ferns are found chiefly between or near the tropics, in airless damp places.
INDIAN FLORA.
The plains of Hindostan are so completely sheltered from the Siberian blasts by the high table-lands of Tartary and the Himalaya mountains, that the vegetation at the foot of that range already assumes a tropical character. In the jungles and lower ridges of the fertile valley of Nepal, and on the dark and airless recesses of the Silhet forests, arborescent ferns and orchideous plants are found in profusion, scarcely surpassed even in the islands of the Indian Archipelago—indeed the marshy Tariyane is full of them. The lowest ranges of the Himalaya, the pestilential swamp of the Tariyane, the alluvial ridges of the hills that bound it on the south, and many parts of the plains of the Ganges, are covered with primeval forests, which produce whole orders of large timber-trees, frequently overrun with parasitical loranths.
The native fruits of India are many; the orange tribe is almost all of Indian origin, though some of the species are now widely spread over the warmer parts of the other continents and the more distant countries of Asia. Two or three species are peculiar to Madagascar; one is found in the forests of the Essequibo, and another in Brazil, which are the only exceptions known. The Limonia laureola grows on the tops of the high Asiatic mountains, which are covered with snow several months in the year; and the wampee, a fruit much esteemed in China and the Indian Archipelago, is produced by a species of this order. The vine grows wild in the forests; plantain, banana, jambrose, guava, mango, mangosteen, date, areca, palmyra, cocoa-nut, and gameto-palms are all Indian, also the gourd family. The Scitamineæ, or ginger tribe, are so numerous, that they form a distinguishing and beautiful feature of Indian botany: they produce ginger, cardamoms, and turmeric. The flowers peculiar to India are brilliant in colours, but generally without odour, except the rose and some jessamines.
The greater part of the trees and plants mentioned belong also to tropical India, where vegetation is still more luxuriant; a large portion of that magnificent country, containing 1,000,000 square miles, has been cultivated time immemorial, although vast tracts still remain in a state of nature. Those extensive mountain-chains which traverse and surround the Deccan are rich in primeval forests of stupendous growth with dense underwood. The most remarkable of these trees are the Indian cotton-tree and the Dombeya, which is of the same order; that which produces the Trincomalee wood, used for building boats at Madras; the red-wood tree, peculiar to the Coromandel coast, the satin-wood, the superb Butea frondosa, the agallochum tribe, which yields the odorous wood of aloes mentioned in Scripture, the Melaleuca leucadendron and the Melaleuca cajepute, from which the oil is prepared. The dragon’s-blood tree is a native of India, though not exclusively, as some of the best specimens grow in Madagascar, where it is planted for hedges. Sanders-wood and dragon’s-blood are obtained from the Pterocarpus sandalinus and Draco; the sappan-tree gives a purple dye: these are all of the leguminous or bean tribe, of which there are 452 Indian species: ebony grows in these tropical regions, in Mauritius, and the south coast of Africa.
Some of the fig tribe are among the most remarkable vegetable productions of India for gigantic size and peculiarity of form, which renders them valuable in a hot climate from the shade which their broad-spreading tops afford. Some throw off shoots from their branches, which take root on reaching the ground, and, after increasing in girth with wonderful rapidity, produce branches which also descend to form new roots, and this process is continued till a forest is formed round the parent tree. Mr. Reinwardt saw in the island of Simao a large wood of the Ficus Benjamina which sprang from one stem. The Ficus Indica, or banyan-tree, is another instance of this wide-spreading growth; it is found in the islands, but is in greatest perfection around the villages in the Circar mountains: there is a tree of it on the banks of the Nerbudda, in the province of Guzerat, with 350 main stems, occupying an area of 2000 feet in circumference, independent of its branches, which extend much farther. The camphor genus is mostly Indian, as well as many more of the laurel tribe of great size. The banana is the most generally useful tree in this country; its fruit is food, its leaves are applied to many domestic purposes, and flax fit for making muslin is obtained from its stem. Cotton is a hairy covering of the seeds of several species of the mallow tribe which grow spontaneously in tropical Asia, Africa, and America; it is, however, cultivated in many countries beyond these limits. That grown in China and the United States of America is an herbaceous annual from 18 inches to 2 feet high: there are also cotton-trees, native and cultivated, in India, China, Africa, and America. Herodotus mentions cotton garments 445 years before the Christian era, and the Mexicans manufactured cotton cloth before the discovery of America.
