Physical Geography of the Sea and its Meteorology

Front matter

THE

PHYSICAL GEOGRAPHY

OF

Chapter 1

THE

PHYSICAL GEOGRAPHY OF THE SEA,

AND ITS METEOROLOGY.

CHAPTER 1.

1-68.—THE SEA. AND THE ATMOSPHERE.

§ 1.The two oceans of air and water.—Our planet is invested with two great oceans; one visible, the other invisible; one is underfoot, the other overhead; one entirely envelops it, the other covers about two-thirds of its surface. All the water of the one weighs about 400 times as much as all the air of the other.

2.Their meeting.—It is at the bottom of this lighter ocean where the forces which we are about to study are brought into play. This place of meeting is the battle-field of nature, the dwelling-place of man; it is the scene of the greatest conflicts which he is permitted to witness, for here rage in their utmost fury the powers of sea, earth, and air; therefore, in treating of the Physical Geography of the sea, we must necessarily refer to the phenomena which are displayed at the meeting of these two oceans. Let us, therefore, before entering either of these fields for study, proceed first to consider each one in some of its most striking characteristics. They are both in a state of what is called unstable equilibrium; hence the currents of one and the winds of the other.

3.Their depth.—As to their depth, we know very little more of the one than of the other; but the conjecture that the average depth of the sea does not much exceed four miles is probably as near the truth as is the commonly received opinion that the height of the atmosphere does not exceed fifty miles. If the air were, like water, non-elastic, and not more compressible than this non-elastic fluid, we could sound out the atmospherical ocean with the barometer, and gauge it by its pressure. The mean height of the barometer at the level of the sea in the torrid and temperate zones, is about 30 inches. Now, it has been ascertained that, if we place a barometer 87 feet above the level of the sea, its average height will be reduced from 30.00 in. to 29.90 in.; that is, it will be diminished one-tenth of an inch, or the three hundredth part of the whole; consequently, by going up 300 X 87 ( = 26,100) feet, the barometer, were the air non-elastic, would stand at 0. It would then be at the top of the atmosphere. The height of 26,100 feet is just five miles lacking 300 feet.

4. Weight of the atmosphere.—But the air is elastic, and very unlike water. That at the bottom is pressed down by the superincumbent air with the force of about 15 pounds to the square inch, while that at the top is inconceivably light. If, for the sake of explanation, we imagine the lightest down, in layers of equal weight and ten feet thick, to be carded into a pit several miles deep, we can readily perceive how that the bottom layer, though it might have been ten feet thick when it first fell, yet with the weight of the accumulated and superincumbent mass, it might now, the pit being full, be compressed into a layer of only a few inches in thickness, while the top layer of all, being uncompressed, would be exceedingly light, and still ten feet thick; so that a person ascending from the bottom of the pit would find the layers of equal weight thicker and thicker until he reached the top. So it is with the barometer and the atmosphere: when it is carried up in the air through several strata of 87 feet, the observer does not find that it falls a tenth of an inch for every successive 87 feet upward through which he may carry it. To get it to fall a tenth of an inch, he must carry it higher and higher for every successive layer.

5. Three-fourths below the mountain tops.—More than three-fourths of the entire atmosphere is below the level of the highest mountains; the other fourth is rarefied and expanded in consequence of the diminished pressure, until the height of many miles be attained. From the reflection of the sun's rays after he has set, or before he rises above the horizon, it is calculated that this upper fourth part must extend at least forty or forty-five miles higher. 6. Its height.—At the height of 26,000 miles from the earth, the centrifugal force would counteract gravity; consequently, all ponderable matter that the earth carries with it in its diurnal revolution must be within that distance, and consequently the atmosphere cannot extend beyond that. This limit, however, has been greatly reduced, for Sir John Herschel has shown, by balloon observations, that at the height of 80 or 90 miles there is a vacuum far more complete than any which we can produce by any air-pump. In 1783 a large meteor, computed to be half a mile in diameter and fifty miles from the earth, was heard to explode. As sound cannot travel through vacuum, it was inferred that the explosion took place within the limits of the atmosphere. Herschel concludes that the aerial ocean is at least 50 miles deep.

