XL. THE FIXED STARS
FIXED STARS—MAGNITUDE OF THE STARS—CONSTELLATIONS—DESCRIPTIONS OF THE ZODIACAL CONSTELLATIONS—NORTHERN AND SOUTHERN STAR GROUPS—DISTANCE OF STARS.
We have been considering the planets so far as they are known to astronomers, but no doubt we shall find out others some day beyond Neptune in space, for it must be assumed that there are other planets wandering about in the infinite firmament. At present, however, we cannot spare time for such speculation; we have got to peep at the stars and their groupings.
“What little bits of things the stars are,” a child said once in our hearing; and there were others present who were inclined to believe that the tiny light spots we could see looked small—not because they were distant, but because they were of no great magnitude; and when those children were told that the tiny stars were “suns” like our sun, giving heat and light millions and millions of miles away,—and, so far as we can tell, some are much bigger and hotter than our own sun,—they were very much surprised indeed, and one little girl aptly quoted Dr. Watts:—
“Twinkle, twinkle, little star, How I wonder what you are”!
Now let us endeavour to learn something about these apparently tiny specks, and why they “twinkle.”
At a very early period in the history of astronomy the observers of the heavens grouped stars together in fancied resemblances to men or animals; and these “constellations,” as they are termed, are combinations of fixed stars—that is, of stars which do not wander about as the planets do. But these so-called fixed stars have motions; they are only relatively fixed with reference to their positions to each other as they appear to revolve daily round the earth. But stars have a movement of their own, which is termed their “proper motion.”
It is to Halley that the discovery of these real star motions is due. He saw three very bright stars (Sirius, Aldebaran, and Arcturus) were not in the places they had been assigned. The sun also has been found to possess a “proper motion,” and, with the planets, is travelling as determined by Sir J. Herschel, to a particular place in the constellation called Hercules. There are now star catalogues and star maps, for the heavens have been as closely surveyed as the earth, and by accurate observations it has now become possible to find the position of every star usually visible. Some of the stars are used as “clock” stars, by which sidereal time can be calculated accurately, and the clocks thereby corrected. The stars, though termed “fixed,” are in perpetual movement—Arcturus at the rate of fifty miles a second, and others less. Only the rates of a few are known.
The number of the stars is beyond our calculation, and even the number of stars only visible in the telescope amount to millions, and these are called telescopic stars. The visible stars amount to about six thousand, and of course these are the brightest up to the sixth magnitude. There are more visible in the southern than in the northern hemisphere. The magnitudes of the stars range in classes according to the brightness of the stars observed, for this is really the test from the first magnitude to the sixth; after that the telescopic stars are seen up to the fifteenth or sixteenth. We can only see about three thousand stars at any one time from any place, although, as remarked above, many millions may be observed with a good telescope, and as many more, probably twenty millions, are invisible.
We will now proceed to detail the constellations, which are familiar by name to everybody. We have already given the names of the zodiacal groups, which consist of many stars, each designated by a letter of the Greek alphabet so far as possible, then the Roman letters and numerals are employed. Thus α (Alpha) is the most brilliant star; β (Beta) the next bright γ (Gamma) the next, and so on; so the relative brilliancy of the stars in the constellation is indicated, but not the very biggest star of the first magnitude is intended by α, for the star δ in one constellation may equal α in another. John Bayer originated this method in 1603.
The arrangement of the constellations is plunged in the obscurity of ages, but B.C. 370 there were forty-five thus grouped. There are northern and southern constellations which are visible above our old friends Aries, Taurus, Cancer, etc. We will, as in duty bound, consider our old acquaintances first, and then give a list of the northern and southern groups of stars; but we shall find that the forms are in the greater part due to the imagination of the ancients, and do not bear out our ideas of the animals they are supposed to represent, while at the same time they cross and recross with other constellations in the skies in a very puzzling way.
The first constellation is ARIES, the Ram, which is celebrated in mythology as the proud possessor of the Golden Fleece, which we may remember was seized and carried away by Jason and the Argonauts. The Hellespont is so called from Helle, who fell from the Ram’s back when being carried upon it over the Black Sea. The Ram is here represented with the equinoctial ring.
We perceive in Aries two very bright stars near the head. These are (α) Arietis and (γ) Sheretan. The signs and constellations do not now correspond as they used to do, because of the change in the position of the stars, which gives rise to the Precession of the Equinoxes (vide ante., p. 497), so that the stars which two thousand years ago were in conjunction with the sun, are much more to the eastward. In olden time (when astronomy was young), the sun entered Aries on the 21st March, and now a change has taken place. But in about another twenty thousand years, they will all come right again. This will be perceived by reference to the celestial globe. The Ram has sixty-six stars in his constitution.
TAURUS, the Bull, is the next constellation. He received his name from the celebrated animal into which Jupiter transformed himself when he wished to carry away Europa. The star Aldebaran (α) is the end of a kind of V in the Bull’s face. The Pleiades are on the shoulder to the right. This cluster of twinkling stars is well known, and will guide the observer towards the imaginary Bull, which we must nowadays describe as rather a fanciful delineation. Europe is called after Europa, because Jupiter, as a Bull, carried her to this continent. There are 141 stars in Taurus, according to the number found in the list of Aratus, and probably more.
GEMINI, the Twins, which are supposed to be Castor and Pollux, though it is believed that two goats were the original sign—which statement, taken in connection with the ram and the bull, that were also turned out in the spring-time, may have something to recommend it. But now Castor and Pollux are generally recognized as the constellation. During the expedition for the Golden Fleece, the electric appearance, now known as St. Elmo’s Fire, became visible upon them, and their effigies were placed in the forepart of ships as a good omen. This led to the adoption of the “figure-head.” They were made into stars when Pollux was immortalized by Jupiter, for he divided the boon with his brother. The planet Uranus was discovered near this constellation, which contains eighty-five stars.
CANCER, the Crab, is the next in order, and the only derivation we can find for this is that Juno sent a crab to attack Hercules when he was busily engaged with the many-headed Hydra. The crab was directed to pinch the hero’s foot, but it appears rather a lame device for the Queen to adopt. The crab, however, was killed by Hercules, and placed amongst the stars by Juno as a reward; so he gained immortality cheaply. He, Cancer, contains more than eighty stars, but none of them of any particular note. Some writers explain the sign as reminding the ancients of “the retrograde movement of the sun to the north”; but as a crab does not move “backwards,” we will still adhere to mythology as equally satisfactory at any rate. Cancer, however, was termed the “northern gate of the sun.”
The next is LEO, the Lion, which came round in summer and at the period of much heat, so this fierce animal may have been chosen to represent that season. But mythology will have us credit the Nemæan Lion sent against Hercules by Juno as the origin of this constellation. The lion was, like the crab, placed amongst the stars when he was killed. He is a very brilliant constellation, and a very bright star called Regulus is to be seen in his chest—“Cor Leonis.” Another very fine star of the second magnitude is observable in the tail. The Lion consists of ninety-five stars, the principal ones being of the first and second magnitudes.
VIRGO is supposed to be outlined by a very rich cluster of stars, and one of the first magnitude. The Virgin is by some supposed to be Astræa, the goddess, but is more likely referable to a girl gleaning, or holding an ear of corn in semblance of the harvest. This constellation contains more than one hundred stars. One of them in the wheat-ear is a particularly brilliant one, and noted for its “solitary splendour,” as no star of large magnitude is near it. The Arabs used to call it the Solitary Simak; Spica Virginis is the modern name.
LIBRA, which follows, may either indicate the balance, or scales of justice, of Astræa, or the equal day and night at the autumnal equinox. Virgil mentions Astræa’s balance, and thus we have a classical authority for the very mythological view of the two foregoing constellations. Libra is not very distinct; it contains fifty-one stars, four of which are very bright.
SCORPIO, the Scorpion, according to classical writers, encountered Orion, who is also met with in the stellar universe. The scorpion stung Orion because he declared there was no living creature he could not overcome by force. On the other hand, this sign may have some reference to the unhealthy time of year, and the prevalence of disease about the time that Scorpio appeared. A beautiful star of reddish hue and of the first magnitude is prominent amongst the brilliant assembly of the Scorpion’s forty-four stars.
SAGITTARIUS, the Archer, is, as one can see, a Centaur, and said to be Chiron, who was wounded by Hercules, and cured by being taken up to Heaven by Jupiter. This Chiron is represented as a great patron of the arts, and thus the fable may be said to exemplify the proverb, “Art is long, time is fleeting”; for readers of mythology will find much more in the legends than is apparent on the surface. But we can now only regard the Centaur from an astronomical, and not a philosophical standpoint. Sagittarius has no very brilliant stars. He is close to the Milky Way, and contains sixty-nine stars, five forming a sort of V in the bow, sometimes compared to a ladle or “dipper.”
CAPRICORNUS, the Goat, is supposed to be Pan—“the great god Pan,” who turned himself into a goat. The sun was in Capricornus at mid-winter, so the “southern gate of the sun” was a title bestowed upon him. But now the constellation is later. It does not include any very striking stars, of which there are fifty-one in the “Goat.”
AQUARIUS, or the Waterbearer, may have referred to wet weather, or as others declare, to Ganymede, the Cupbearer. There are four stars in the waterpot like a Y; and more than one hundred stars of small brilliancy are included in this constellation. But here again fancy must come to our assistance, for without a diagram the ordinary observer could not distinguish the Waterbearer.
PISCES, the Fishes, are not plainly defined. It is supposed that Venus and Cupid turned themselves into fish when the Titans assailed Heaven. This Constellation occupies a triangle in the sky.
The foregoing are the zodiacal constellations, and may be more easily remembered by repeating an old rhyme, which runs as follows:—
“The Ram, the Bull, the Heavenly Twins; Then, next the Crab, the Lion shines, The Virgin, and the Scales; The Scorpion, Archer, and the Goat, The Man who holds the Watering Pot, The Fish with Glittering tails.”
The arrangement of the various Constellations at which we have so rapidly glanced, as well as of those that follow, has been the work of many different periods. Aratus and Ptolemy are the oldest enumerators, but modern research has added immensely to the store of knowledge. Many of the most prominent stars were named by Grecian and Arabian observers, and many of the names are still retained—such as Arcturus, Rigel, Capella, and others.
THE NORTHERN CONSTELLATIONS.
There are, altogether, thirty-five of these, as per list on next page. It is of course impossible to describe them all, but we will make a few remarks respecting those which will be distinguished most readily, and the manner of finding out particular stars. There are star maps published, and with a little attention and reading, a great many very pleasant evening excursions may be made across the sky, with or without a telescope. The following is the list of the northern constellations. We have put them in various types to indicate the most important.
+----------------------------------------------+-------------+--------+ | | | NO. OF | | NAME OF CONSTELLATION. | “AUTHOR.” | STARS. | +----------------------------------------------+-------------+--------+ | URSA MAJOR The Great Bear | Aratus | 87 | | Ursa Minor The Lesser Bear | “ | 24 | | Perseus Perseus | “ | 59 | | Auriga The Waggoner | “ | 66 | | Boötes The Herdsman (Boötes) | “ | 54 | | Draco The Dragon | “ | 80 | | Cepheus Cepheus | “ | 35 | | Canes Venatici {The Greyhounds} | Hevelius | 25 | | {Hunting Dogs } | | | | Cor Caroli Heart of Charles | Halley | 3 | | Triangulum The Triangle | Aratus | 16 | | Triangulum Minor The Little Triangle | Hevelius | 10 | | Musca The Fly | Bode | 6 | | Lynx The Lynx | Hevelius | 44 | | Leo Minor The Lesser Lion | “ | 53 | | Coma Berenices Berenice’s Hair | Tycho Brahé | 43 | | Camelopardalis The Giraffe | Hevelius | 58 | | Mons Menelaus Mount Menalaus | “ | 11 | | Corona Borealis The Northern Crown | Aratus | 21 | | Serpens The Serpent | “ | 64 | | Scutum Sobieski Sobieski’s Shield. | Hevelius | 8 | | Hercules Hercules | Aratus | 113 | | Serpentarius The Serpent-bearer | “ | 74 | | Taurus Poniatowski Poniatowski’s Bull | Poczobat | 7 | | LYRA The Harp, or Lyre | Aratus | 22 | | Vulpecula et Anser Fox and Goose | Hevelius | 37 | | Sagitta The Arrow | Aratus | 18 | | AQUILA The Eagle | “ | 71 | | Delphinus The Dolphin | “ | 18 | | Cygnus The Swan | “ | 81 | | CASSIOPEIA The Lady’s Chair | “ | 55 | | Equuleus The Little Horse | Ptolemy | 10 | | Lacerta The Lizard | Hevelius | 16 | | Pegasus Pegasus (Flying Horse) | Aratus | 89 | | Andromeda Andromeda | “ | 66 | | Tarandus The Reindeer | Lemounier | 12 | | (There are a few others marked in continental maps.) | | +----------------------------------------------+-------------+--------+
The GREAT BEAR, or “Charles’s Wain,” or the “Plough,” as Ursa Major is variously called, is of great value in indicating the pole star, which, when once known, can never be mistaken. This constellation has also been termed the “Dipper,” and is very conspicuous in the northern hemisphere. The three stars form the bear’s tail, or the handle of the “plough”; the others form the body, Charles’s Wain, or “Karl-Wagen,” the German term for peasant’s cart, is represented by the quadrangle forming the cart, and the other three stars are the horses.
The “Pointers” are the two end stars, and if a line be followed northwards from them it will lead close to Polaris, the principal star in the lesser bear. This pole star is of a very great brightness, and peeps out, almost isolated, with a pure lustre. The names of the pointers are Dubhe and Menak. The star at the tail-tip is Benetnasch, then Mizar and Alioth. Megrez and Phad are the remaining pair. We append a rough outline of the bear, for the information of those who have not yet noticed it.
The Lesser Bear is not so important as his elder brother as regards size, but he is very useful to astronomers. He resembles the Great Bear in appearance, but is smaller, and the positions of the stars are inverted. In the cut on page 555 (fig. 629) you see the little bear swinging round the polar star, which is at the tip of the Lesser Bear’s tail, so any one will be enabled to find him if they look for the polar star, and then count the three stars away from it, and the four in the body. The Great Bear’s tail points in the other direction. This movement of the earth’s axis by displacing the equinoctial points, alters the “declination” and “right ascension” of the stars (compare page 473). So Polaris is gradually approaching the actual polar point. In about 200 years he will have got as close as he can, and will then begin to recede from it, and in about 12,500 years after he will reach his most distant point.