Palms, the most stately and graceful of the vegetable productions of tropical regions, are abundant in India, in forests, in groups, and in single trees. Some species grow at the limit of perpetual snow, some 900 feet above the sea, others in valleys and on the shores of the continent and islands. They decrease in number and variety as the latitude increases, and terminate at Nice, in 44° N. lat., their limit in the great continent. The leaves of some are of gigantic size, and all are beautiful, varying in height from the slender Calamus rotang, 130 feet high, to the Chamærops humilis, not more than 15 or 20. Different species yield wine, oil, wax, flour, sugar, thread, and rope; weapons and utensils are made of their stems and leaves; they serve for the construction of houses; the cocoa-nut palm gives food and drink; sago is made from all except the Areca catechu, the fruit of which, the betel-nut, is used by the natives for its intoxicating quality.
Though palms in general are very limited in their distribution, a few species are very widely spread; for example, the cocoa-nut palm, which grows spontaneously on the southern coasts of the Indo-Chinese peninsula and the Sunda Islands, from whence it has been carried to all the intertropical regions of the globe, where it has been extensively cultivated from its usefulness. So luxuriant is its growth in Ceylon, that in one year nearly 3,000,000 of nuts were exported; in parts of that island, on the Malabar and Coromandel coasts, and in some districts in Bengal, the Borassus flabelliformis supplies its place.
The island of Ceylon, which may be regarded as the southernmost extremity of the Indian peninsula, is very mountainous, and rivals the islands of the Indian Archipelago in luxuriance of vegetable productions, and, in some respects, bears a strong resemblance to them. The laurel, the bark of which is cinnamon, is indigenous, and one of the principal sources of the revenue of Ceylon. The taleput leaves of a species of palm are of such enormous size, that they are applied to many uses by the Cingalese; in ancient times strips of the leaf were written upon with a sharp style, and served as books. The sandalwood of Ceylon is of a different species from that of the South Sea islands, and its perfume more esteemed. Indigo is indigenous, and so is the choya, whose roots give a scarlet dye. The mountains produce a great variety of beautiful woods used in cabinet-work. It is a remarkable circumstance in the distribution of plants, that the orchideæ are very numerous in this island.
ARABIAN VEGETATION.
The third division of the tropical flora of Asia is the Arabian, which differs widely from the other two, and is chiefly marked by trees yielding balsams. Oceans of barren sand extend to the south, from Syria through the greater part of Arabia, varied only by occasional oases in those spots where a spring of water has reached the surface; there the prevalent vegetation consists of the grasses, Holcus and Panicum dicotomum growing under the shade of the date-palm; mimosas and stunted prickly bushes appear here and there in the sand. There is verdure on the mountains, and along some of the coasts, especially in the province of Yemen, which has a flora of its own. The Keura odorifera, a superb tree, with agreeable perfume, eight species of figs, the three species of Amyris—gileadensis, or balm of Gilead, opobalsamum also yielding balsam, and the kataf, from which myrrh is supposed to come—are peculiar to Arabia. Frankincense is said to be the produce of the Boswellia serrata; and there are many species of Acacia, among others the Acacia arabica, which produces gum arabic. The arak and tamarind trees connect the botany of Arabia with that of the West Indies, while it is connected with that of the Cape of Good Hope by Stapelias, mesembryanthemums, and liliaceous flowers. The character of Arabian vegetation, like that of other dry hot climates, consists in its odoriferous plants and flowers.
Arabia produces coffee, which, however, is not indigenous, but is supposed to have come from the table-land of Ethiopia, and to have its name from the province of Kaffa, where it forms dense forests. It was introduced into Arabia in the end of the fifteenth century, and grows luxuriantly in Arabia Felix, where the coffee is of the highest flavour. Most of that now used is the progeny of plants raised from seed and brought from Mocha to the Botanic Garden at Amsterdam in 1690, by Van Hoorn, Governor of Batavia. A plant was sent to Louis XIV., in 1714, by the magistrates of Amsterdam—it was from this plant that the first coffee-plants were introduced in 1717 into the West India islands. A year afterwards the Dutch introduced coffee-trees into Surinam, from whence they spread rapidly over the warm parts of America and the West India islands. Many thousands of people are now employed in its cultivation there, in Demerara, Java, Manilla, the isle of Bourbon, and other places. More than 3,000,000 pounds of coffee-beans are produced, and 100,000 tons of shipping are annually employed in its transport across the Indian and Atlantic Oceans. Coffee was not known till many centuries after the introduction of sugar. The first coffee-house was opened in London in 1652, and the first in France, at Marseilles, in 1671.