7. Data conjectural.—The data from which we deduce our estimate, both as to the mean height of the atmosphere and average depth of the ocean, are, to some extent, conjectural; consequently, the estimates themselves must be regarded as approximations, but sufficiently close, nevertheless, for the present purposes of this work.

8. Analysis of air.—Chemists who have made the analysis, tell us that, out of 100 parts of atmospheric air, 99.5 consist of oxygen and nitrogen, mixed in the proportion of 21 of oxygen to 79 of nitrogen by volume, and of 23 to 77 by weight. The remaining half of a part consists of .05 of carbonic acid and .45 of aqueous vapour.

9. Information respecting the depth of the ocean.—The average depth of the ocean has been variously computed by astronomers, from such arguments as the science affords, to be from 26 to 11 miles. About ten years ago I was permitted to organize and set on foot in the American navy a plan for "sounding out" the ocean with the plummet. Other navies, especially the English, have done not a little in furtherance of that object. Suffice it to say that, within this brief period, though the undertaking has been by no means completed—no, not even to the tenth part—yet more knowledge has been gained concerning the depths and bottom of the deep sea, than all the world had before acquired in all previous time.

10. Its probable depth. "The system of deep-sea soundings thus inaugurated does not thus far authorize the conclusion that the overage depth of ocean water is more than three or four miles (§ 3) nor have any reliable soundings yet been made in water over five miles deep.

11. Relation between its depth and the waves of the sea.—In very shallow pools, where the water is not more than a few inches deep, the ripples or waves, as all of us, when children, have observed, are small; their motion, also, is slow. But when the water is deep, the waves are larger and more rapid in their progress, thus indicating the existence of a numerical relation between their breadth, height, and velocity, and the depth of the water. It may be inferred, therefore, that if we knew the size and velocity of certain waves, we could compute the depth of the ocean.

12. Airy's wave tables.—Such a computation has been made, and we have the authority of Mr. Airy, the Astronomer Royal, that waves of given breadths will travel in water of certain depths with the velocities as per table:

Depth of the Water in Feet. | Breadth of the wave in feet.

|1 | |10 | |100 | |1000 | |10,000 | |100,000 | |1,000,000 | |10,000,000

| | Corresponding Velocity of Wave per Hour in Miles.

|1 | |1.54 | |3.81 | |3.86 | |3.86 | |3.86 | |3.86 | |3.86 | |3.86

|10 | |1.54 | |4.87 | |11.51 | |12.21 | |12.22 | |12.22 | |12.22 | |12.22

|100 | |1.54 | |4.87 | |15.18 | |36.40 | |38.64 | |38.66 | |38.66 | |38.66

|1,000 | |1.54 | |4.87 | |15.18 | |48.77 | |115.11 | |122.18 | |122.27 | |122.27

|10,000 | |1.54 | |4.87 | |15.18 | |48.77 | |154.25 | |364.92 | |386.40 | |386.66

|100,000 | |1.54 | |4.87 | |15.18 | |48.77 | |154.25 | |487.79 | |1150.00 | |1222.70

13. The earthquake of Simoda.—Accident has afforded us an opportunity of giving a quasi practical application to Mr. Airy's formulæ. On the 23rd of December, 1854, at 9.45 a.m., the first shocks of an earthquake were felt on board the Russian frigate "Diana," as she lay at anchor in the harbour of Simoda, not far from Jeddo, in Japan. In fifteen minutes afterwards (10 o'clock), a large wave was observed rolling into the harbour, and the water on the beach to be rapidly rising. The town, as seen from the frigate, appeared to be sinking. This wave was followed by another, and when the two receded—which was at 10h. 15m.—there was not a house, save an unfinished temple, left standing in the village. These waves continued to come and go until 2.30 P.M., during which time the frigate was thrown on her beam ends five times. A piece of her keel 81 feet long was torn off, holes were knocked in her by striking on the bottom, and she was reduced to a wreck. In the course of five minutes the water in the harbour fell, it is said, from 23 to 3 feet, and the anchors of the ship were laid bare. There was a great loss of life; many houses were washed into the sea, and many junks carried up—one two miles inland—and dashed to pieces on the shore. The day was beautifully fine, and no warning was given of the approaching convulsion; the barometer standing at 29.87 in., thermometer 58°; the sea perfectly smooth when its surface was broken by the first wave. It was calm in the morning, and the wind continued light all day.