POLARIS, the Pole Star, was called “Cynosure” by the ancients, and thus we can understand the quotation, “Cynosure of neighbouring eyes,” when a person or object is very attractive. The pole star was the point to which all looked. There are some other very important stars in these constellations. For instance, in—
Perseus we have Algenib and Algol, of second magnitude. Auriga we have Capella, of the first magnitude. Boötes we have Arcturus, of the first magnitude. Lyra we have Vega, a very large and bright star. Aquila, Altair, also a very beautiful star. In Cygnus there is Deneb, of the first magnitude.
These stars are also designated by the Greek letters—α, lyræ, or the first in the Lyre—that is, Vega; and so on for all, according to rank, as already explained.
SOUTHERN CONSTELLATIONS.
We must pass on to the southern constellations, of which there are forty-six; the principal ones are in capital letters:—
+---------------------------------------------------+-----------+--------+ | | | NO. OF | | NAME OF CONSTELLATION. | “AUTHOR.” | STARS. | +---------------------------------------------------+-----------+--------+ | * Phœnix The Phœnix | Bayer | 13 | | Apparatus Sculptoris The Sculptor’s Tools | Lacaille | 12 | | Eridanus Fluvius The River Po | Aratus | 84 | | * Hydrus The Water-snake | Bayer | 10 | | Cetus The Whale | Aratus | 97 | | Fornax Chemica The Furnace | Lacaille | 14 | | * Horologium The Clock | “ | 12 | | * Rheticulus Rhomboidialus The Rhomboidal Net | “ | 10 | | * Xiphias Dorado The Sword-fish | Bayer | 7 | | * Celapraxitels The Engraver’s Tools | Lacaille | 16 | | Lepus The Hare | Aratus | 19 | | Columba Noachi Noah’s Dove | Halley | 10 | | ORION Orion | Aratus | 78 | | Argo Navis The ship Argo | “ | 64 | | CANIS MAJOR The Great Dog | “ | 31 | | Equuleus Pictoris The Easel | Lacaille | 8 | | Monoceros The Unicorn | Hevelius | 31 | | Canis Minor The Lesser Dog | Ptolemy | 14 | | * Chameleon The Chameleon | Bayer | 10 | | Pyxis Nautica The Mariner’s Compass | Lacaille | 4 | | * Piscis Volans The Flying Fish | Bayer | 8 | | Hydra The Snake | Aratus | 60 | | Sextans The Sextant | Hevelius | 41 | | * Rober Carolinum Charlie’s Oak | Halley | 12 | | Antlia Pneumatica The Air Pump | Lacaille | 3 | | Crater The Cup | Aratus | 31 | | Corvus The Crow | “ | 9 | | *Crux Australis The Southern Cross | Royer | 6 | | Apis Musca The South Fly | Bayer | 4 | | * Avis Indica The Bird of Paradise | “ | 11 | | * Circinus The Compass | Lacaille | 4 | | Centaurus The Centaur | Aratus | 35 | | Lupus The Wolf | “ | 24 | | Norma The Square | Lacaille | 12 | | * Triangulum Australis The Southern Triangle | Bayer | 5 | | *Ara The Altar | Aratus | 9 | | * Telescopium The Telescope | Lacaille | 9 | | Corona Australis The Southern Crown | Ptolemy | 12 | | * Pavo The Peacock | Bayer | 14 | | * Indus The Indian | “ | 12 | | Microscopium The Microscope | Lacaille | 10 | | * Octans Hadliensis Hadley’s Octant | “ | 43 | | * Grus The Crane | Bayer | 14 | | Toucan The Toucan | “ | 9 | | Pisces Australis The Southern Fish | Aratus | 24 | | * Mons Mensa Table Mountain | Lacaille | 30 | +---------------------------------------------------+-----------+--------+
We need only describe Orion and Canis Major, the principal groups. The former certainly constitutes the most glorious group, and it is visible to all the world, because the equinoctial passes through it.
ORION, as we have said, can be viewed from either hemisphere, and so can some others; but those marked with an asterisk in the foregoing list are not visible in the latitude of London.
Orion is a very brilliant constellation, and contains two fine stars of the first magnitude, and some of the second. The former are Betelgeux and Rigel. Bellatrix is the third in order. The “belt” is formed of three bright stars, and the sword is visible as five stars just below. Canis Major possesses Sirius, a very fine star (the dog star). Canis Minor has two of the first and second magnitude, and Hydra has one of the first. The Southern Cross is a beautiful constellation, invisible in our latitude, but familiar to sailors in the Southern Seas.
THE STARS’ DISTANCE AND MAGNITUDE.
When we gaze up into the sky at night, we see the stars twinkling far away, and we may remark here that this twinkling of the stars is due to the atmosphere and the changes in its power of refraction, and of course the star’s light changes its direction. But if we ascend in a balloon into very high and rarefied strata of the air, we will find the twinkling less. We have given the number of the stars according to Flamstead, but the larger the telescope the greater will be the number of stars we shall see, numbers again being too far even for our perfected instruments.
But we can gain some idea of the magnitude of the stars when we consider the distance to arrive at, which is a most difficult task, for figures seem scarcely long enough to count the millions of miles, and no instrument can detect the parallax. Even supposing the parallax to be a very small fraction of a degree we should get a result equalling trillions of miles. NO. 61 in Cygni was at one time continually observed by Professor Bessel, and he found that its distance—and it is the nearest star—is sixty-two and a half trillions of miles.
Let us consider what this means. Light comes to us from the sun (91,000,000 of miles) in about eight minutes, and travels at the rate of something like 186,000 miles in a second. But even at that astounding rate the light from the star called 61 Cygni took ten years to reach the earth; and there are stars whose light has never yet reached the earth, although the gleam may have been travelling at 186,000 miles a second for thousands of years. And we may presume that though we still see the light of stars, some of them may be dead, but the light left is still progressing to us through space.
So we must conclude that some stars which look large, as do Vega and Sirius for instance, must be enormous “suns,” a great deal larger than our sun, and the stars are each the centre of invisible systems just as our sun is the centre of the “solar” system. Vega is a tremendous star, and shines with her own light as do all other visible stars; for reflected light, so very visible in the moon, which is close to us, would be quite invisible at such tremendous distances. So we must call these stars “suns,” and may add an apparently astonishing fact, that our own sun is merely one of the stars in the Galaxy, or “Milky Way”!
XLI. THE STARS—(continued)
DOUBLE AND MULTIPLE STARS—COLOURED AND VARIABLE STARS—CLUSTERS, GROUPS, AND NEBULÆ—THE GALAXY, OR MILKY WAY—HOW TO FIND OUT THE PRINCIPAL STARS.
Although not very clearly visible to the naked eye, there are in the sky some pairs of stars very close together apparently; but when these double stars are examined with a good telescope we find that though we fancy they are two stars very close, in reality an immense distance separates them. By Vega, which we have already mentioned, there is apparently a star, which on examination will be found really to be two stars. It is also in the constellations of the Lyre, but of much lower magnitude than Vega. But in some instances there are three or four stars thus placed together, and the frequency of the occurrence of this fact establishes the farther fact that these combinations are not accidental—that the stars are interdependent and physically connected.
There are now at least six thousand double stars known,[27] and this is a very small proportion of the forty millions or so of suns which are believed to exist in space. But of these six thousand a larger proportion have been ascertained to be physically connected. More than six hundred of these pairs are double suns, while again there are other combinations of three and perhaps more. When two are thus connected we have what are termed binary systems, and when more are associated they are called triple and multiple stars. An example of the last-mentioned class is the small star above mentioned near Vega. It is ε Lyræ, and is a double of a double. In ordinary telescopes this will not be perceived, but with a high power the combination will be noticed. The same phenomenon is observable in one of the stars of Hercules and in Andromeda.
The revolution of these double suns, or binary systems, has been closely observed, and Professor Newcomb has given us a list of the binary systems of short period which are well determined. These are as follows:—
42 Coronæ 26 years. ζ Herculis 35 years. Struve 3,121 40 years. η Coronæ 40 years. Sirius 50 years. ξ Cancri 58 years. ξ Ursæ Majoris 63 years. η Coronæ Borealis 67 years. α Centauri 77 years. μ Ophiuchi 92 years. λ Ophiuchi 96 years. ξ Scorpii 98 years.
It must be borne in mind that although these double stars appear close together from our standpoint, they may be far apart—one behind the other in a straight line. When such “pairs” exist they are known as optical pairs, or optically double stars, as distinguished from the actually physical “pairs” which revolve round the centre of a system. In Orion there has been discovered a regular system, and the θ in Orion, which appears in a common telescope a moderate star, and to the unaided eye only a speck of light, is really composed of seven stars—four are set in the form of a trapezium, as figured in the diagram in the margin by dots and asterisks. Two of these have been ascertained to possess attendants indicated by dots, and a seventh star was discovered by Lassell, and Humboldt remarks that in all probability this apparently tiny star in the constellation Orion constitutes a real system, for the five smaller stars have the same proper motion as the principal one.
Thus our imagination almost fails to grasp the infinity of the systems with our single sun, and with the distant double and even triple suns round which planets revolve perfectly independent of the other systems, as we are independent of them possessing heat and light from their own sun or suns as we receive it from ours, day and night seasons succeeding each other, and the wondrous varieties of the light produced by the appearance or withdrawal of a sun or two in the firmament of those most distant planets. These suns being double or triple would affect each other; the composition of the light given forth would produce—as we may assume—varying effects. We know something about the light of the stars by the spectroscope, and the colours of stars are due to the vapour which takes away a certain part of the light emitted, leaving the remainder to descend through the atmosphere to us.
Binary stars are most numerous of the doubles; for instance Castor, η Coronæ, Rigel, Polaris, Mivac, γ Leonis, γ Virginis, ξ Ursæ Majoris, α Hercules, 36 Andromedæ, λ Ophiuchi, and π Aquilæ. The illustration in the margin is Castor (or α Geminorum), the most northerly of the Twins. The η Coronæ is also figured, as are Polaris (see fig. 610 ante.), Boötes, Rigel, and γ Leonis.
The cuts herewith illustrate the relative positions at the periods named of the “doubles,” and of the revolution of suns around other suns as mentioned. As a consequence of their proper motion the binary stars appear to vary in their distances from each other, as in the topmost of the three cuts on the (opposite) page representing γ Virginis. The stars have gradually approached each other, and so are the stars in Castor approximating, and when they have closed, and have appeared almost as a single star, as they will do, they will take open order again.
The shortest time occupied by a double star in its revolution is thirty-five years, and we have already given some of those which have been ascertained. We will close this section with a few other examples. For instance, γ Virginis revolves in one hundred and fifty years, Castor in two hundred and forty years, 4 Aquarii in three hundred years, 37 Pegasi in five hundred years. There are numerous other instances up to a period of three thousand years, and about eight hundred of these binary systems are known. We have mentioned that there are two or more suns in the multiple systems. These suns are the cause of the different colours of the stars.
COLOURS OF THE STARS.
The question of star-colour follows naturally the consideration of the multiple stars; for although single stars have been observed of a ruddy colour, there are no instances of a blue or green one unattended by a companion. This colouring has been attributed to the contrast between multiple stars, for the colours are frequently complementary; but investigation has shown that this cannot be the case. For instances have been known in which, when two are thus associated, and one is concealed from us, the other is just as bright, and retains its former colour.
Of course in cases in which colour is apparent to the unaided vision, only the brightest stars betray colour. Antares, Betelgeuse and Aldebaran are red (orange) colour. Sirius and Canopus are white. Arcturus and Capella are yellowish, so is Pollux. Vega is bluish-white. These appearances are, of course, much more marked when the stars are examined through the telescope, and telescopic stars—which are stars unobservable without a glass—are very much coloured, and the multiple stars give us blue, green, violet, and other tints, besides those already mentioned.
Again, these coloured stars do not always remain the same colour. Sirius was once red; Mars was at times white. Spectrum analysis shows that the colours of many are due to absorption by the vapours of some of the rays; and the existence of certain vapours may cut off some, and at other times other vapours may exist and cut off other rays, and so the colours may be changed. Struve gives the following list of binary complements of “multiple” stars:—
Pairs consisting of a blue and white principal star 53 “ “ and a light yellow principal star 52 “ “ a yellow or red principal star 52 “ “ a green principal star 16
We need scarcely pursue this question farther, though many ideas concerning the coloured stars will arise in every thoughtful reader’s mind. Supposing that every system has its sun or suns, can we fancy the effects of a green or blue or violet sunlight—a light unmixed? To employ the words of Sir John Herschel—“It may be more easily suggested in words than conceived in imagination what variety of illumination two suns, a red and a green, or a yellow and blue one, must afford to a planet circulating round either—or what charming contrasts and graceful vicissitudes a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon.”
LOST AND NEW STARS.
We may have perhaps read the “Lost Pleiad,” and wondered what has become of the star supposed to have dropped out of the cluster so well known in the constellation Taurus—the Pleiades. There are seven stars, of which six are visible to the average eye, and the ancients used to declare that one of the seven sisters (the daughters of Atlas and Pleione) hid herself because she had married a mortal, while all her sisters wedded gods. It is not improbable that one of the seven, formerly distinguishable with the unassisted eye, may have disappeared or been lost; but it is certain that strong eyesight can see more than seven now, and in the telescope there are about one hundred.
And it is a fact that some stars whose places have been carefully marked in the catalogue have subsequently disappeared. Many errors may have been made, and stars put down where no star existed, so a succeeding observer has not been able to find the star indicated. But, on the other hand, we may admit that stars have been lost to sight, and to compensate us for any such disappearances new stars are frequently observed, and these are very remarkable phenomena. About 121 B.C. Hipparchus perceived a new star, which was visible even in the daytime, and on subsequent occasions others came into existence—viz., in the years 945, 1264, and 1572. In the last-mentioned year Tycho Brahé suddenly perceived the new star, which was at first very brilliant. It grew fainter and fainter, after first gaining in intensity, and disappeared entirely in 1574; and at other times stars have been seen which remained only for a short time, and then disappeared.
The star discovered by Tycho Brahé was seen by him when walking across the fields one night, and he encountered peasants who were gazing at the new luminary. It was so bright that it threw a shadow from Brahé’s stick. The new arrival appeared in “Cassiopeia,” under the lady’s chair, forming, as pictured in the diagram, an irregular square. The strange star is the largest.