Chapter XXVI
African Flora—Flora of Australia, New Zealand, Norfolk Island, and of Polynesia.
THE northern coast of Africa, and the range of the Atlas generally, may be regarded as a zone of transition, where the plants of southern Europe are mingled with those peculiar to the country; half the plants of northern Africa are also found in the other countries on the shores of the Mediterranean. Of 60 trees and 248 shrubs which grow there, 100 only are peculiar to Africa, and about 18 of these belong to its tropical flora. There are about six times as many herbaceous plants as there are trees and shrubs; and in the Atlas mountains, as in other chains, the perennial plants are much more numerous than annuals. Evergreens predominate, and are the same as those on the other shores of the Mediterranean. The pomegranite, the locust-tree, the oleander, and the palmetto abound; and the cistus tribe give a distinct character to the flora. The sandarach, or Thuia articulata, peculiar to the northern side of the Atlas mountains and to Cyrenaica, yields close-grained hard timber, used for the ceiling of mosques, and is supposed to be the shittim-wood of Scripture. The Atlas produces seven or eight species of oak, various pines, especially the Pinus maritima, and forests of the Aleppo pine in Algiers. The sweet-scented arborescent heath and Erica scoparia are native here, also in the Canary Islands and the Azores, where the tribe of house-leeks characterizes the botany. There are 534 phanerogamous plants, or such as have the parts of fructification evident, in the Canary Islands; of these, 310 are indigenous, the rest African: the Pinus canariensis is peculiar, and also the Dracænæ, which grow in perfection here. The stem of the Dracæna Draco, of the Villa Oratava in Teneriffe, measures 46 feet in circumference at the base of the tree, which is 75 feet high. It is known to have been an object of great antiquity in the year 1402, and is still alive, bearing blossoms and fruit. If it be not an instance of the partial location of plants, there must have been intercourse between India and the Canary Islands in very ancient times.
Plants with bluish-green succulent leaves are characteristic of tropical Africa and its islands; and though the group of the Canaries has plants in common with Spain, Portugal, Africa, and the Azores, yet there are many species, and even genera, which are found in them only; and the height of the mountains causes much variety in the vegetation.
On the continent, south of the Atlas, a great change of soil and climate takes place; the drought on the borders of the desert is so excessive that no trees can resist it, rain hardly ever falls, and the scorching blasts from the south speedily dry up any moisture that may exist; yet, in consequence of what descends from the mountains, the date-palm forms large forests along their base, which supply the inhabitants with food, and give shelter to crops which could not otherwise grow. The date-palm, each tree of which yields from 150 to 160 pounds weight of fruit, grows naturally, and is also cultivated, through northern Africa. It has been carried to the Canary Islands, Arabia, the Persian Gulf, and to Nice, the most northern limit of the palm-tribe. Stunted plants are the only produce of the desert, yet large tracts are covered with the Pennisetum dichotomum, a harsh prickly grass, which, together with the Alhagi maurorum, is the food of camels.
The plants peculiar to Egypt are acacias, mimosas, cassias, tamarisks, the Nymphæa Lotus, the blue Lotus, the Papyrus, from which probably the first substance used for writing upon was made, and has left its name to that we now use; also the Zizyphus or jujub, various mesembryanthemums, and most of the plants of Barbary grow here. The date-palm is not found higher on the Nile than Thebes, where it gives place to the doom-palm or Cucifera Thebaica, peculiar to this district, and singular as being the only palm that has a branched stem.