14. The propagation of leaves by it.—In a few hours afterwards, at San Francisco and San Diego, the tide-gauges showed that several well-marked and extraordinary waves had arrived off the coast of California. The origin of these waves, and those which destroyed the town of Simoda, in Japan, and wrecked the "Diana," was doubtless the same. But where was their birthplace? Supposing it to be near the coasts of Japan, we may, with the tide-gauge observations in California and Mr. Airy's formulæ, calculate the average depth of the sea along the path of the wave from Simoda both to San Francisco and San Diego.

15. Their breadth and velocity.—Supposing the waves to have taken up their line of march from some point along the coast of Japan, the San Francisco wave, having a breadth of 256 miles, had a velocity of 438 miles an hour; while the breadth of the San Diego wave was 221 miles, and its rate of travel 427 miles an hour.

16. Average depth of the North Pacific.— Admitting these premises—which are partly assumed—to be correct, then, according to Airy's formulæ, the average depth of the North Pacific between Japan and California is, by the path of the San Francisco wave, 2149 fathoms, by the San Diego, 2034 (say 2½ miles).

17. Specific gravity of sea-water.—At the temperature of 60°, the specific gravity of average sea-water is 1.0272, and the weight of a cubic foot is 64.003 lbs.

18. Of air.—With the barometer at 30 in. and the thermometer at 32°, the weight of a cubic foot of dry atmospheric air is 1.291 oz., and its specific gravity .00129. Such is the difference in weight between the two elements, the phenomena of which give the physical geography of the sea its charms.

19. Unequal distribution of light, land, and air.—There is in the northern hemisphere more land, less sea, more fresh water, more atmospheric air, and a longer annual duration of sunlight, than there is in the southern. And though the two hemispheres receive annually the same amount of heat directly from the sun, yet the northern, without growing cooler, dispenses the greater quantity by radiation.

20. The sun longer in northern declination.—In his annual round, the sun tarries a week (7¾ days) longer on the north than he does on the south side of the equator, and consequently the antarctic night and its winter are longer than the polar winter and night of the arctic regions. The southern hemisphere is said also to be cooler, but this is true only as to its torrid and temperate zones. In the summer of the southern hemisphere the sun is in perigee, and during the course of a diurnal revolution there the southern half of our planet receives more heat than the northern half during the same period of our summer. This difference, however, Sir John Herschel rightfully maintains is compensated by the longer duration of the northern summer. Therefore, admitting the total quantity of heat annually impressed upon the earth by the sun to be equally divided between the two hemispheres, it does not follow that their temperature should be the same, for their powers of radiation may be very different. The northern hemisphere having most land, radiates the more freely—the land and sea breezes tell us that the land dispenses heat more freely than the sea by radiation—but the northern hemisphere is prevented in two ways from growing cooler than the southern:—1. by the transfer of heat in the latent form with the vapours from the southern seas;—2. by the transfer of heat in the sensible form, by currents such as the Gulf Stream, et al., from one climate to another in our hemisphere. Hence we infer that the southern hemisphere is in certain zones cooler than the northern, not by reason of its short summer or long winter, but it is the cooler chiefly on account of the latent heat which is brought thence by vapour, and set free here by condensation.

21. England about the pole of hemisphere with most land.—Within the torrid zone the land is nearly equally divided north and south of the equator, the proportion being as 5 to 4. In the temperate zones, however, the north with its land is thirteen times in excess of the south. Indeed, such is the inequality in the distribution of land over the surface of the globe that the world may be divided into hemispheres consisting, the one with almost all the land in it, except Australia and a slip of America lying south of a line drawn from the desert of Atacama to Uruguay; England is the centre of this, the dry hemisphere. The other, or aqueous hemisphere, contains all the great waters except the Atlantic Ocean; New Zealand is the nearest land to its centre.