Some stars exhibit extraordinary fluctuations, and one discovered by Mr. Birmingham in 1866, decreased rapidly and sank away to about the tenth magnitude, and then got brighter, and again diminished in splendour. The “Eta” Argûs has also been subjected to many fluctuations likewise, and such alterations have gained for these luminaries the name of “Variable Stars.”
In the accompanying little chart there will be perceived two particular stars, named Algol, “the demon,” and Mira “the wonderful.” The latter is the most celebrated for its variable qualities, and its cycle of change occupies nearly one of our years. For a few days it appears very bright, and then fades away for about three months, to disappear for five months, and then it reappears again, increasing in brilliancy up to the second magnitude for another three months or so. Some people account for these phenomena by stating that the sides of the star being less luminous present the dark and light portions in rotation; but we can give no satisfactory explanation of the reason, unless it be caused by an aggregation of spots upon its surface, like sun-spots on our sun, or perhaps by eclipse.
STAR-CLUSTERS AND NEBULÆ.
NEBULÆ and STAR-CLUSTERS are numerous in the heavens. The most important are the Great Nebulæ in Orion and in Andromeda. But there are other very beautiful “patches” of luminous matter or cloud appearances composed of minute stars invisible to the naked eye. We annex specimens of the Nebulæ, one or two having been already inserted. There must be thousands of these star-clouds, and they have been classified by Sir John Herschel from Sir William’s discoveries as follows:—
(1) Clusters of stars, in which the stars are clearly distinguishable, divided again into regular and irregular clusters.
(2) Resolvable Nebulæ, which may be separated into distinct stars under powerful telescopes.
(3) Nebulæ, in which there is no appearance whatever of stars, divided into classes according to brightness, etc.
(4) Planetary Nebulæ.
(5) Stellar Nebulæ.
(6) Nebulous Stars.
We learn also from the foregoing authority that Nebulæ affect a certain district; that is, they have, as it were, a preference for it, and are not distributed in a random manner over the heavens, and are found in Leo, Leo Minor, Ursa Major, Canes Venatici, Coma, Böotes, and Virgo, and more sparingly in Aries, Taurus, Orion, Perseus, Draco, Hercules, Lyra, etc. Nebulæ are found associated with stars, as is the case with η Argus; these are called nebulous stars, and in the case of this particular star many very interesting investigations have been made. The Nebulæ are as equally variable as the stars they surround.
What is termed the Nebular Hypothesis was put forward by La Place, and by it he endeavoured to account for the regular development of the stellar system, which is supposed to have originated from an immense nebular cloud. This immense mass would rotate and contract, and the outer portions would separate and develop in rings like Saturn’s rings. Then the rings break into separate portions, and each portion condenses into a planet, or the small “bits” travel round the sun like asteroids, and in this manner various systems were formed. This theory was considered to be quite exploded when stars were discerned in nebulæ by the more recent telescopes; but then the spectroscope came to our aid, and it was discovered that there were some nebulæ which are simply masses of glowing gas or aggregations of stones which are dashing against each other in so forcible a manner as to produce heat and luminosity. Mr. Lockyer appears to favour the latter theory as to nebulæ.
Mr. Proctor, however, has put forward a hypothesis that the star or meteor showers are the original cause of the sidereal system, and this rain of meteors has fallen for all time, gradually consolidating into orbs. The fact that the constituents of sun, earth and planets, comets and meteors being fundamentally the same lends probability to this hypothesis, which is fully explained by the author.
THE MILKY WAY.
The Galaxy is familiar to all readers, and although visible all the year round, is perceived more plainly in August, September, and October, or at the beginning and ending of that period. This zone of stars was of course well known to the ancients, but it is to Galileo that we owe the first important information about the Galaxy; he decided that it was formed of stars. Sir John Herschel investigated the subject very closely, and to him much of the information concerning the Milky Way is due.
It is not very distinct in the north, but as it advances from Cepheus southwards by the Unicorn, it gets clearer, and opens out in Argo, and descends still south, becoming brighter near the Southern Cross. It then passes northward again, dividing into two branches, one of which dies out, and then over Sagittarius, and so on to Cygnus, then to Casseopeia and the starting-point. The number of stars in the Galaxy is about 18,000,000.
In this wonderful zone of stars the centre of our system, the sun, is placed. It was supposed to be divided as in the diagram above; the inner portion being the stars seen in their thickness, and the outer ring representing the stars viewed in the direction of the length and breadth. But afterwards, Herschel modified his opinions respecting the Milky Way, and since his death many astronomers—and Mr. Proctor more particularly—have devoted considerable time to an examination of this wonderful zone of stars; which, it must be remembered, is not a continuous stream; it is a series of luminous patches. On this point Professor Nichol says:—
“It is only to the most careless glance that the Milky Way appears a continuous zone. Let the naked eye rest thoughtfully on any part of it, and if circumstances are favourable, it will stand out rather as an accumulation of patches and streams of light in every conceivable variety of form and brightness; now side by side, now heaped on each other, again spanning across dark spaces ... and at other times darting off into the neighbouring skies in branches of capricious length and shape, which gradually thin away and disappear.”
The Milky Way has its greatest breadth in the “Swan,” and in the “Eagle” constellation it divides itself. In the “Southern Triangle” the zone is brightest, and in the “Southern Cross” the hole or space, termed by sailors the “Coal Sack,” is very distinct. It then contracts and expands, and there is in Argo another gap. Then it is lost for a space, then it branches out, and soon crosses the Equator, dilates, contracts, opens out again, and so returns to the “Swan” again.
Philosophers have frequently discoursed upon this phenomenon, but all statements must remain more or less speculative. From Kepler’s to the present time astronomers have been considering the Milky Way, and when the Nebular theory was given up, when the Galaxy was found to be composed of stars, there was, as we have noticed, the idea of the ring and the cloven disc. Mr. R. Proctor has likened the Galaxy to a coiled serpent, and considers the openings in the Milky Way as evidence that the stratum of stars is limited, and that here we can see beyond it. In fact, it would appear that it is a very complicated question; and as the zone itself is complicated “with outlying branches beyond the range of our most powerful telescopes,” so an actual knowledge of the Milky Way is beyond us at present. It is composed of most extraordinary aggregations of stars, which appear not only impossible to count, but each one to be independent of the other. Thus we must conclude our rapid survey of the Milky Way, and close with Mr. Proctor’s remark in his “Universe of Stars.” “The sidereal system,” he says, “is altogether more complicated, altogether more varied in structure than has hitherto been supposed. Within one and the same region co-exist stars of many orders of real magnitude, the greatest being thousands of times larger than the least. All the Nebulæ hitherto discovered, whether gaseous or stellar, irregular, planetary, ring-formed, or elliptic, exist within the limits of the sidereal system. They all form part and parcel of that wonderful system, whose nearer and brighter parts constitute the glories of our nocturnal heavens.”
And a little reflection will show how true this is. Not very long ago in the world’s life the solar system was supposed to consist of one sun with a few planets wandering around him. Then some more were found, and they were called “satellites.” For a long time man fancied he had reached the “ultima thule” of astronomy in these depths; but the whole idea was changed when it was discovered that beyond Mars there lie the asteroids and the host of bodies in this solar system which we cannot do more than allude to. Then when we consider that this “sun” of ours, which we think so enormous, and which keeps in subjection so many heavenly bodies, and illuminates them; when we reflect that there are in space, and visible, stars many times larger than our ruling star, each a sun, and that our sun would, if put where the great Sirius glows, be but a speck in the firmament, and his system invisible to our eyes, we may well wonder at the magnitude of the subject, and the Infinite Wisdom and Power “that telleth the number of the stars, and calleth them all by their names.”
HOW TO READ THE SKY.
A few particulars, to enable a reader to identify the most prominent stars, may be given as starting-points from which some few excursions into the spangled heavens may be attempted. But the suggestions must be considered with reference to the ever-varying directions of the supposed lines in consequence of the daily revolution of the sphere. We have illustrated this in the cut in the margin, wherein the Lesser Bear is shown as swinging round the Polar Star in different positions. Sometimes the lines of direction will be vertical, sometimes inclined, but all retaining their relative positions.
We have already learnt that the “pointers” of the Great Bear indicate the Polar Star in the Lesser Bear, and we can (roughly) estimate the distance between the pointers as 5°. This will give us the distance between the pointers and the Polar Star as 29°. By following an imaginary line through the two northern stars of the “Waggon” (the Bear) away from the “horses,” we shall find Capella about 50° away.
If we pass from the first star next the waggon of “Charles’s Wain” to the Pole Star, and past it, we shall arrive at an irregular W. This is Cassiopeia, about as far beyond Polaris as the Bear is below it. When the latter is low, the former is at the zenith, and so on.
A line drawn from the Pole Star through the end star of the Great Bear leads to Arcturus. A line taken from Arcturus for about an equal distance will, with the Pole Star, make a triangle with Vega. The Polar Star may be called the Apex.
Regulus may be found southwards by drawing a line through the two first stars of the square in the Bear (opposite the pointers). From Vega, almost opposite the Pole Star, and through it about twice as far away on the other side, is Sirius, a brilliant “sun.” Procyon will be found to the westward of Regulus about 30°. From Procyon to the Pole Star a line will pass through Pollux and Castor.
Another line from the pole star through the middle of the three “horses” in the “Wain” will reach Spica Virginis about 70° beyond. So we can describe a large triangle with Spica, Regulus, and Arcturus, at the angles. Regulus is the apex, Spica and Arcturus a short base line.
From the pole star through Capella, passing between Betelgeux and Bellatrix, we shall describe a line leading to the three stars of Orion’s belt. Between it and the Pleiades is Aldebaran.
There are many other stars which could be indicated; but on a fine evening, if the observer will mark them upon a piece of paper, placing the pole star in the centre, he will be able to add to his star map very rapidly.
In the foregoing chapters of Astronomy we have seen how the earth and other planets move around the sun; we have glanced at the “fixed” stars and their groups, termed the constellations, and have noted the planets and their characteristics, with many other interesting facts. There is yet a great deal to be learnt, and much study will be required with daily (nightly) observations before the young reader will obtain success as a student of astronomy; but there is no study so interesting. We have seen what a very small portion of the universe is occupied by our solar system, and what a speck our earth is on the plain of creation. We find ourselves on the border-land of the incomprehensible, and we are lost in speculations upon the unseen.
FOOTNOTES:
[27] Mr. Burnham has lately given a list of two hundred and fifty-one new double stars, and in the Astronomical Society’s proceedings there is a list of ten thousand.
XLII. NEW ASTRONOMICAL APPLIANCES
A CELESTIAL INDICATOR—ASTRONOMICAL OR COSMOGRAPHICAL CLOCK—A SIMPLE GLOBE—A SOLAR CHRONOMETER.
Having said something concerning astronomy, we will give a few instructions respecting the instruments not already described, and make some observations, supplementing our directions in the previous chapter, for many people will be glad to learn how to read the evening skies.
Here we have an apparatus which will prove useful to amateurs; it is a sort of celestial indicator by Mauperin, and will facilitate the finding of every star or constellation, when the apparatus has been made ready by pointing the rod, T, in the direction of the object it is desired to view. This rod is mounted upon a rod, S, and is movable upwards or downwards or sideways, and in the last-named movement it will carry with it an indicator, I, which slides over the chart or diagram of the heavens. The two arms of this indicator are always parallel to the plane of the rod, T, no matter in what position they may be on the chart or the inclination of the rod. The extremities of this rod are terminated by an eye-slit, and by a crescent respectively.
When the apparatus is set, all one has to do is to look through the eye aperture, O, and view the star which we have chosen in the centre of the crescent, C. This star will be found named in the space between the arms of the sliding indicator, I.
It is easy to perform the operation inversely—that is to say, to find in the sky, by means of the sighting-rod, T, the stars we have chosen on the chart between the prongs of the indicator. The chart represents exactly the heavens as we see them, and this new mode is opposed to the manner generally adopted with celestial charts, and is very important, for it obviates the necessity of holding the map above one’s head, its face downwards.
As we have already showed, it is not difficult to find the Polar Star and the Great Bear. The latter is readily recognised by its seven stars, and the Lesser Bear glides around the pole as shown in the diagram on the preceding page (fig. 629). Now let us see how the celestial indicator will work.
Let us take the apparatus into the open air, and place it upon its tripod stand. The upper portion will be found movable by loosening the screw, V. By another simple arrangement the table can be slanted, and by turning a screw we can entirely slope the side of the chart where midnight (minuit) is written. Being placed opposite the polar star we take the upper part of the chart by the button, G, and bring it before us by a horizontal rotative movement.
We now place the sliding indicator upon midday (midi), and keep it in that position while working the apparatus—that is, until we have caught sight of the polar star in the centre of the crescent, C, by means of the eye aperture, O. We have now obtained the meridian, and care should be taken to tighten the screw, V. Then the table is raised to its fixed place upon the support; it is regulated according to the latitude of the place, and the apparatus is then “oriented.”
The upper disc is an elliptical opening, or aperture, which contains for every moment the stars visible upon the horizon, and the circumference is furnished with a graduated scale of hours divided into five-minute divisions, and this is fixed upon the apparatus. The dotted line between the midday and midnight points gives the meridian.
The disc placed underneath is the celestial chart, on the circumference of which we shall find the days of the months. It can be moved around the rod, S, which represents the axis of the earth around which the heavens are supposed to revolve. When the stars have to be observed, the day of the month has to be brought to the time at which the observation is about to be made. We can easily read off the chart by looking through the eye-piece as already explained. Every five minutes it is necessary to move the chart one division, which indicates that five minutes have passed; (other stars are, of course, arriving). The apparatus can be packed away when done with, or the bearings taken, and then the trouble of getting it into position again need not be repeated.
A small lamp, L, throws its light upon the chart in such a way that the eyes of the observer are not incommoded, while the table is fully illuminated. It can be placed at L´ if necessary. The inclination varies according to the latitude of the place when the observations are made. There is an arrangement underneath which admits of this inclination according to longitude.
The apparatus can also be made available to ascertain what the aspect of the heavens will be upon any particular evening of the month. We have only to place the chart at the day and hour, and we shall then see upon it all the stars visible above the horizon. We can thus find out at what time the stars rise and set, and those which do not set—to find the hour at which they pass the meridian (the line drawn between midday and midnight upon the chart), and the time of their appearance on the horizon. When the sliding indicator, I, does not show a star that is discoverable in the sky, the observer may conclude that he is viewing a planet. This apparatus is well adapted for beginners in astronomy, as no deep preparatory study is necessary, and the tyro can read the sky as easily as he could read a book.