The eastern side of equatorial Africa is less known than the western, but the floras of the two countries, under the same latitude, have little affinity: on the eastern side the Rubiaceæ, the Euphorbiæ, a race peculiarly African, and the Malvaceæ, are most frequent. The genus Danais of the coffee tribe distinguishes the vegetation of Abyssinia, also the Dombeya, a species of vine, various jessamines, a beautiful species of honeysuckle; and Bruce says a caper-tree grows to the height of the elm, with white blossoms, and fruit as large as a peach. The daroo, or Ficus sycomorus, and the arak-tree, are native. The kollquall, or Euphorbia antiquorum, grows 40 feet high on the plain of Baharnagach, in the form of an elegant branched candelabrum, covered with scented fruit. The kantuffa or thornby shrub, is so great a nuisance from its spines, that even animals avoid it. The Erythrina Abyssinica bears a poisonous red bean with a black spot, used by the shangalla and other tribes for ages as a weight for gold, and by the women as necklaces. Mr. Rochet has lately brought some seeds of new grain from Shoa, that are likely to be a valuable addition to European cerealia.
The vegetation of tropical Africa on the west is known only along the coast, where some affinity with that of India may be observed. It consists of 573 species of flower-bearing plants, and is distinguished by a remarkable uniformity, not only in orders and genera, but even in species, from the 16th degree of N. lat. to the river Congo in 6° S. lat. The most prevalent are the grasses and bean tribes, the Cyperaceæ Rubiaceæ, and the Compositæ. The Adansonia, or baobab of Senegal, is one of the most extraordinary vegetable productions; the stem is sometimes 34 feet in diameter, though the tree is rarely more than 50 or 60 feet high; it covers the sandy plains so entirely with its umbrella-shaped top, that a forest of these trees presents a compact surface, which at some distance seems to be a green field. Cape Verde has its name from the numbers that conceal the barren soil under their spreading tops; some of them are very old, and, with the dragon-tree at Teneriffe, are supposed to be the most ancient vegetable inhabitants of the earth. The Pandanus candelabrum, instead of growing crowded together in masses like the baobab, stands solitary on the equatorial plains, with its lofty forked branches ending in tufts of long stiff leaves. Numerous sedges, of which the Papyrus is the most remarkable, give a character to this region, and cover boundless plains, waving in the wind like corn-fields, while other places are overgrown by forests of gigantic grasses with branching stems.
A rich vegetation, consisting of impenetrable thickets of mangrove, the poisonous manchineel, and many large trees, cover the deltas of the rivers, and even grow so far into the water, that their trunks are coated with shell-fish; but the pestilential exhalations render it almost certain death to botanize in this luxuriance of nature.
Various kinds of the soap or sapodilla trees are peculiar to Africa; the butter-tree of the enterprising but unfortunate Mungo Park, the star-apple, the cream-fruit, the custard-apple, and the water-vine, are plentiful in Senegal and Sierra Leone. The ibraculea is peculiarly African; its seeds are used to sweeten brackish water. The safu and bread-fruit of Polynesia are represented here by the musanga, a large tree of the nettle tribe, the fruit of which has the flavour of the hazel-nut. A few palms have very local habitations, as the Elais Guineensis, or palm-oil plant, found only on that coast. That graceful tribe is less varied in species in equatorial Africa than in the other continents. It appears that a great part of the flora of this portion of Africa is of foreign origin.
The flora of south Africa differs entirely from that of the northern and tropical zones, and as widely from that of every other country, with the exception of Australia and some parts of Chile. The soil of the table-land at the Cape of Good Hope, stretching to an unknown distance, and of the Karoo plains and valleys between the mountains, is sometimes gravelly, but more frequently is composed of sand and clay; in summer it is dry and parched, and most of its rivers are dried up; it bears but a few stunted shrubs, some succulent plants and mimosas, along the margin of the river-courses. The sudden effect of rain on the parched ground is like magic: it is recalled to life, and in a short time is decked with a beautiful and peculiar vegetation, comprehending, more than any other country, numerous and distinctly-defined foci of genera and species.
Twelve thousand species of plants have been collected in the colony of the Cape in an extent of country about equal to Germany. Of these, heaths and proteas are two very conspicuous tribes; there are 300 species of the former, and 200 of the latter, both of which have nearly the same limited range, though Mr. Bunbury found two heaths, and the Protea cynaroides, the most splendid of the family (bearing a flower the size of a man’s hat), on the hills round Graham’s Town, in the eastern part of the colony. These two tribes of plants are so limited that there is not one of either to be seen north of the mountains which bound the Great Karoo, and by much the greatest number of them grew within 100 miles of Cape Town; indeed at the distance of only 40 miles the prevailing Proteaceæ are different from those at the Cape. The Leucadendron argenteum, or silver-tree, which forms groves at the back of the table-mountain, is confined to the peninsula of the Cape. The beautiful Disa grandiflora is found only in one particular place on the top of the table-mountain.