22. Effects of inequality in distribution of land and water.—This unequal distribution of land, light, air, and water is suggestive. To it we owe, in a measure, the different climates of the earth. Were it different, they would be different also; were it not for the winds, the vapours that rise from the sea would from the clouds be returned in showers back to the places in the sea whence they came; on an earth where no winds blow we should have neither green pastures, still waters, nor running brooks to beautify the landscape. Were there no currents in the sea, nor vertical movements in the air, the seasons might change, but climates would be a simple affair, depending solely on the declination of the sun in the sky.

23. Quantity of fresh water in American lakes.—About two-thirds of all the fresh water on the surface of the earth is contained in the great American lakes; and though there be in the northern, as compared with the southern hemisphere, so much less sea surface to yield vapour, so much more land to swallow up rain, and so many more plants to drink it in, yet the fresh-water courses are far more numerous and copious on the north than they are on the south side of the equator. 24. Southern seas the boiler, and northern lands the condenser.—These facts have suggested the comparison in which the southern hemisphere has been likened to the boiler and the northern to the condenser of the steam-engine. This vast amount of steam or vapour rising up in the extra-tropical regions of the south, expels the air thence, causing the barometer to show a much less weight of atmosphere on the polar side of 40° S., than we find in corresponding latitudes north.

25. Offices of the atmosphere.—The offices of the atmosphere are many, marvellous, and various. Though many of them are past finding out, yet, beautiful to contemplate, they afford most instructive and profitable themes for meditation.

26. Dr. Buist.—When this system of research touching the physics of the sea first began—when friends were timid and co-labourers few, the excellent Dr. Buist stood up as its friend and champion in India; and by the services he thus rendered, entitled himself to the gratitude of all who, with me, take delight in the results which have been obtained. The field which it was proposed to occupy—the firstlings of which were gathered in this little book—was described by him in glowing terms, and with that enthusiasm which never fails to inspire zeal. They are apropos, and it is a pleasure to repeat the substance of them.

27. The sea and the atmosphere contrasted.—"The weight of the atmosphere is equal to that of a solid globe of lead sixty miles in diameter. Its principal elements are oxygen and nitrogen gases, with a vast quantity of water suspended in them in the shape of vapour, and commingled with these a quantity of carbon in the shape of fixed air, equal to restore from its mass many fold, the coal that now exists in the world. In common with all substances, the ocean and the air are increased in bulk, and, consequently diminished in weight, by heat; like all fluids, they are mobile, tending to extend themselves equally in all directions, and to fill up depressions wherever vacant space will admit them; hence in these respects the resemblance betwixt their movements. Water is not compressible or elastic, and it may be solidified into ice, or vaporized into steam; the air is elastic; it may be condensed to any extent by pressure, or expanded to an indefinite degree of tenuity by pressure being removed from it; it is not liable to undergo any change in its constitution beyond these, by any of the ordinary influences by which it is affected.

28. Influence of the sun.—"These facts are few and simple enough; let us see what results arise from them: As the constant exposure of the equatorial regions of the earth to the sun must necessarily there engender a vast amount of heat, and as his absence from the polar regions must in like manner promote an infinite accumulation of cold, to fit the entire earth for a habitation to similar races of beings, a constant interchange and communion betwixt the heat of the one, and the cold of the other, must be carried on. The ease and simplicity with which this is effected surpass all description. The air, heated near the equator by the overpowering influences of the sun, is expanded and lightened; it ascends into upper space, leaving a partial vacuum at the surface to be supplied from the regions adjoining. Two currents from the poles toward the equator are thus established at the surface, while the sublimated air, diffusing itself by its mobility, flows in the upper regions of space from the equator toward the poles. Two vast whirlpools are thus established, constantly carrying away the heat from the torrid toward the icy regions, and, there becoming cold by contact with the ice, they carry back their gelid freight to refresh the torrid zone.