A COSMOGRAPHICAL CLOCK.
We have, in the foregoing chapters upon Astronomy, endeavoured to give the reader some idea respecting the inclination of the earth and its rotation, and writers have often endeavoured to devise an apparatus which shall show the position of the globe in space, its diurnal motion—even its inclination and the succession of seasons in its revolution round the sun. But such reproductions of simultaneous movements have hitherto been obtained only on a very large scale, which find their place well enough in the museum or lecture-room, but which it is quite impossible to utilize in our sitting-rooms, on our tables, or chimney-pieces. Besides, the usual apparatus employed is a very costly one, and only serves for occasional representation; it will not keep the facts constantly before the observer in the manner of a clock showing the time.
But for all who are interested in Astronomy, or in Cosmography, or even for a young person who desires merely to understand the reality of the earth’s motion and how our earth is placed in the universe—for any one who deems it of use that he or she should be able to see the signs and the seasons, and the days and years, and how the earth revolves, may obtain an astronomical or cosmographical clock, which will tell him or her how the “world wags”; a useful as well as an ornamental timepiece.
Now this is precisely the result which the talented inventor of the astronomical clock has arrived at. M. Mouret devoted a great portion of his life and all his available means to the realization of his great idea, and, sad to say, he died miserably in an attic the very day before his great and deserving effort brought him the reward for which he had so painfully striven and devoted himself to by a life of self-denial and labour.
M. Mouret communicated to his globe the astronomical movement, which our earth possesses, by the aid of clock-work, which conveys to it, second by second, at each stroke of the pendulum, the double movement of rotation and progression. The globe turns upon its axis in twenty-four hours, and thus one can perceive, without any mental effort, the rotation of our planet, and the portions of the globe which come under the influence of the sun in rotation, just as they do actually on the earth. Not the least interesting attribute of this ingenious arrangement is the fact that during breakfast or dinner one can see the displacement and revolution of the earth with reference to the sun to all people in the world. Here, on the meridian, all are at midday. There, on the left, near the circle which defines the limit between day and night, the sun is rising and day is beginning; opposite, on the right, the sun is setting and day is closing. Yonder is the Pacific Ocean in full daylight, while almost every continent is in darkness and the inhabitants wrapped in slumber. Now the Chinese are opening their eyes, and the Asiatic and European continents will soon be illuminated and awake. This is the movement of the world as it has ever been since time came into its calculations.
The ingenious inventor, who wished to make a clock of his apparatus, and not being able to change its place on the earth from day to day as the time changed, very cleverly reproduced the sun’s movement of declination by making it describe a double cone at the axis of the globe. At the equinoxes the poles are in a plane, and equal day and night are shown. At the winter solstice the north pole is inclined backwards at an angle 23° 28´, and our hemisphere is in the winter season. We have then only eight hours’ daylight and sixteen of darkness; six months later the pole is inclined towards the sun, and the southern pole is plunged in darkness. We have the long days and the southerners the long nights. An upright dial shows the time of the country in which the globe may happen to be, and one can ascertain at any moment what time it is anywhere else. A horizontal dial indicates the day of the month, and changes every day in a manner corresponding with the movement of the earth around the sun, reproduced by means of the arrangement with the double cone. The spectator is supposed to be turning his back to the sun.
We may add that these movements are all self-acting, and there is no need to interfere with the clock, which is wound up like ordinary timepieces. By an ingenious forethought the inventor provided that the sphere should be independent of the other movements, and it can be used for demonstration in the hands of the lecturer, and be explained with all its motions without in any way disarranging the clock-work. The globe must, of course, when replaced, be put exactly at the correct day and hour.
A SIMPLE TERRESTRIAL GLOBE.
A terrestrial globe without any mechanism, so long as its axis is parallel to that of the earth, exposed to the direct rays of the sun, represents our planet with its recurrence of day and night.
The figure (634) shows us a globe without any support. The axis is north and south, and makes, with the horizon, an angle equal to the latitude of Paris, if the support, A B, be horizontal. To make the axis of the globe parallel to the axis of our earth, the line, N S, must correspond to the meridian of the place; this can be done with the compass, for instance.
The solar rays always illuminate one-half of a sphere, no matter what its dimensions. If we look at the illustration we shall see that the line of separation between the light and dark portions of the globe corresponds with that in our earth. This globe, then, tells us the passage of light and darkness for the day, and even for the moment of the day, when it is turned as the earth moves. The place examined should be placed in the meridian of that place (Paris, for instance), and occupy the most elevated spot on the globe. The earth is then in just the same position, and daylight and darkness are shown exactly as they exist on the earth at the time.
If this globe be then observed for a few minutes, the sun will be seen rising and setting, as it were, in various places (we must remember we have concentrated the sun’s rays, not a lamp, upon the globe). The places on the right, if the observer be placed facing the sun, will come out of the shade, and those on the left will enter it. The former are then really enjoying the sunrise, and the others are actually witnessing his setting.
The globe represented, making the double revolution of our planet in the year, will reproduce all the actual phenomena of day and night as taking place on the earth itself if we stand at a little distance so as to observe it all at once.
Of course, the employment of this simple apparatus should not exclude more complicated ones, for the former can only be used on a fine bright day. But the advantage claimed for it is that in it we can imitate nature exactly. Illuminated, as it is, by the real sun, the portions of light and shade are indicated by the rays and not by a metallic circle.
In order that the line of demarcation may be exactly defined, it will be necessary that the sun’s rays be concentrated upon the globe, and that no lateral or vertical light be admitted. The curtains should therefore be so arranged, and the blinds pulled down to a certain point, and if the stand or support be painted black it will be found an advantage. If the globe be a small one, it will be sufficient to place the stand upon an ordinary table, without verifying the horizontal plane. With a large globe the arrangement must be very exact.
A SOLAR CHRONOMETER.
M. Flechet’s chronometer, of which we give an illustration, is a kind of equitorial reduced to its most simple form. It is possible to ascertain the exact time by it very easily. It consists of a disc, AB, divided into twenty-four hours and fractions of hours. This disc turns upon itself around an arc, CD, which has a direction parallel to the axis of the world, and can be moved on a joint, E, according to the latitude of the place; F is a lens which can be moved and presented to the sun at any time, forming the centre of a concave and exactly spherical plate represented at GH.
When the instrument is fixed so that the axis, CD, is parallel to the axis of the globe, the disc, AB, is turned so that the centre of the image of the sun, produced by the lens, shall fall at m. The real time is found by an examination of the position of the index, A, upon the hour graduations of the disc. A French writer, Ch. Delounay, has mentioned this instrument, and considers it easy of arrangement, exact in time, and very useful.
XLIII. PHYSICAL GEOGRAPHY.—I. GEOLOGY
GEOGRAPHY AND GEOLOGY—THE EARTH’S CRUST—ORIGIN OF THE EARTH—DENUDATION AND EXCAVATION BY WATER—ROCKS, GRAVEL, AND SAND—CLASSES OF ROCKS.
When we were at school, and learnt the various countries of Europe, we had maps showing us the several divisions of one realm from another; the mountains, lakes, and other prominent features of the continent were learned and repeated, but we, maybe, seldom, perhaps never, bestowed a thought upon the formations of the mountains, and the manner in which rivers ran down into and through lakes to the ocean. There were the mountains, there were the lakes and rivers, and capes, and headlands, and there they are still, to all intents and purposes, the same to see, to climb up, to sail down, as the case may be. But the map of Europe has undergone a visible change. Territory has changed hands. Germany has gripped France, England has got Cyprus, Turkey has been dismembered, and Austria is annexing territory. This study is called Geography,—POLITICAL GEOGRAPHY,—for it marks the political boundaries. The knowledge of the formation of hills, headlands, lakes, rivers, seas, their causes, constitution, and effects, how they rose, how they exist and wax or wane during the course of centuries is PHYSICAL GEOGRAPHY, which we propose to consider.
This tree of knowledge includes some very important branches, almost parent stems. As a magnificent oak spreads forth its brawny arms, with smaller branches and twigs, each of these great branches being as large as an ordinary tree, so Physical Geography includes other arms such as Geology, Meteorology, Botany, and Physiology—even Astronomy in its comprehensive embrace. We find it is a difficult task to separate these kindred sciences from the great tree. We may have therefore to refer to earth, air, and water, and their various forms in hills and mountains, wind, vapour, rain, glaciers, and sea. We must learn how this earth has been gradually cooled, and what the various stages of its growth have done. We must consider plant and animal life upon our planet, and how the atmosphere affects them. All this is Physical Geography, and its satellite sciences of Geology, Meteorology, “Climatology,” Botany, and Physiology.
“Everything must have a beginning,” and the earth must have had a beginning, although the actual manner of the physical creation of the planet is a disputed fact. We are not about to discuss the religious side of the question, although we should undoubtedly find that Biblical and Geologic teaching run side by side towards the same end, and the testimony of the earth and sky bears witness to the Divine hand that created the universe, which we can trace back to the dim and distant ages when “the earth was without form and void, and darkness was upon the face of the deep.”
With this brief preface, let us consider some of these aspects, and pick up interesting facts from the ground.
GEOLOGY.
In the chemical and mineral sections of this volume we have heard something concerning the formation of the globe and its composition, its clays, rocks, etc. With these internal arrangements Chemistry and Mineralogy have dealt. Geology tells us about the external surface of the earth, its stones and rocks, and how they were formed, and generally something about the conformation of the crust of the earth, and its history.
When we speak of “crust” of the earth, we do not simply mean the exterior layer of gravel, clay, or stone. The crust is a thick one, and our crust extends just so far as we can cut into it. The surface of the sea can scarcely be termed a crust, but we must penetrate that ever-moving liquid boundary, and touch upon (and examine) what lies down below far beyond the “full fathom five” of the lead line. In this study we must not forget our Book of Nature, which is always open and inviting us to read. We shall see how things are produced, and how our physical surroundings will continue to be produced until the age of miracles returns, and Providence sees fit to interfere with the otherwise immutable laws which He in His wisdom has laid down for the universe.
It will be of no use to go back into space and imagine the world a red-hot fragment of matter, whirling through the heaven around the sun which, as a larger aggregation of burning atoms, kept it, as it now keeps it, in its place. The earth was a globe of liquid fire, or in gaseous state, and the atoms gradually cooled on the surface; the fire is still under our feet. The outer part would by degrees lose all its heat, while the interior remained hot; the planet must then have been surrounded by a steamy atmosphere, and enveloped in vapours condensed from the air through which no light of the sun could by any appreciable degree penetrate.
We can give an example of this, and it will be seen how the surface of the earth gradually became formed from the vaporous condition. If any one will take the pains to evaporate any saline solution in a capsule till it is about to crystallise, and observe attentively the pellicle of salt as it forms on the surface; first a partial film will show itself in a few places, floating about and joining with others, then when nearly the whole surface is coated, it will break up in some places and sink into the liquid beneath, another pellicle will form and join with the remains of the first, and as this thickens it will push up ridges and inequalities of the surface from openings and fissures in which little jets of steam and fluid will escape; these little ridges are chains of mountains, the little jets of steam those volcanic eruptions which were at that period so frequent, the surface of the capsule is the surface of the earth, and the five minutes which the observer has contemplated it, a million years.
The next effect of the cooling of the earth would be the gradual condensation of the vapour of water with which it was surrounded; this falling upon the earth formed seas and oceans, leaving only the higher portions exposed above its level. The clearing up of the dense dark clouds for the first time let in upon the earth’s surface the glorious and vivifying rays of the sun, and this great effect possibly accords with the earliest record in the Bible of the acts of creation: “And God said, let there be light, and there was light.”
This clearing up of the vapours and the subsequent rain no doubt gave rise to terribly grand electrical phenomena—thunderings and lightnings. By degrees the waters got their own way, and then many changes took place, land and water fighting, as it were, for the mastery, as they are fighting to this day.
But perhaps some reader may not think that the land and water of our earth are thus engaged. A very few minutes’ reflection will suffice to confirm our assertion. Look at the lofty crags in the Alps, for instance; what has shattered those peaks, and sent the masses toppling down in stone avalanches to the lower slopes, and then into the valleys?—Water. Water has been in the crevices, and was frozen there; in freezing it expanded and loosened the crags, which, forced asunder, gave an opening to more snow and ice, and so this powerful leverage, aided by the wind and storm, is disintegrating our mountains.
It is the same by the seashore; the cliffs are wearing away, and the sea approaches; at other places the sea recedes from the land, as coral formation and embryo chalk cliffs are rising under the surface of the ocean. Lakes dry up, and the meadow or farm arises on the site, while other old spots are submerged. No rest, no change of idea, but ever changing in physical appearance, Nature goes on her wondrous way, working now as steadily, as harmoniously, and as surely as she did before time was, and as she will continue to do when time shall be no more!
In our investigations into Geology we cannot enter into many technical details. Our object in these pages is recreation; but we shall, even under these circumstances, find plenty to interest, and sufficient to lead any one who wishes to pursue the study for himself. We will endeavour so to put the features of the stones and rocks before him that they will be recognized by the passer by. We will try to show how the earth has been built up, and how the great and terrible changes through which our little globe has passed have been effected. In our own islands, Great Britain and Ireland, we shall find traces of all the materials likely to be useful to us in our quest, As has been well said, “Geology is the Physical Geography of the past.”
CONSTITUTION OF THE EARTH.
The descent from rocks to stones, from stones to gravel, from gravel to sand, is evident to everyone, so we need not insist upon the fact that sand is powdered rock, and that an aggregation of sand particles makes stones. We have heard in the Mineralogy section that there are certain “earths”—silica, alumina, lime, etc. Of these “earths,” the two former constitute the greater portion of the ROCKS. Lime, also, is very evident, and in limestones fossils or organic remains are abundant. Now we must entirely put away from our minds the old idea that the earth we live on was created at once, or as it appeared to the first human beings. Our planet was prepared for man by degrees during millions of years. We conclude that the earth was originally composed of certain elements, and we find the same elements in the sun. Therefore, supposing (as is supposed) that the earth came from the sun, we have all the material of the globe in a fused state. As the earth cooled, rocks were formed by pressure, and then water came, and now we can read “books in the running brooks, and sermons in stones” at our leisure.
Perhaps as someone reads this he may be walking by the seashore kicking the pebbles or seated upon the sands, the grains of which are so very tiny. He will probably find sand, shingle, and gravel within reach, and perhaps the curious-looking “pudding stone.” Now what can we learn from these stones or sand grains or that curious bit of conglomerate? Perhaps the reader may be at Ramsgate or Margate or another place where the “white cliffs of Albion” glisten in the sun. Take up a piece of chalk and examine it. It is soft and soils your hands, and you will throw it away, perhaps—but don’t. Take it home and put it under the microscope or a good magnifying glass. What do you see?