The dry sand of the west coast, and the country northward through many degrees of latitude, is the native habitation of Stapelias, succulent plants with square leafless stems, and flowers like star-fish, with the smell of carrion. A great portion of the eastern frontier of the Cape colony and the adjacent districts is covered with extensive thickets of a strong succulent and thorny vegetation, called by the natives the bush: similar thickets occur again far to the west, on the banks of the river Gauritz. The most common plants of the bush are aloes of many species, all exceedingly fleshy and some beautiful: the great red-flowering arborescent aloe, and some others, make a conspicuous figure in the eastern part of the colony. Other characteristic plants of the eastern districts are the spek-boem, or Portulacaria afra, Schotia speciosa, and the great succulent euphorbias, which grow into real trees 40 feet high, branching like a candelabrum, entirely leafless, prickly, and with a very acrid juice. The Euphorbia meloformis, three feet in diameter, lies on the ground, to which it is attached by slender fibrous roots, and is confined to the mountains of Graaf Reynet. Euphorbias, in the Old World, correspond with the Cactus tribe, which belong exclusively to the New. The Zamia, a singular plant, having the appearance of a dwarf-palm, without any real similarity of structure, belongs to the eastern districts, especially to the great tract of bush on the Caffir frontier.
Various species of Acacia are indigenous and much circumscribed in their location: the Acacia horrida, or the white-thorned acacia, is very common in the eastern districts and in Caffirland. The Acacia cafra is strictly eastern, growing along the margins of rivers, to which it is a great ornament. The Acacia detinens, or hook-thorn, is almost peculiar to Zand valley.
It appears, from the instances mentioned, that the vegetation in the eastern districts of the colony differs from that on the western, yet many plants are generally diffused of orders and genera found only in this part of Africa:—Nearly all the 300 species of the fleshy succulent tribe of Mesembryanthemum, or Hottentot’s fig; a great many beautiful species of the Oxalis, or wood-sorrel-tribe; every species of Gladiolus, with the exception of that in the cornfields in Italy and France; ixias innumerable, one with petals of apple-green colour; geraniums, especially the genus Pelargonium, or stork’s bill, almost peculiar to this locality; many varieties of Gnaphalium and Xeranthemum; the brilliant Strelitzia; 133 species of the house-leek tribe, all fleshy, attached to the soil by a strong wiry root, and nourished more or less from the atmosphere: Diosmas are widely scattered in great variety; shrubby Boragineæ with flowers of vivid colours, and Orchideæ with large and showy blossoms. The leguminous plants and Cruciferæ of the Cape are peculiar; indeed all the vegetation has a distinct character, and both genera and species are confined within narrower limits than anywhere else, without any apparent cause to account for a dispersion so arbitrary.
Notwithstanding the peculiarity of character with which the botany of the Cape is so distinctly marked, it is connected with that of very remote countries by particular plants; for example, of the seven species of bramble which grow at the Cape, one is the common English bramble or blackberry. The affinity with New Holland is greater: in portions of the two countries in the same latitude there are several genera and species that are identical: Proteaceæ are common to both, so are several genera of Irideæ, Leguminosæ, Ficoideæ, Myrtaceæ, Diosmeæ, and some others. The botany of the Cape is connected with that of India, and even that of South America, by a few congeners.
The vegetation of Madagascar, though similar in many respects to the floras of India and Africa, nevertheless is its own: the Brexiaceæ and Chlenaceæ are orders found nowhere else: there are species of Bignonia, Cycadeæ, and Zamias, a few of the mangosteen tribe, and in the mountains some heaths. The Hydrogeton fenestralis is a singular aquatic plant, with leaves like the dried skeletons of leaves, having no green fleshy substance, and the Tanghinia veneniflua, which produces a poison so deadly that its seeds are used to execute criminals, and one seed is sufficient.