29. Of diurnal rotation.—"Did the earth, as was long believed, stand still while the sun circled around it, we should have had directly from north and south two sets of meridional currents blowing at the surface of the earth toward the equator; in the upper regions we should have had them flowing back again to the place whence they came. On the other hand, were the heating and cooling influences just referred to to cease, and the earth to fail in impressing its own motion on the atmosphere, we should have a furious hurricane rushing round the globe at the rate of 1000 miles an hour—tornadoes of ten times the speed of the most violent now known to us, sweeping everything before them. A combination of the two influences, modified by the friction of the earth, which tends to draw the air after it, gives us the trade-winds, which, at the speed of from ten to twenty miles an hour, sweep round the equatorial region of the globe unceasingly.

30. Currents.—"Impressed with the motion of the air, constantly sweeping its surface in one direction, and obeying the same laws of motion, the great sea itself would be excited into currents similar to those of the air, were it not walled in by continents and subjected to other control. As it is, there are constant currents flowing from the torrid toward the frigid zone to supply the vast amount of vapour there drained off, while other whirlpools and currents, such as the gigantic Gulf Stream, come to preform their part in the same stupendous drama. The waters of this vast ocean river are, to the north of the tropic, greatly warmer than those around; the climate of every country it approaches is improved by it, and the Laplander is enabled by its means to live and cultivate his barley in a latitude which, everywhere else throughout the world, is condemned to perpetual sterility. There are other laws which the great sea obeys which peculiarly adapt it as the vehicle of interchange of heat and cold betwixt those regions where either exists in excess.

31. Icebergs.—"In obedience to these laws water warmer than ice attacks the basis and saps the foundations of the icebergs—themselves gigantic glaciers, which have fallen from the mountains into the sea, or which have grown to their present size in the shelter of bays and estuaries, and by accumulations from above. Once forced from their anchorage, the first storm that arises drifts them to sea, where the beautiful law which renders ice lighter than the warmest water, enables it to float, and drifts southward a vast magazine of cold to cool the tepid water which bears it along—the evaporation at the equator causing a deficit, the melting and accumulation of the ice in the frigid zone giving rise to an excess of accumulation, which tends, along with the action of the air and other causes, to institute and maintain the transporting current. These stupendous masses, which have been seen at sea in the form of church spires, and Gothic towers, and minarets, rising to the height of from 300 to 600 feet, and extending over an area of not less than six square miles, the masses above water being only one-tenth of the whole, are often to be found within the tropics.

32. Mountain ranges.—"But these, though among the most regular and magnificent, are but a small number of the contrivances by which the vast and beneficent ends of nature are brought about. Ascent from the surface of the earth produces the same change, in point of climate, as an approach to the poles; even under the torrid zone mountains reach the line of perpetual congelation at nearly a third less altitude than the extreme elevation which they sometimes attain. At the poles snow is perpetual on the ground, and at the different intervening latitudes reaches some intermediate point of congelation betwixt one and 20,000 feet. In America, from the line south to the tropics, as also, as there is now every reason to believe, in Africa within similar latitudes, vast ridges of mountains, covered with perpetual snow, run northward and southward in the line of the meridian right across the path of the trade-winds. A similar ridge, though of less magnificent dimensions, traverses the peninsula of Hindoostan, increasing in altitude as it approaches the line, attaining an elevation of 8500 feet at Dodabetta, and about 6000 in Ceylon. The Alps in Europe, and the gigantic chain of the Himalayas in Asia, both far south in the temperate zone, stretch from east to west, and intercept the aerial current from the north. Others of lesser note, in the equatorial or meridional, or some intermediate direction, cross the paths of the atmospherical currents in every direction, imparting to them fresh supplies of cold, as they themselves obtain from them warmth in exchange: in strictness the two operations are the same.