You will find the remains of animals—that is, shells and tiny bits of coral packed close together. Under the microscope they will become more separated, and the grains will be distinct fragments of shells, etc. If this little bit of chalk be composed of marine animals’ shells, of course the whole cliff is composed of the same kind of material. But how did the shells get into the chalk? Shells are chalk—carbonate of lime; lime was deposited at the bottom of the sea, and the infinite millions of minute animals formed themselves shells, and left them to be piled up by Nature’s forces into cliffs during countless ages.
Yes, but how did the lime get into the water to make the shells? We will endeavour to explain. Rain, when falling, takes some carbonic acid from the air, which we know contains it. This acts upon the lime in the rocks (lime is oxide of calcium, and calcium is an element in the earth), so we get a bi-carbonate of lime (soluble in water), which rises from the rocks in springs. These springs and their streams deposit lime, as we can see in caverns where we find stalactites and stalagmites. The lime is transmitted to the ocean, and absorbed by the crinoideans and molluscs which produced shells. These shells hardened and crystallized became limestone, and whole mountains are formed of this “organic” rock, which is used for so many purposes.
We have spoken of ORGANIC ROCKS, but there are others, and we ought, perhaps, to have spoken of that kind before the chalk put them aside. Let us go back to our sandy shore again and look at the SEDIMENTARY ROCKS, which are the very first formation. We have all seen sandstone, and visitors to the South Devon Coast will remember the red cliffs near Dawlish and Teignmouth. These are red sandstone—not the very “old red” so pleasantly written of by Hugh Miller, but at any rate sandstone, and composed of grains of sand. When we were at Dawlish last year a piece of the sandstone had fallen on to the beach, and when the waves came up that stone was no doubt gradually washed away into sand, and then fell to the earth as sediment.
We said something a few pages back about the wear and tear which is always going on: the mountain is worn away—a mass falls, it is broken into smaller pieces; these are carried by a river; the mud is deposited, and the finer particles are ground and rounded into gravel, and finally sand. Beneath the current of the river, and at the bottom of a lake or sea, these sediments (mud, etc.) accumulate one on the top of the other in regular series called strata, and then the weight and pressure acting with the soluble mineral deposits always washing down, consolidate and bind the loose sand-grains into stone, which, in the course of ages, hardens. The stones thus formed from sediments such as gravel, mud, and sand, are termed Sedimentary Rocks; they have become rocks by enormous and continuous pressure. Thus:—
Sands have become “Sandstones”; Gravel has become “Pudding-stone” (Conglomerate); Mud and clay have become “Shale”; Calcareous deposits have become “Limestone”; Vegetable deposits have become “Coal.”
So we have sandy, clayey, limy, flinty, and corally rocks under long names respectively—Arenarious, Argillaceous, Calcareous, Silicious; and we may add Bitumenous and Ferruginous—Irony Rocks—to the list.
Speaking of sediments, it is curious to note the different colours of the Arve and the Rhone which meet near Geneva. The white sedimentary Arve can be traced for a long distance beside, not mixing with, the blue Rhone. The same effect can be traced where the latter river enters the Lake of Geneva. So the land is being perpetually carried away and deposited; and where water gains on land there is somewhere else always a corresponding elevation to compensate it. Thus places disappear, and the sea washes over the site, as on the Kentish Coast, where Earl Godwin’s land was inundated, and new land is reclaimed or is elevated from the sea to make up the balance.
“There rolls the deep, where grew the tree; Oh, earth, what changes hast thou seen! There, where the long street roars, has been The stillness of the central sea.”
We have spoken of sedimentary and organic rocks. There is yet another kind called igneous, or fiery rocks—those upraised by volcanic action. Of the igneous rocks the crystalline have been evidently in a fused condition. Granite is an example; lava or basalt is the usual term for volcanic rock, and the basaltic caves of Staffa and the Giants’ Causeway bear testimony to the igneous or volcanic origin of the surroundings. The pillars and fantastic rocks of Ireland and Scotland which are so remarkable, are simply lava, which was erupted in a molten state, now cooled and contracted into blocks of curious regularity of form.
Granite, already referred to, is another igneous rock, and must have been forced upwards; for as an igneous rock granite has cooled beneath the crust of the earth, throwing out arms, while melting, into other formations, and frequently being found in mountains. There is another kind of igneous rocks formed by the continuous accumulation of the ashes, etc., vomited forth from volcanoes. Masses of mountain are thus produced in the course of years, and the material thus formed is called tufa, or tuff, when consolidated; and this (now solidified) is what caused the destruction of Pompeii and Herculaneum.
So we have two classes of IGNEOUS ROCKS, the Crystalline and Fragmental (or the “Plutonic” and “Volcanic”), including basalts, pitchstones, pumice, trachytes, granite, syenite, etc.; and, on the other hand, tuff and “volcanic” breccia, with felstones, porphyries, etc., which have been classed as intermediary.
Of these three classes of rocks, the sedimentary and the organic compose the greater portion of the earth. We will now glance at the crust of the earth and its various formations.
Chapter XLIV
CRUST OF THE EARTH—GEOLOGICAL SYSTEMS—EOZOIC, PRIMARY, SECONDARY, TERTIARY, PREHISTORIC FORMATIONS.
The crust of Great Britain has been carefully examined, and from the results of investigations at various periods, the earth has been divided into a series of strata which follow the same order of succession. Sometimes certain strata may not be present, and they may be replaced by others, but the same order of succession will be found. The order is as follows, commencing at the lowest. The illustration is taken in the opposite direction:—
{ Laurentian } Until recently believed to “Eozoic.” { Cambrian } be without traces of living { } creatures; hence “Eozoic.”
Palæzoic, { Silurian } or { Old Red Sandstone } Shell-fish, seaweed, Primary. { Carboniferous } ferns, fish, low reptiles. { Permian }
Mesozoic, { Triassic (Upper } or { Red Sandstone) } Secondary. { Oolitic } Birds, marsupials, reptiles. { Cretaceous }
Kainzoic, { Eocene } Superior life. Mammals, or { Miocene } with great vegetable life, on Tertiary. { Pliocene } to plants and animals now { Post-tertiary } existing. Man.
Quaternary. Recent—Prehistoric
THE PALÆZOIC SYSTEMS.
LAURENTIAN SYSTEM. It will be perceived from the above list that the Laurentian Rocks are the oldest. The name is derived from the St. Lawrence formations, and was given to the strata by Sir William Logan. They are metamorphosed rocks older than the Cambrian. These rocks are sedimentary, of very old deposition, and of a crystalline nature, consisting of quartz, gneiss, etc. The granite was probably formed by the fusion of its component constituents, quartz, mica, and felspar, which become crystallized by the excessive heat. For a long time no traces of organisms could be detected in this or the Cambrian systems, but modern research has been rewarded with a little success. The original deposits of micaceous gneiss, etc., have been altered, and many true igneous rocks, such as syenite and granite, are found in them. These very old rocks must have been originally deposited in strata converted by heat and pressure into crystalline rocks. These rocks have been divided into two series, under the names of lower and upper Laurentian. They are metamorphic, and consist “mainly of gneiss interstratified with mica-schist, with great beds of quartz, and massive beds of crystalline limestone, of which one varies from 700 to 1,500 feet in thickness. Conglomerates also occur, and there are vast deposits of magnetic and specular iron. Graphite, or blacklead, is disseminated in strings, veins, and beds through hundreds of feet of the lower Laurentian, and its amount is calculated by Dr. Dawson to be equal in quantity to the coal seams of an equal area of the carboniferous rocks” (Nicholson).
Hitherto, no distinctly recognisable fossil has been discovered, with the important exception of the Eozoön Canadense, which has been pronounced to have been a gigantic foraminifer, growing layer upon layer, and thus forming reefs of limestone; the subject, however, is still a matter of dispute. The eozoön was discovered by Mr. J. M’Mullen in 1858 in Canada.
Granite was at one time considered to be the true primitive rock. Gneiss is a word of Saxon origin, and consists of the same materials as granite in different proportions. Mica-schist is made up of two of the same constituents as the granite and gneiss. They are without fossil traces.
The CAMBRIAN system of aqueous origin may be said to contain evidence of the dawn of organic life. It is part of the clay-slate system, and the term “Cambrian” is taken from the ancient name of Wales, where slate is plentiful. Mica-slate is also very important. These Cambrian rocks are of the next oldest formation to the Laurentian, and all the various deposits may be examined in Wales, where also traces of volcanic and ice action may readily be perceived. In the pass of Llanberis one immense ice-borne block is very prominent; no agency but ice could have put it there as it rests. It is estimated that the Cambrian and “Lower” Silurian rocks are from 20,000 to 30,000 feet in thickness, and must embrace a very lengthened period. The fossils of these formations show that zoophytes and certain primitive crustacea lived in the remote ages when these rocks were formed by sedimentary deposition. We have scarce a trace of plant-life. The organic remains include annular worms, the first arrangement of the articulated animals according to Cuvier. The flora and fauna are, of course, very low in the scale of creation, when land and sea were so differently arranged.
The SILURIAN system was so named by Sir. R. Murchison, after the territory formerly occupied by the Silures, but the system is, of course, universal. We have here sandstones, limestones, and shales, deposits lying upon the Welsh slate. The Upper and Lower Ludlow beds, and the “May Hill” sandstone, then the Lower Silurian, with Caradoc beds, and the Tremadoc slate, etc. In this system volcanic action is observable, and all the organic remains are those of marine animals, such as corals, shell-fish, marine worms, encrinites, molluscs, and other zoophytes in great variety. We find also a number of graptolites, trilobites, echinus (sea-urchin), terebratula, and many other forms.
The Trilobites were amongst the first creatures inhabiting our globe, and it is a curious fact to contemplate, that their eyes (fig. 655) should have been preserved perfect; they present one of those wonderful objects which carry one’s thoughts backwards to the early ages of the world, probably many millions of years, and yet it is found by the peculiar structure of the eyes of these Trilobites that they were placed at the bottom of the sea with perfect power to look upwards at the light of the sun through the transparent waters. The same hand and the same power had then Divine care and solicitude for the well-being of His creatures, as great as He has for those of later ages, and these animals are mentioned in Genesis—“Let the waters bring forth abundantly the moving creature that hath life.”
Thus we see that ages of comparative quiet succeeded the first great contraction of the earth’s crust, probably millions of years, during which time the tides and currents of the ocean had to wash and wear down all the thousands of projecting rocks or inequalities, and dissolve (as before described) all the lime, depositing the sand and clay in those immense strata which form the old-named “transition series”; this appears to have taken place over nearly the whole world at that time, and ages upon ages must have elapsed to form such deposits as the sandstone, claystone, and limestone, in alternation, forming the “Llandilo,” “Caradoc,” and “Wenlock” strata, more than a mile in thickness; these are by some geologists reckoned among the primary series (by some called the “transition rocks”), and in England form the “Cambrian” and “Silurian” systems which are so rich in minerals and metallic veins.
The OLD RED SANDSTONE, or DEVONIAN System, is the next of the series of layers which built up our earth, but a great gap of years separates it from the Silurian. We find the “Old Red” in the Mendips and in Scotland. The rocks in the West of England are apparently of later deposition and of marine origin, while the “Old Red” is apparently a fresh-water deposit. It is very “arenaceous,” and owes its tint chiefly to iron, although there are circumstances in which it appears neither as a sandstone nor with a red colour; but red sandstone describes the true formation very accurately.
In this water-deposited system, whether in lakes or by the sea, we find a considerable advance upon the Silurian. We have flora in more variety—seaweeds and ferns. The remains are all aquatic. We have nothing higher in the scale of creation than the fish, the first vertebrates; and judging by varieties a very considerable time must have elapsed during which the Old Red Sandstone was deposited. The dipterus—or double-winged—is herewith shown as an example of the fossil fish of the Old Red Sandstone period.
It is curious that no remains of any land-inhabiting animals have been discovered in this system—whether in the Old Red or the Devonian formations (the Lower and Upper Red Sandstones). We can only distinguish the remains of aquatic animals or plants. We may picture the great cuttle-fish, the nautilus, and the dipterus, with various orders of mollusca, and the gradual approach to the crustacea, but no terrestrial animals have been discovered. We may take it for granted then that the Old Red Sandstone and Devonian systems are different—the former being found in Scotland and parts of England, and formed of deposits in fresh or brackish water, while in the Devonian system the marine deposits are corals, and all the indications of ocean life, separated from the great inland lakes by a range of hills. Neither of the terms (Old Red Sandstone or Devonian) limit geographically or descriptively the formations of this system. All the rocks are clearly distributed between the Silurian and the Carboniferous. The invertebrates in this last system have not developed very much, but corals are very abundant, and fish of some armoured species are plentiful and curious, while the crustacea were enormous.
We now arrive at the most important of all the rock formations, the one to which we owe our national prosperity—we mean the COAL System.
THE CARBONIFEROUS FORMATION.
While the foregoing depositions were being made the earth was still undergoing changes. The sandstones were deposited, and the corals making use of the lime carried into the waters began to build and form masses of limestone under the sea, pushing back the water and changing the forms and positions of land and water. All this went on apparently very quietly—volcanic action was not very frequent—the water was warm. But sometimes earthquakes would heave up the submarine formations into mountains, and therefore we find the fossils of the tiny sea-animals on the hills. Extensive swamps were formed by partially retreating sea-water, and their vegetation became luxuriant. Tree ferns and all the floral appearance of the tropics grew up and formed dense forests, far thicker than any we know of at the present time. It will readily be understood that the condition of the atmosphere must have been particularly favourable to the growth of plants, and therefore not suitable for air-breathing animals. Heat and carbonic acid must have been greatly developed.
We can now perceive how the gradual filling up of the earth for man’s reception was taking place. The rain was taking carbonic acid from the air, for the dead plants gave it out in enormous volumes. The carbonate of lime dissolved in the water-springs, etc., was carried to the sea for polypi to build shells from. The trees were absorbing carbonic acid, too, and while purifying the air, were retaining the carbon in their stems and leaves and branches, which (when they decayed) remained untouched, and accumulated in thick layers to sink down, and by pressure be turned into coal. This great effect was carried out several times; and it is a remarkable fact that we find coal, limestone, and ironstone so near together, all useful to us and to each other in the course of the working of the minerals—so we come to the Carboniferous system.