Some genera and species are common and peculiar to Madagascar, the Isle of Bourbon, and Mauritius; yet of the 161 known genera in Madagascar only 54 grow on the other two islands. The three islands are rich in ferns. The Pandanus, or screw-pine genus, abounds in Bourbon and the Mauritius, where it covers sandy plains, sending off strong aërial roots from the stem, which strike into the ground and protect the plant from the violent winds. Of 290 genera in Bourbon and Mauritius, 196 also grow in India, though the species are different: there is also some resemblance to the vegetation of South Africa, and there is a solitary genus in common with America.
Eight or ten degrees north of Madagascar lies the group of the Seychelles Islands, in which are groves of the peculiar palm which bears the double cocoa-nut, or coco de mer, the growth of these islands only. Its gigantic leaves are employed in the construction of houses, and other parts of the plant are applied to various domestic purposes.
FLORA OF AUSTRALIA.
The interior of the Australian continent is so little known, that the flora which has come under observation is confined to a short distance from the coast; but it is of so strange and unexampled a character, that it might easily be mistaken for the production of another planet. Many entire orders of plants are known only in Australia, and the genera and species of others that grow elsewhere assume new and singular forms. Evergreens, with hard narrow leaves of a sombre, melancholy hue, are prevalent, and there are whole shadowless forests of leafless trees; the foot-stalks, dilated and set edgewise on the stem, supply their place and perform the functions of nutrition; their altered position gives them a singular appearance. Plants in other countries have glands on the under side of the leaves, but in Australia there are glands on both sides of these substitutes for leaves, which make them dull and lustreless, and the changes of the seasons have no influence on the unvarying olive-green of the Australian forests; even the grasses are distinguished from the gramineæ of other countries by a remarkable rigidity. Torres Straits, in the north, only 50 miles broad, separates this dry, sombre vegetation from the luxuriant jungle-clad shores of New Guinea, where deep and dark forests are rich in more than the usual tropical exuberance—a more complete and sudden change can hardly be imagined.
The peculiarly Australian vegetation is in the southern part of the continent of New Holland distributed in distinct foci in the same latitude, a circumstance of which the Proteaceæ afford a remarkable instance. Nearly one-half of the known species of these beautiful shrubs grow in the parallel of Port Jackson, from which they decrease in number both to the south and the north. In that latitude, however, there are twice as many species on the eastern side of the continent as there are on the western, and four times as many as in the centre. Although the Proteaceæ at both extremities of the continent have all the characters peculiar to Australia, yet those on the eastern coast resemble the South American species, while those on the western side have a resemblance to African forms, and are confined to the same latitudes.
Species of this family are numerous in Van Diemen’s Land; where they thrive at the elevation of 3500 feet, and also on the plains. The myrtle tribe form a conspicuous feature in Australian vegetation, particularly the genera Eucalyptus, Melaleuca, Beaufortia, and others, with splendid blossoms—white, purple, yellow, crimson: 100 species of the Eucalypti, most of them large trees, grow in New Holland; they form great forests in the colony of Port Jackson. The leafless acacias, of which there are 93 species, are a prominent feature in the Australian landscape. The leaves, except in very young plants, are merely foliaceous foot-stalks, presenting their margin towards the stem; yet these and the Eucalypti form the densest shade of any trees in the country. The genus Casuarina, with its strange-jointed, drooping branches, called the marsh-oak, holds a conspicuous place; it is chiefly confined to the principal parallel of this vegetation, and produces excellent timber; it grows also in the Malayan peninsula and South Sea islands. The Oxleya xanthoxylon or yellow wood, one of the mahogany tribe, grows to great size, and the Podocarpus aspleniifolia forms a new genus of the cone-bearing trees. Some of the nettle tribe grow 15 or even 20 feet high. The Epacrideæ, with scarlet, rose, and white blossoms, supply the place of, and very much resemble, heaths, which do not exist here. The purple-flowering Tremandreæ; the yellow-flowering Dilleniaceæ; the Doryanthes excelsa, the most splendid of the lily tribe, 24 feet high, with a brilliant crimson blossom; the Banksia, the most Australian of all the Proteaceæ; with Zamias of new species, are all conspicuous in the vegetation of Port Jackson.