33. Water.—"Magnificent and stupendous as are the effects and results of the water and of air acting independently on each other, in equalizing the temperature of the globe, they are still more so when combined. One cubic inch of water, when invested with a sufficiency of heat, will form one cubic foot of steam—the water before its evaporation, and the vapour which it forms being exactly of the same temperature; though in reality, in the process of conversion, 1100 degrees of heat have been absorbed or carried away from the vicinage, and rendered latent or imperceptible; this heat is returned in a sensible and perceptible form the moment the vapour is converted once more into water. The general fact is the same in the case of vapour carried off by dry air at any temperature that may be imagined; for, down far below the freezing-point, evaporation proceeds uninterruptedly.

34. Latent heat.—"The air, heated and dried as it sweeps over the arid surface of the soil, drinks up by day myriads of tons of moisture from the sea—as much, indeed, as would, were no moisture restored to it, depress its whole surface at the rate of eight or ten feet annually. The quantity of heat thus converted from a sensible or perceptible to an insensible or latent state is almost incredible. The action equally goes on, and with the like results, over the surface of the earth, where there is moisture to be withdrawn. But night and the seasons of the year come round, and the surplus temperature, thus withdrawn and stored away at the time it might have proved superfluous or inconvenient, is rendered back so soon as it is required; thus the cold of night and the rigour of winter are modified by the heat given out at the point of condensation by dew, rain, hail, and snow.

35. Effects upon the earth.—"The earth is a bad conductor of heat; the rays of the sun, which enter its surface and raise the temperature to 100° or 150°, scarcely penetrate a foot into the ground; a few feet down, the warmth of the ground is nearly the same night and day. The moisture which is there preserved free from the influence of currents of air is never raised into vapour; so soon as the upper stratum of earth becomes thoroughly dried, capillary action, by means of which all excess of water was withdrawn, ceases; so that, even under the heats of the tropics, the soil two feet down will be found, on the approach of the rains, sufficiently moist for the nourishment of plants. The splendid flowers and vigorous foliage which burst forth in May, when the parched soil would lead us to look for nothing but sterility, need in no way surprise us; fountains of water, boundless in extent and limited in depth only by the thickness of the soil which contains them, have been set aside and sealed up for their use, beyond the reach of those thirsty winds or burning rays which are suffered to carry off only the water which is superfluous, and would be pernicious. They remove it to other lands, where its agency is required, or treasure it up, as the material of clouds and dew, in the crystal vault of the firmament, the source, when the fitting season comes round again, of those deluges of rain which provide for the wants of the year. Such are some of the examples which may be supplied of general laws operating over nearly the whole surface of the terraqueous globe. Among the local provisions ancillary to these are the monsoons of India, and the land and sea breezes prevalent throughout the tropical coasts.

36. The tides.—"We have not noticed the tides, which, obedient to the sun and moon, daily convey two vast masses of water round the globe, and which twice a month, rising to an unusual height, visit elevations which otherwise are dry. During one half of the year the highest tides visit us by day, the other half by night; and at Bombay, at spring tide, the depths of the two differ by two or three feet from each other. The tides simply rise and fall, in the open ocean, to an elevation of two or three feet in all; along our shores, and up gulfs and estuaries, they sweep with the violence of a torrent, having a general range of ten or twelve feet—sometimes, as at Fundy, in America, at Brest and Milford Haven, in Europe, to a height of from forty to sixty feet. The tides sweep our shores from filth, and purify our rivers and inlets, affording to the residents of our islands and continents the benefits of a bi-diurnal ablution, and giving a health, and freshness, and purity wherever they appear. Obedient to the influence of bodies many millions of miles removed from them, their subjection is not the less complete; the vast volume of water, capable of crushing by its weight the most stupendous barriers that can be opposed to it, and bearing on its bosom the navies of the world, impetuously rushing against our shores, gently stops at a given line, and flows back again to its place when the word goes forth, 'Thus far shalt thou go, and no farther;' and that which no human power or contrivance could have repelled, returns at its appointed time so regularly and surely that the hour of its approach, and measure of its mass, may be predicted with unerring certainty centuries beforehand.