Coal we have already treated of in Mineralogy, and coal looks at times very different from our preconceived ideas of a sedimentary rock, which we know is regularly deposited in layers. But when we split or break the coal we find its cleavage in a certain direction. Coal is wood squeezed and petrified by ages between enormous layers of sedimentary rocks, and the coal-seam rests upon the soil in which the plants once grew—perhaps more than six hundred thousand years ago!
Of course coal, as we burn it, was not all made at once. We can trace it from the swamp as Peat, on to “Lignite,” or woody coal, through the Tertiary and New Red Sandstone to the coal measures themselves. Even lower down we find the remains in more or less pure carbon forms—the anthracite and the graphite of the primary formations.
Coal appears not to have been formed equally in all places during the period in which it originated. The remains of plants found in these strata lead us to infer, that, during that period there existed an exceedingly vigorous and crowded vegetation, consisting principally of tree ferns and equisetaceæ, of which the Sphenopteris Hœninghausii (fig. 662), Pecopteris aquilina (fig. 663), and Neuropteris Loshii (fig. 664), are amongst the most beautiful that have been found, and the flora and fauna of this period were of a more or less primitive kind or low order, but very luxurious. The former display a decided advance, and reptiles of aquatic forms appear with large and predaceous fishes. Mountain limestone, which is usually found in the coal formations, includes metallic deposits, and organic remains are very abundant in it. The following are specimens of the fossils—
The Carboniferous system is a very important one, as may be seen. In these beds we have coal, limestone, sandstone, and shale. The Coal Measures consist of grit and sandstone and shale, with coal seams. The Carboniferous Limestone, or “Mountain” Limestone, has no coal in it. The sandstone has been termed “Millstone Grit,” because millstones are made from it. We then have limestone shales, and the sandstone beds. Each coal seam indicates a subsidence of the land and a regular series of underclay or soil in which the plants grew,—the plants themselves,—iron, coal, and then the shale, and so on again, indicating frequent changes and a long lapse of ages.
These “Coal Measures” occupy an area of five hundred square miles in Great Britain alone; and as we have already said, the period which elapsed while these deposits were being laid down represents hundreds of thousands of years. The deposit would increase at the rate of about three feet in a thousand years. And this is only one period of the many changes to which our world has been subjected since it first began its revolution in space around the sun.
Coal is usually found in “basins” or depressions—a sort of trough owing to the upheaval of surrounding strata which became in time denuded (or washed away) with any coal that was there. So it is in places where it is concealed by overlying beds that protect it, that we now find the coal saved from disturbance. When we search and come upon red sandstone and grey-wacke, we may be almost certain that we are near coal, particularly if the surrounding rocks form a “basin.”
We have now briefly sketched the Carboniferous system, for in our recreations we can do little more. We shall find in many places in England tree trunks in the sandy shore, and ample evidence that a forest has been at one time submerged in that spot. So inland the land sank down again and again in successive periods—water, mud, soil, vegetable growth succeeded, to be again submerged and form a new coal seam for the use of man, who was destined to appear after the lapse of ages.
THE PERMIAN PERIOD.
[Illustration: Fig. 669.—Impressions of feet of Cheirotherium].
At length the land remained undisturbed. It sank no more, and the trees waved luxuriously over the buried forests of past ages, and another cycle set in called the PERMIAN, from the ancient kingdom of Permia, where all the features of that period are exhibited on a very grand scale, and which extended for several hundred miles between the Ural mountains and the Volga. In the Permian period we have the progression of animal life more distinctly developed than in the Carboniferous system, which immediately preceded it. It is true there are indications in the latter that curious animals, called the Cheirotherium and the Labyrinthodon, were alive then, and the remains of numerous insects, such as beetles and crickets, have been found; but the Permian developed them, and reptiles, saurians, and lizards have been traced; but as Sir R. Murchison states, throughout the whole extent the animals are of a single type. We have the hand-like impressions of the feet of the Cheirotherium, so called from the Greek “cheir,” a hand. The soil appears to have been very soft, and peculiarly adapted to receive impressions, and which having been in many places covered over with a stratum of fine sand, and then abandoned by the sea, the whole has hardened into stone, and being now separated, the one contains their footprints, and the other perfect casts of them! Nor are these footmarks all that these sandstones have to tell us of their day; for the ripples of the waves, and even the little pits made by drops of rain as they fell, are in this most marvellous manner preserved, forming objects of wonder and admiration.
The organic remains during this period are not very abundant, and many of the fauna of the previous systems appear to have died out, while others appeared to meet with fuller development in succeeding ages. The Permian is also known as the New Red Sandstone, or Magnesian Limestone group. “Dias” has also been suggested with reference to the “Trias” group, “the Upper New Red Sandstone,” which comes next. The Permian rocks are very varied, and contain minerals, such as copper and sulphur.
As the strata below the new sandstone formation was called the “Carboniferous” system, from its containing much carbon in the form both of coal and carbonic acid, so this has been called the “Saliferous” system, from the occurrence in many places of strata of “rock-salt,” or crystallised chloride of sodium, and (where the rain finds its way down and dissolves it) of brine springs; these (in England) exist chiefly in Cheshire and Warwickshire, but in Poland and Hungary they exist on a much larger scale, the rock-salt being nearly a thousand feet thick. It has been said that these strata of salt were formed by the evaporation of salt lakes, but it is much more probable that salt is one of the natural materials of the earth, and that both salt lakes and oceans have become salt from dissolving out these strata wherever they have come into contact.
It is supposed that during the Permian period the greater portion of the continent of Europe was raised above the ocean, and the deposits were formed in salt lakes, for the appearance of the organic remains tends to establish the fact that the creatures of that period were not far from dry land even in their watery existence, and the reptiles found confirm this view. We have now to examine the Mesozoic, or Secondary System.
Chapter XLV
THE MESOZOIC SYSTEM—THE TRIASSIC, OOLITIC, AND CRETACEOUS FORMATIONS—THE EOCENE, MIOCENE, AND PLIOCENE—THE GLACIAL PERIOD—PRE-HISTORIC MAN.
We trust that the general reader has gleaned from the foregoing chapter some few ideas concerning the growth of plant and animal life in the early periods of the world’s existence. From the Laurentian System we have briefly traced the conformation of the globe at the dawn of organic life through the Silurian Old Red Sandstone and Carboniferous formations, indicating as we proceeded the chief points in the world’s history, and the gradual development of life through many ages. There is no real or bold line of demarcation drawn between these systems. As seam unites to seam, and layer to layer, stratum upon stratum, so the systems almost insensibly unite, and forms of life appear, mature, and die away as the babe grows into the man, and dies away again to old age and final extinction. So one system merges into another, each and all a factor in the great work which was intended to prepare the earth for the greatest and latest development of Nature—MAN!
But all this while the earth had been, as it still is, undergoing continual change. Sometimes gradually, in the wearing away, or elevation of beach or headland; sometimes suddenly, as when mighty hills were upheaved and the deeply-laid granite or limestone was lifted to the summits of the mountains from the depths of the sea. Land and water came and went, and the ever-changing earth still brought forth abundantly “the herb yielding seed after its kind,” and the “moving things upon the ground after their kind,” ever improving and developing till they culminated in the splendid vegetation and immense animals of the Tertiary period, and lay silent afterwards in the cold grasp of the great ice age for the thousands of years of the glacial epoch.
We now enter upon the TRIAS, or New (Upper) Red Sandstone, which is divided into Upper and Lower Trias, “Keuper” and “Bunter.” We have three principal headings in the Secondary System—the TRIASSIC (the oldest), the OOLITIC, and the CRETACEOUS. In the first we find red sandstones and shelly limestone; in the second, clays and shale; in the last, chalk, or white limestone. In some districts there are traces of volcanic action.
On the top of the “Upper Trias,” or “Keuper” formation, we have the Lias, which succeeds the Rhætic beds, and in this we find many rich traces of reptiles and birds which come now before us in the rising scale of creation. In the seas of this period we have numerous crustacea, the nautilus and the cuttle-fish. The Saurians now come before our retrospective vision. It is the “Reptile Age” in all its development, and the huge labyrinthodon, the iguanodon, pterodactyle, and ichthyosaurus testify to the magnitude of the fauna of the period. The first mammal specimen, a marsupial, has been traced back to this time; and the tropical temperature was favourable for luxuriant vegetation, pines, and palms.
In the swamps or shallow waters the great reptiles disported themselves, and seized their prey, the water-fowl, which now appeared in numbers, and of enormous size. Nor were insects absent. Numbers of remains have been discovered; beetles, dragon-flies, grasshoppers, etc., in multitudes yield us information, while the marine fossils, star-fish, mollusca, and various fishes, are of frequent occurrence. Animal and vegetable life during this period must have been very rich and varied—literally leaving “footprints in the sands of time.”
The “Blue Lias” is a term familiar to every reader. It is a kind of limestone mixed with clay, of a blue colour, and upon this we find the Oolitic, or Oolite System—so called because it somewhat resembles the roe of a fish. The Lias clays are used for bricks, and Whitby “jet” is also obtained from the Upper Lias. Jet is really a lignite, or wood in the process of transmutation. In this Lias formation, besides the numerous fossil remains already mentioned, we find the “snakestones” (ammonites), the stone-lily, and belemnites, with many nautili and shells.
The OOLITE, or JURASSIC, underlies the chalk, and overlies the Trias formation. The term “Jurassic” originates from the Jura range, which is almost entirely composed of Oolitic strata. These strata are greatly distorted by pressure, and when we reach Switzerland and the familiar Alps, we find gneiss, crystalline, limestones, and schists, into which the Oolite has been metamorphosed. The Oolite is divided into Upper, Middle, and Lower, consisting of the following:—
{ Pembroke Beds. Upper Oolite { Portland “ { Kimmeridge Clay.
{ Calcareous Grit. Middle Oolite { Coral Rag. { Oxford Clay.
{ “Cornbrash”—Forest Marble. { Stansfield Slate. Lower Oolite { Bath Oolite. { Fuller’s Earth. { Inferior Oolite.
The Oolite formation (see Map) occupies a stretch of country in England extending from Yorkshire into Dorset. The Great Oolite holds the Fuller’s earth, and the Bath stone is also well known. The Stonesfield slate holds many remains of reptiles. It is a kind of shelly limestone, and is used for roofing purposes. The Forest Marble (so called from Wychwood Forest) is a sandy limestone holding marine fossils. It is used for ornaments. The Coral Rag and Oxford clay are rich in fossils, and the former, as its name implies, is composed of ancient coral reef. The Portland beds produce the well-known building stone. The Purbecks, of which there are three divisions, appear to have been deposited in fresh water, and occur in Dorsetshire.
All the Oolite strata supply organic remains. We have plants and ferns, reptiles, and a number of new genera of conchifera and cephalopods, star-fish, urchins, and the enormous bats, and the terrible megalosaurus, and the cetiosaurus, steneosaurus, and pliosaurus, of enormous size. One very remarkable bird has been found in the Bavarian limestone of this period; it is called the Archæopteryx, which is described as having a leg-bone and foot like the familiar birds, but the tail is lizard-like, with feathers springing from each joint. Sponges, corals, and fish, and many other forms of animal life are found in the Oolites. The reptiles must have had it all their own way in this period, for there were both carnivorous and vegetable feeders, and teeth of the pliosaurus have been found which measure fifteen inches, the jaws being six feet long. We have seen that corals must have built up their reefs in the waters, which then overlaid the land we call the United Kingdom.
There must have been great changes during this period, and the strata are chiefly marine. The Wealden formations are the exceptions, and in the fresh-water deposits insect forms abound. The appearance and variety of animal and vegetable life must have been curious and interesting.
The Weald or “Wold” of Kent is often spoken of, and it extends with the Surrey and Sussex Wealden formations for some distance. The strata are of fresh-water deposition, differing in this from the chalk, although the Wealden beds are included in the Cretaceous Group, which is composed as follows:—
{ Wealden. Lower { Greensand. { “Gault.”
{ Upper Greensand. { Chalk Marl. Upper { Chalk (without Flint). { Chalk (with Flint). { Maestricht.
The “Wealden” formation is divided into Hastings sand and Weald clay. The former consists of clay and sandy beds, and is observable at Hastings, and in the neighbourhood of Tunbridge. The Weald clay consists of blue and brown clays, with sandstone, and the limestone known as “Sussex Marble,” which is formed by the paludina of the rivers. There is another division often seen in Dorsetshire, and called the Punfield beds, which partake both of marine and fresh-water remains, which are distinct in the true Wealden and cretaceous formations, the former being of fresh, and the latter of salt-water origin.
The remains of enormous reptiles are numerous in the Wealden formations; crocodiles, lizards, turtles of gigantic size have been discovered, and most curious fossils have been disinterred in the Hastings district. The “Greensands” are separated by what is termed gault, a stiff blue clay found in Norfolk, Essex, and Kent. The Lower Greensand includes the well-known Kentish rag, or limestone, of which so many churches are built. The Upper Greensand is supposed to be a seashore deposit on the sides of an extensive ocean or sea, at the bottom of which the chalk was formed. After the Wealden beds were formed, they were covered by these greensand estuary-beds, or littoral strata. In these series new forms of life appeared, and the waters became the receptacle of myriads of mollusca, etc., which in time formed the great chalk cliffs and downs so often referred to. The chalk is interstratified with sand, which as “gault” and “greensand” was probably the sand of the ocean bed before the chalk was formed upon it, and the seas must have supported many marine reptiles, for stony “nodules,” or coprolites, which are the fossil excreta of the animals, are found, and now used for manure, after being buried for thousands of years. Examination of these remains has resulted in the discovery of the teeth and bones of fish which had been devoured by the gigantic reptiles. An illustration of a shell thus discovered is annexed.
We have in a former chapter spoken of the chalk and its formation. We know that it is composed of the minute foraminifera. The fossil remains are very numerous in chalk and all of a marine kind, such as the ammonites, belemnites, and such cephalopods, and the echinus, bivalve mollusca, crustacea, etc. We have occasionally flints appearing in the chalk, and this circumstance has given rise to some speculation as to how the flints got there, for they consist of nearly pure silica; and the theory of the petrifaction of sponges, madrepores, etc., has been started to account for their presence. Dr. Carpenter says: “It may be stated, as a fact beyond all question, that nodular flint and other analogous concretions (such as agates) may generally be considered as fossilised sponges or alcyonian zoophytes, since not only are their external forms and their superficial markings often highly characteristic of those organisms, but when sections of them are made sufficiently thin to be transparent, a spongy texture may be most distinctly recognised in their interior.”