There is a change on the north-eastern coast of New Holland. The Castanospermum Australe is so plentiful that it furnishes the principal food of the natives; a caper-tree of grotesque form, having the colossal dimensions of the Senegal baobab, and extraordinary trees of the fig genus, characterize this region. It sometimes occurs, when the seeds of these fig-trees are deposited by birds on the iron-bark-tree, or Eucalyptus resinifera, that they vegetate and enclose the trunk of the tree entirely with their roots, whence they send off enormous lateral branches, which so completely envelop the tree, that at last its top alone is visible in the centre of the fig-tree, at the height of 70 or 80 feet. The Pandanus genus flourishes within the influence of the sea-air. There are only six species of palms, equally local in their habitations as elsewhere, not one of which grows on the west side of the continent. The Araucaria excelsa, or Norfolk Island pine, produces the best timber of any tree in this part of Australia: it, or others of the same genus, extends from the parallel of 29° on the east coast towards the equator, and grows over an area of 900 square miles, including New Norfolk, New Caledonia, and other islands, some of which have no other timber-tree: they are supposed to exist only within the influence of the sea. The Asphodeleæ abound and extend to the southern extremity of Van Dieman’s Land.
The south-western districts of New Holland exhibit another focus of vegetation, less rich in species than that of Port Jackson, but not less peculiar. The Kingia Australis, or grass-tree, rises solitary on the sandy plains, with bare blackened trunks as if scathed by lightning, occasioned by the fires of the natives, and tufts of long grassy leaves at their extremities; Banksias, particularly the kind called wild honeysuckle, are numerous; the Stylidium, whose blossoms are even more irritable than the leaves of the sensitive mimosa, and plants with dry, everlasting blossoms, characterize the flora of these districts. The greater part of the southern vegetation vanishes on the northern coasts of the continent, and what remains is mingled with the cabbage-palm, various species of the nutmeg tribe, sandal-wood, and other Malayan forms—a circumstance that may hereafter be of importance to our colonists.
Orchideæ, chiefly terrestrial, are in great variety in the extra-tropical regions of New Holland, and the grasses amount to one-fourth of the monocotyledonous plants. Reeds of gigantic size form forests in the marshes, and kangaroo-grass covers the plains.
Beautiful and varied as the flora is, New Holland is by no means luxuriant in vegetation. There is little appearance of verdure, the foliage is poor, the forests often shadeless, and the grass thin; but in many valleys of the mountains, and even on some parts of the plains, the vegetation is vigorous. It is not the least remarkable circumstance in this extraordinary flora, that, with the exception of a few berries, there is no edible fruit, grain, or vegetable indigenous either in New Holland or Van Diemen’s Land.
The plants of New Holland prevail in every part of Van Diemen’s Land; yet the coldness of the climate and the height of the mountains permit genera of the northern hemisphere to be mixed with the vegetation of the country. Butter-cups, anemones, and polygonums of peculiar species grow on the mountain-tops, together with Proteaceæ and other Australian plants. The plains glow with the warm golden flowers of the black wattle, a Mimosa, emblematic of the island, and with the equally bright and orange blossom of the gorse, which perfumes the whole atmosphere. Only one tree-fern grows in this country; it rises 20 feet to the base of the fronds, which spread into an elegant top, producing a shadow gloomy as night-fall, and there are 150 species of orchis. The southern extremities both of New Holland and Van Diemen’s Land are characterized by the prevalence of evergreen plants: but the trees here, as well as in the other parts of the southern hemisphere, do not shed their leaves periodically as with us.
The botany of New Zealand appears to be intimately allied to that of New Holland, South America, and South Africa, but chiefly to that of New Holland. Noble trees form impenetrable forests, 60 of which yield the finest timber, and many are of kinds to which we have nothing similar. Here there are no representatives of our oak, birch, or willow, but five species of beech and ten of Coniferæ have been discovered that are peculiar to the country. They are all alpine, and only descend to the level of the sea in the southern parts of the island. The Coniferæ of the southern hemisphere are more local than in the northern; of the ten species peculiar to New Zealand it is not certain that more than two or three are found in the middle island, or that any of them grow south of the 40th parallel. The Kauri pine, or Dammara australis, is indigenous in all the three islands, but it is the only cone-bearing tree in North Island, where it grows in hilly situations near the sea, shooting up with a clean stem 60 or 90 feet, sometimes 30 feet in diameter, with a spreading but thin top, and generally has a quantity of transparent yellow resin imbedded at its base. This fine tree does not grow beyond the 38th degree of S. lat. The Metrosideros tomentosa, with rich crimson blossoms, is one of the greatest ornaments of the forests, and the Metrosideros robusta the most singular. It grows to a very great size, and sends shoots from its trunk and branches to the ground, which become so massive that they support the old stem, which to all appearance loses its vitality; it is in fact an enormous epiphyte, growing to, and not from, the ground. Many of the smaller trees are of the laurel tribe, with poisonous berries. Besides, there is a cabbage-palm, the Areca sapida, elder, the Fuchsia excorticata, and other shrubs. This country is probably the southern limit of the orchideous plants that grow on trees. Before New Zealand was colonized, the natives lived chiefly on the roots of the edible fern, Pteris esculenta, with which the country is densely covered, mixed with a shrub that grows like a cypress, and the tea-plant, which is a kind of myrtle whose berries afford an intoxicating liquor. More than 140 species of fern are natives of these islands, some of which are arborescent and 40 feet high; the country is chiefly covered with these and with the New Zealand flax, Phormium tenax, which grows abundantly both on the mountains and plains. The vegetation is so vigorous on these volcanic islands that it grows richly on the banks of hot springs, and even in water too hot to be touched.[161]
In Norfolk Island, 152 species of plants are already known, and many, no doubt, are yet to be discovered. The Cape gooseberry or Physalis edulis, the guava-tree, pepper, white and swamp oak, iron, blood-wood, and lemon trees, are native; also the bread-fruit tree, which blossoms, but does not bear fruit. The Araucaria excelsa and some palms are indigenous, and there are three times as many ferns as of all the other plants together.
The multitude of islands of Polynesia constitute a botanical region apart from all others, though it is but little varied, and characterized principally by the number of syngenesious plants with arborescent terms and tree-ferns. In continental India and the tropical parts of New Holland, the proportion of ferns to conspicuously-flowering plants is as 1 to 26, while on the Polynesian islands it is as 1 to 4, and perhaps even as 1 to 3.[162]
The cocoa-nut palm and the pandanus are common to all the islands, but the latter thrives only when exposed to the sea-air. This archipelago produces Tacca pinnatifida, which yields arrow-root; the Morus papyrifera, whose bark is manufactured into paper; and one of the Dracæna tribe, from which an intoxicating liquor is made. Fifty varieties of the bread-fruit tree are indigenous, which produce three or four crops annually. It is most abundant in the Friendly, Society, and Caroline groups, from whence it has been taken to America, where it thrives in very low latitudes. The Sandwich group is peculiar in the number of Goodenias and Lobelias; while the Coral Islands, whose flora is entirely borrowed, rarely have two species belonging to the same genus; the fragrant suriana and sweet-scented Tournfortia are among their scanty vegetation.
The two species of banana-trees which are natives of southern Asia have been introduced at an unknown and probably early period into the Polynesian islands, and all tropical countries in the eastern and western hemispheres. Syria is their northern limit, where the Musa paradisaica grows to 34° N. lat. The sweet fruit of these trees produces, on the same extent of ground, 44 times as much nutriment as the potato, and 133 times more than wheat.
St. Helena, the Sandwich group, New Zealand, Juan Fernandez, and above all the Galapagos islands, are more peculiar in their floras than any other tracts of their size. The Galapagos archipelago consists of 10 principal islands lying immediately under the equator, 600 miles from the coast of America. They are entirely volcanic, and contain 2000 extinct craters. The vegetation is so peculiar that, of 180 plants which have been collected, 100 are found nowhere else; of 21 species of Compositæ all but one are new, and belong to 10 genera, 8 of which are confined to these islands exclusively.
This flora has no analogy to that of Polynesia, but it bears a double relation to the flora of South America. The plants peculiar to the Galapagos islands are, for the most part, allied to those on the cooler part of the continent or on high lands, while the others are the same with those that abound in the hot damp intertropical regions of the continent. The greatest number of peculiar plants grow on the tops of the islands where the sea vapour is condensed, and many of them are confined to some one islet of the group. Though this flora is singular, it is poor compared with that of the Sandwich group, or the Cape de Verde Islands.[163]