It is now generally admitted that the decaying animal matter acts upon the silicious spiculæ of sponges, etc., and the silica is thus deposited.
We may then surmise that at some very distant period the whole extent of the British Isles was submerged, as well as portions of the continent, and after the strata had been deposited the sea and land were disturbed by volcanic action. While the secondary strata were being deposited, very little relative alteration took place, as the deposits are seen to lie “conformably.” But when the great convulsion which upheaved the Apennines occurred, the chalk was raised as we find it in the cliffs and downs, which were the beds of seas. This is the last of the great convulsions which the earth has undergone, for the tertiary strata, which afterwards began to be deposited, rest in the hollows or basins (chiefly in the chalk) then left; the alterations in and since these deposits appear to consist chiefly of the upheaval of certain localities, the depression of others, the evaporation of inland lakes, and the wear and tear of the land from these causes, which are still in continuous action (as from the washing down of cliffs by the sea, and the formation of mud deposits at the mouths of rivers), or the volcanic agencies which in some places (as in Ireland) have cast up basalt over the chalk.
There is a sort of transition formation which is classed with the Cretaceous System, and termed “Maestricht,” after the town in Belgium. It appears that this is an upper chalk layer, an intermediary between the Secondary and Tertiary, and here on the banks of the Meuse we find the Mosasaurus, the “lizard of the Meuse,” of whose remains we give specimens in the illustration. This transition chalk—as we may call it to distinguish it—must have been laid down at a later period than the flinty chalk, and we find it in many places. It serves therefore as a fitting introduction to the Tertiary Period of Geological time.
THE TERTIARY PERIOD.
We now enter upon a period when the animal creation attained its greatest development, the “Age of Mammals”; for they were then the kings of creation. The Tertiary Period is divided into three stages, viz.—
The EOCENE, or the Dawning of the now existing creation.
The MIOCENE, or the Middle, or “minority” of existing creation.
The PLIOCENE, or the Recent, or still more developed period.
We will glance at them in that order, which Sir C. Lyell introduced.
The Eocene formation is shown in what is termed the “London Basin,” here illustrated by a section in which we find soft sands without fossils (Thanet Beds), and a kindred kind in Surrey, in which fossils (marine) are found. After these we get the “Reading and Woolwich” beds as we ascend. These are of clay and pebbles, etc., with river fossils. The Oldhaven beds are included on the map; they occur towards Blackheath and Herne Bay. The London clay is very stiff, and in some places blue. It is full of fossils of birds, beasts, fruits, and vegetables, trees, reptiles, and fish, and the variety of the organic remains appears to indicate the fact that at one time the Thames flowed through swampy ground to the sea, in which dwelt, in a warm climate, immense mammalia, such as the megatherium, glyptodon, tapir, etc., and some turtles of enormous size.
It is also on record from late observations that these immense animals were even mixed up, and almost fabulous creatures inhabited the land where England now is. We read of antelope-horses, lion-like bears, and camel-stags. The vegetation was then of a tropical kind, and in the deep forests and jungles these enormous animals—the mammoth dinotherium, and such species—roamed and plunged in the swamps at the mouth of the Thames. At length these types died away, and gave place to the elephant and the hippopotamus, and the climate by degrees became less warm, and still slowly decreased in temperature.
A glance at Sir C. Lyell’s “Principles of Geology” will show us how, as we examine the more modern strata, we find a great increase in the European lands, which may have been compensated by the submersion of the Pacific islands. During the period of the vegetation of the Secondary epochs, our climate (between the lias and the chalk) was favourable to a tropical growth. Enormous rivers flowed through our islands, and gigantic crocodiles, etc., with flying reptiles, were masters of the land. There were numerous fishes, but the reptiles did not appear in such very great numbers.
These large and elephantine animals must have existed while the climate of Northern Europe underwent some very considerable changes. We read of the woolly rhinoceros, and the hairy elephant, or mastodon, which has been found in Siberia. Reindeer appeared in England, and we know now that these animals inhabit cold countries. The mountains were considerably elevated during the latter Tertiary period; snow fell and ice formed upon the summits of the mountains, while glaciers crept down the sides. The warm, almost tropical climate of the prior ages was gradually but surely giving way to the Ice Age; the earth was slowly dipping, and the sun’s rays had less power.
Professor Ramsay says the “assemblage of fossils found in the London clay point to the fact that the whole of these strata were deposited in the estuary of a great continental river comparable to the Amazon and the Ganges. The palm-nuts and the host of other plants help to prove it, and the remains of river tortoises, crocodiles, snakes, marsupials, and several tapir-like mammals, all point in the same direction. The estuarine conditions begun during the deposit of the Woolwich and Reading beds were still going on when the London clay was thrown down; with this difference, that by sinking of the area the estuary had become longer, wider, and deeper, but still remained connected with a vast continent, through which the Eocene river flowed.”
The Miocene deposits are not so generally important in the United Kingdom, but in America very valuable fossils have been discovered in these strata. The Pliocene strata extend along the east of Great Britain, where they are denominated “Crag,” as Norfolk Crag, Red Crag, Coralline Crag. Underneath these mammalian remains have been discovered. After the Pliocene we come to the Post-Pliocene, which really closes the long Tertiary period. During these ages the gradual development of created beings apparently reached its height. It was towards the end of the Middle Eocene that the great mountain chain of Europe came into existence, which is connected, as any casual observer may see, with the Himalaya. In fact, the whole chain, from the Thibetian range through India, the Caucasus, Alps, and Pyrenees, is continuous, and formed of the same material (“nummulitic limestone”). There is no doubt that the whole northern hemisphere enjoyed at the commencement of the Tertiary period a warm, not to say tropical, climate, which got colder and colder.
We find the increase of animals and plants more fitted to the requirements of man and our present climate. There are many signs of the successive increase of land in Europe generally, while the contrast the Tertiary period bears to the Secondary is very marked. In the former we have extensive deposits in the waters of wide, open seas; in the latter the depositions were evidently made where dry land, with its accompanying bays and lakes, were extensive and numerous. The former is marine, the latter lacustrine and marine. The seas of the Tertiary period have lately been defined.
Sir Charles Lyell, in his “Principles of Geology,” shows us this, and defines the European features at the commencement of the Tertiary epoch. At that time, the British islands, with the exception of the basins of London, the Isle of Wight, and Norfolk, had wholly emerged from the deep. But a third part of France was still under water. Italy consisted only of a long and narrow ridgy peninsula, branching off from the Alps near Savona. Turkey and Greece, south of the Danube, were laid dry; and a tract of land extended from the Vosges, through central Germany, Bohemia, and the north of Hungary, perhaps to the Balkan. But the whole of the north of Europe and Asia, from Holland eastward to central Tartary, and from Saxony and the Carpathians northward to Sweden, Lapland, and the Ural chain, lay beneath the ocean. The same subterranean movements, which have subsequently raised the wide plains of our northern continents above the sea-level, have given great additional elevation to the then existing land. Thus the Alps have certainly acquired an increased height of from two thousand to four thousand feet since the commencement of the Tertiary period. The Pyrenees, whose highest ridge consists of marine calcareous beds, of the age of our chalk and greensand series, while the Tertiary strata at their foot are horizontal, and reach only the height of a few hundred feet above the sea, seem to have been entirely upheaved in the comparatively brief interval between the deposition of the chalk and these Tertiary strata. The Jura, also, owe a great part of their present elevation to convulsions which happened after the deposition of the Tertiary groups. On the other hand, it is possible that some mountain-chains may have been lowered by subsidence, as well as by meteoric degradation, during the same series of ages in this quarter of the globe; and on some points shallows may have been depressed into deep abysses. But, on the whole, everything tends to show that the great predominance of land which now distinguishes the northern hemisphere has been brought about only at a recent period, and Sir Charles holds that the shifting of the continents is sufficient to account for the variations of climate. We have every reason to believe that before the Glacial epoch England and the Continent were united, and during the Glacial period England and North America were joined, viâ Greenland and Ireland. Mr. Dawkins says that England at that time was six hundred feet above its present level. If so—and we cannot question his conclusions—the Channel was then dry.
The “Great Ice Age” then came upon the world. For the information of readers who wish to peruse the whole history of this epoch and its causes, we may add that in Professor Geikie’s most interesting work, they will find full details. We can only refer to it.
The gradual decrease of temperature upon the earth, which was the cause of the Glacial period extending over the north of Europe, has been attributed to the eccentricity of the earth’s orbit; and here astronomy steps in to our assistance. We have read in the chapters on Astronomy, how the movement of the earth, like a top near the end of its “spin,” causes the “precession of the equinoxes,” and in connection with this phenomenon the earth’s orbit becomes more and more circular at certain periods of thousands of years, and goes away from the sun. We therefore receive the light and heat at a greater angle. Consequently, less heat is received, and ice is formed, as at the North and South Poles at present.
Doctor Croll has pointed out that the great eccentricity of the earth’s orbit existed about 210,000 years ago, when there was a difference between the nearest and farthest position of the earth and the sun of 12,000,000 of miles at least.[28] This is a very considerable distance even in the enormous spaces which intervene between us and the other planets of the solar system, and about that time the Glacial period arrived. Perhaps we may make this clearer by going back to the precession of the equinoxes.
The earth moves in an orbit called an ellipse, and the sun is not in the centre of this nearly circular path. We can now understand that the earth comes nearer to the sun sometimes and recedes at others. These points of nearest approach and greatest distance are termed perihelion and aphelion. In the latter case we are about 9,000,000 of miles farther from the sun than when in perihelion—that is, when the greatest “eccentricity” is reached. In addition to this the axis of the earth is continually changing in direction by reason of solar attraction at the equator. This shifting, as explained in the astronomical section, is very slight every year, and in the course of 24,000 years the conditions of the seasons will have completely changed round and back again,—for the northern and southern conditions will be reversed in our hemisphere. Day and night come twenty minutes earlier every year. We are now nearer the sun in winter as shown in diagram (page 497); when we change we shall be nearest the sun in summer and farthest in winter.
Doctor Croll, who has done much in his most interesting paper on changes of climate[29], tells us how this eccentricity of the earth’s orbit produced indirectly the Glacial epoch. He shows how, if in a period of the greatest “eccentricity” our winter came in aphelion, we should receive one-fifth less heat than now, but a correspondingly greater heat in summer. But if our winter under such circumstances fall (as now) in perihelion, the difference between winter and summer would be practically nil, because the sun during a period of the earth’s great eccentricity “could not warm the hemisphere whose summer happened to arrive in perihelion.” No doubt the sun’s rays would be very powerful, but the earth being covered with ice and snow could not be warmed; fogs would accrue and hide the sun, as at present in Antarctic summers, when the cold is very great. The warm ocean currents would be stopped, and the northern portion of our hemisphere would be, as it undoubtedly was, frozen over and covered with snow.
When we consider the millions of years since the earth is supposed to have been launched into space, we can imagine that the Glacial periods would occur frequently, and considering the very slow “precession” movement there, and the alternating tropical climate with graduations of temperature for thousands of years they would last long. The great Glacial period is computed to have begun 240,000 years ago and lasted 160,000 years with alternations of comparative summer; and so the years went on, season succeeding season, altering the appearance of the earth, and causing successive changes in the distribution of animal and vegetable life. Then the great mammalia, the mammoth and hippopotamus, with the hyæna, lion, and other felidæ came, and went when Arctic animals usurped their places. At the later Glacial epoch man must have arrived in Britain, and “this being so,” says Professor Geikie, “it is startling to recall in imagination those grand geological revolutions of which he must have been a witness.... He entered Britain at a time when our country was joined to Europe across the bed of the German Ocean; at a time when the winters were still severe enough to freeze over the rivers in the south of England; at a time when glaciers nestled in our upland and mountain valleys, and the Arctic mammalia occupied the land. He lived here long enough to witness a complete change of climate, to see the Arctic mammalia vanish from England, and the hippopotamus and its congeners take their places. At a later date, and while a mild and genial climate still continued, he beheld the sea slowly gain upon the land, until, little by little, step by step, a large portion of our country was submerged—a submergence which, as we know, reached in Wales to the extent of 1,300 feet or thereabouts.”
We find that the land underwent many subsequent changes; it rose from the sea, was again covered with ice, and many parts of Europe were devastated by immense glaciers—that of the Rhone extending for more than two hundred miles. Then came vegetation as the ice gave way, and luxuriance of the tropics reigned; more cold after that, then more heat, till the ice was finally driven to its mountain fastnesses, and “Britain for the last time became continental. Neolithic man came upon the scene; his palæolithic predecessor had, as far as Britain and northern Europe are concerned, vanished for ever.” The inquiry respecting the arrival and presence of man in Britain would lead us too far in pursuit. The fact has been established that man was living in the Thames valley while tropical animals were in the country, and he has been classed by Professor Boyd-Dawkins amongst the mid-pleistocene mammalia, and at that distant period, man as man, and not as an intermediate form connecting the human race with the lower animals, was present in Europe.
The stone implements which have been found in river beds and in caverns, associated with the bones of various animals, such as the elephant, rhinoceros, hyæna, bear, and others prove this. These very ancient and rudely-fashioned implements have been divided into two classes, the Palæolithic and the Neolithic, by Sir John Lubbock. First the stone implements were used, and stone was superseded by bronze and iron. Then we come to the historic period. In the neolithic period we find stone implements in the lake dwellings of Switzerland and Constance (as well as the Lake of Neuchatel), all of which have lately developed many treasures. Bronze tools have also been found, and so the gradual progress of man as a fashioner of weapons can be traced from age to age.
From the “river-drift” man we descend to the cave-man, who is supposed to have been identical with the Esquimaux. When Britain became an island the cave-man seems to have disappeared from our country, and in the prehistoric age the earliest of the present inhabitants came here, and brought with them domestic animals; then the Celts of the bronze age, and then the iron. The wild beasts gradually disappeared, and domestic ones occupied their places under civilized conditions.[30]
So we come from the “Glacial period” to the open door of history through the antechamber of the prehistoric time.
The prehistoric is the arbitrary division between the post-pliocene or pleistocene and the known “historic” periods of the world’s history, and we must dismiss it with a few general remarks, for the changes which we have attempted to follow are still taking place in the earth; volcanoes and earthquakes are unsettling the strata, and adding to the physical and geographical record which will some day have to be written by posterity and future geologists. We can see in those prehistoric times traces of men (hunters and fishers) existing with difficulty, mayhap, in the midst of enormous quadrupeds, and fighting for existence with the bears and many other formidable foes. We have noticed the stone ages, the rough and the smooth as they may be called, and we can picture the primitive agriculture and work of the neolithic man. But it is by no means to be believed that neolithic man in Britain was a race all over the world. We may assume that in eastern climes the human race were in a more civilized condition as improvements made their way slowly westward. Our island history commences in the time of Julius Cæsar. Eastern chronicles go back many thousands of years farther.
It is so short a time, geologically speaking, since man appeared within the limits of history, that the earth’s changes, except from direct volcanic action or water erosion, are very trifling. The change is, as we said, continually proceeding; ceaselessly the earth is wearing away, and depositing her riches where she is undisturbed by civilization and man’s excavations and intrusions. The rock is worn by water; the grit is carried down and deposited to form sedimentary rocks as of old; the lime will continue to assist the coral to be built up; and the chalk cliffs will be born under the sea, and our organic remains shall be found to tell remote ages that we were an enlightened people. For all we can tell, and it is by no means unlikely another recurring cycle of Arctic and Tropical periods will in time pass over our earth; the bear and reindeer, the hippopotamus and the rhinoceros, may again inhabit our islands. If our generation be destroyed, the purely animal creation with the vegetable world will reign over the land, and new forests will deposit new coal measures for the support and comfort of a new generation of highly organized beings, when our remains shall have passed away to the borders of a “prehistoric” age.
We have seen in the foregoing brief sketch how the world has arrived at its present beautiful condition,—how it has been step by step prepared for us, how nature’s forces have been and are still working according to the immutable laws of the Universe. And, after all, how little we know! What scraps of intelligence only are we able to gather up from the boundless quantity of material which must have been laid down, yet what wondrous results scientists have been able to adduce from even these comparatively scanty specimens! The sea and land are ever telling us the same old story. Man’s research and Bible teaching are found hand in hand in cordial and reverent agreement. Nothing is altered since the day that the Divine command, “Let there be light,” went forth into space, and till the earth be destroyed the same forces will continue in operation, guided by the Hand that made it—“ever faithful, ever sure.”
FOOTNOTES:
[28] It might reach 14,000,000 of miles at a maximum.
[29] “Physical Causes of Change of Climate,” Phil. Mag., 1864.
[30] See Dawkins’ “Early Man in Britain.”
XLVI. PHYSICAL GEOGRAPHY
IGNEOUS ROCKS—LAND AND WATER—SPRINGS, WELLS, AND GEYSERS—SNOW AND ICE—THEIR EFFECTS.
In the foregoing pages we have chiefly considered the stratified rocks, but we are now approaching another branch of our subject—viz., “Physiography,” which, as distinguished from the usual so-called Physical Geography, will deal with the phenomena of the earth, air, and water, thus leading us to Meteorology as a conclusion.
We have arrived at a certain knowledge concerning the Earth as a planet, her place in the universe, and the composition of the “Crust,” as it is termed. We have examined the stratified rocks, which include sand and gravel, stones, and boulders equally. To a geologist they are all “rocks.” We must now examine the igneous rocks, which bear an important part in the structure of the Earth, whose surface we have now more minutely to examine. It has already been stated (p. 571) that igneous rocks have been upheaved while in a state of fusion—that is, while in a melted condition. These igneous, or fire-produced rocks, are divided into classes, just as the unstratified rocks are, and the divisions are called the VOLCANIC and PLUTONIC, including “Basic” and “Acidic,” according as they are possessed of less silica or more.
Sometimes the igneous rocks are classed as volcanic, trappean (from trappa, a stair, such as in the Giant’s Causeway), and granitic. The volcanic in such case being the modern or upper rocks, such as lava, scoria, etc., which, having been cast up by volcanoes, are of comparatively recent formation.
The VOLCANIC rocks, then, are of recent date, comparatively speaking; they form the constituent portions of the volcanoes of the present day, and are found as basaltic formations. They are traced as far back as the Tertiary period of the globe. Amongst the volcanic rocks we find basalt, augite, porphyry, serpentine, pumice, pitchstone, felspar, etc. But no doubt volcanic action has been going on ever since the beginning of the world as it is now, and will continue to do. It is somewhat curious that the very old igneous rocks should not be more evident.
The PLUTONIC rocks do not differ essentially from the foregoing. There is less quartz and more hornblende; and if the ages during which these formations have been existent in the earth-depths after they became solidified be considered, the differences will be fully accounted for. The greenstones and syenites are prominent amongst the plutonic series.
These plutonic and volcanic rocks are separated into basic and acidic, as already remarked, but the line cannot be drawn very distinctly. Granite is the chief plutonic (acidic) rock, and we frequently find it forced upwards into other strata, its essentially eruptive character being thus decided. That granite must have taken an immense time to solidify and crystallize is evident, for no new granites are ever found. We find granite in all the old mountain chains—such as the Grampians in Scotland, and the Wicklow mountains. Our chief European (active) volcanoes are, so to speak, modern, as may be supposed when their constituents are known. It may be said that granite was first deposited as sediment heated by subterranean fire, and forced up by thermal action of water to the mountains, where it is uncovered by a slow process of denudation and surface washings of the earth.
Now without at present going any farther into the causes of volcanoes we can see at a glance that the eruption of the igneous rocks must have created a marked and essential difference in the physical geography of the globe. It is to these eruptions that the dislocation and disturbance of the stratified formations are due. The igneous rocks present ridges in the mountains; sometimes they are rounded at the summits, while the aqueous and metamorphic rocks are disposed in layers.
These two classes in their varieties form the land and the crust of the earth, which is ever being acted upon by air and water. The ice, again, polishes and scratches the valleys in which it moves. The loosened boulders that tumble from the mountains are carried down by the ice, and deposited in the glacier moraine, whence flows a stream. By degrees the stone is ground up, and carried away in the water to form sediment in a “delta” at the embouchure, or to lie beneath the surface and form rock once more. The igneous rocks, composed of lavas and ashes, are volcanic rocks, deposited deep down, and then after the lapse of ages disclosed by the action of air and water.
The consideration of the land and water upon the globe shows us that they are distributed over the earth very unequally. There is nearly three times as much water in our planet as there is land, and these proportions could not be altered without giving rise to phenomena, the results of which cannot be properly estimated. Our earth has an area of 197,000,000 of square miles; about 52,000,000 of this is land, and about 145,000,000 of it water; so about three-quarters of the globe is made up of water. The first portion of our subject therefore should be directed to the examination and consideration of water, and the phenomena which arise from its presence upon the earth.
We need not go into details which every geography indicates. We will try to trace the sources, not the plain effects, which all can afterwards study from special books. In a preceding portion of this volume we have explained the chemical composition of water, and we showed by experiment that it is a fluid composed of oxygen and hydrogen gases, in the proportions of one to two volumes respectively. No matter in what form water may appear,—as water, as ice, or as steam,—these proportions never vary in pure water (see p. 352). But water on the earth is seldom, or never, pure. We know the difficulty we have to procure good drinking water, and though it may be filtered, there will remain natural salts, which are found in different degrees in all water upon the globe. We know the rain, which is perfectly pure when condensed from the clouds, absorbs carbonic acid, etc., from the atmosphere. We have shown how this water as soon as it comes upon the earth attacks the rocks, and as it progresses carries away lime. After descending deep down, it rises again in the form of SPRINGS.
Now what are these springs? They are the result of percolation of rain-water through certain strata. When water falls it is absorbed into the ground, unless it happens to rest upon an impermeable rock, in which case it becomes a rivulet. But it can penetrate between the atoms of many rocks, and thus falls through sand and harder rocks, till it reaches a stratum which will not receive it—like clay. We then find that it will flow away in a spring, or if tapped will be an Artesian well. These water-wells are of very ancient date, but the name is more modern.[31] The springs flow out, and develop, with the assistance of tributaries, into rivers. These again receive more tributaries, which swell the volume of their waters, and widen out, carrying millions of gallons hourly to the sea with sediment and gravel and stone.
Water has enormous power of disintegration. We have only to cast our eyes upon the illustrations in any volume of continental travel in Europe or America to perceive the gorges and cañons worn out by the resistless and frequently gently-flowing river to estimate the part which water plays in Physical Geography and Meteorology.
But springs occur not only in the case mentioned; there are mineral springs, hot springs, and oil springs, all following the same rules of nature. The Artesian well has been mentioned. The Geysers of Iceland have often been portrayed, and are amongst the most wonderful phenomena of nature. These will serve as a type of the other thermal springs, of which the districts of the Yellowstone in North America afford perhaps the most extraordinary instances. These are intermittent springs, and the water rises to a great height, at intervals of about an hour and a half; and after many successive attempts, or trials, as it were, the geyser shoots up to a great height enveloped in steam.
The cause of these well-known phenomena have been explained by Bunsen, and it has already been referred to. We know that at a certain air-pressure water boils at 212° (Fahr.), but on mountains at less pressure it will boil before that degree, because the air is rarefied. So conversely, under the ground, it may reach 212° without boiling. So the surface (warm) water falls, and reaches a high temperature before it is converted into steam. When it is so converted, the vapour is formed very rapidly, and the expansive force is tremendous, shooting up the water and all the contents of the tube with terrific violence, and with a beautiful effect. Pressure therefore alters the boiling point of water.
The mineral springs of Bath and many continental towns owe their properties to the solvent power of water, which assimilates the mineral atoms and gases. They arise just in the same way as the ordinary spring, the taste and smell depending upon the soil and strata. Perhaps the oil wells are the most curious phenomena of this kind. They are excavated upon the Artesian principle. The petroleum is bored for, as we bore for water, and the oil rushes up with great force, and in enormous quantities. Gas wells are also to be found in Pennsylvania, and have supplied towns with gas for years. Both these Artesian wells are caused by the decay of vegetation. The gas is in the coal formation, and the oil has been pressed out from vegetable deposit, and as anthracite is a stony coal, petroleum is a kind of coal-tar, of natural formation.
We have alluded to the river, which emerges from the spring, which has fallen as rain. But there is another, and, to many minds, a much more interesting form of the universal fluid we call water. This is ice. Familiar as ice is, either to the stay-at-home invalid, the skater, and the traveller, there is a great deal to be said about it. It is a subject we would dwell upon had we space, for the remembrance of many a pleasant hour passed upon snow and glacier call upon us to go back again, even though only in imagination. No one who has not climbed the glacier—even the Mer de Glace to the Jardin, now such a common excursion—can fail to be struck with the beauty and grandeur of the scene presented to him, and to carry away a fond recollection of the icy regions he penetrated.
For the ordinary hard-working man there is no change, no rest so truly beneficial as a trip amongst the mountains and snowfields of Europe. He need not be a climber; that is, a climber like Tyndall or Whymper, those giants of the Alpine Club. But a stroll up to the Bel Alp, the Æggishhorn, the Riffel, the Montanvert, or the Grimsel, will give the average pedestrian some of the finest glacier scenery in Europe, and which may, we believe, compare with any in the world for beauty. These glaciers—ice-rivers—we will now consider briefly. We may take the Mer de Glace as an example (see the illustration, p. 596). That gives us a very fair idea of the ice-river, but the cut below is a good specimen of a glacier.
Suppose we start up from Chamouni, or come across from Argentières, we shall reach the Montanvert by ascending through the wood, or by the “Chapeau,” across the ice-sea. As we take the former course, we walk alongside a white-flowing and rapid river, the Arve, which unites with the Rhone below Geneva. This river divides, and if we keep alongside one (the right or south branch), we shall reach the moraine and the icy grotto, from which the water issues. It is in this way many large rivers are born. The Rhine, the Rhone, the Aar, the Ticino, have all of them their sources in the ice. The Visp and the Sass waters are other almost equally well-known examples.
There used to be a grotto or cavern, into which the tourist could enter at the source of the Arveiron, and here the beautiful blue of the ice could be studied. From this place the Chapeau is reached, up a stony path amid the trees, and from the top outside the hut we can see the Mer de Glace all broken and contorted. The frequently occurring roar of a falling rock which heat has deprived of its icy support, or the cracking and tumbling of ice-blocks, may be seen and heard in the forenoon. But the grandeur and majesty of the ice and snow-clad mountains is best enjoyed by moonlight.
On the ice we shall see huge stones and gravel and grit, which have been carried down by the ever-moving glacier, which is denuded in its course, and worn down upon the surface as it slides, scraping and grinding the valley through which it flows. By passing along a path now made easy by irons, but formerly without supports or guards, the surface of the glacier will be reached, and a man with a hatchet will cut steps for the timid traveller. We are now upon the deep ice-river, which has its springs in the snowy regions of the Col de Géant, in the snow which is continually falling upon the heights, and draining away to water again to form a river.
Thus the circle of events is completed,—snow, névé, ice (glacier), water, which last is again absorbed into the atmosphere, and again descends as rain or snow. And this is always going on by the action of the sun. It may here fairly be asked how snow becomes ice. Why does not the snow turn into ice at once, and form a glacier at the top of the mountain as well as at the bottom? We will endeavour to make this clear. Snow is composed of crystals, which assume certain definite forms, and when first the flakes fall they are soft and powdery. By degrees they melt a little, and when unconsolidated form what is termed névé, the border line between ice and snow. This semi-icy snow descends under pressure, and, as it increases, the glacier is formed by huge blocks and masses being pressed together on the steep slopes of the mountains. Thus the glacier descends, rounding off rocks, and scouring as it goes, moving at a certain estimated rate daily,—about twenty inches on the average,—carrying stones and débris which form the moraine, and finally when the high temperature in the valley melts the ice, it issues forth as a river into the plain, or bounds down the mountain side in a cascade. An excursion—and one by no means dangerous if a guide be taken—to the Jardin, near Chamouni, will reveal many interesting features of glacier formation, and of the glaciers themselves.
Physical Geography is therefore very much indebted to the action of water as a fluid or as a solid. In the former condition it erodes the rocks, carries down the stones and gravel and sand, forms deltas at the mouths of rivers, and elevates plains by overflowing its banks and depositing sediment. Water gives beautiful scenery, and the ever-changing features of the landscape are due to it. From the time the spring emerges to the time when it has developed into a river, bearing fine ships upon its restless waters, the universal fluid is always at its work of destruction and benefit combined. From the limpid stream we pass to the salt ocean, the reservoir of all the waters of the globe.
We have in this chapter briefly considered two very important forces which have much to do with the varying conformation of the earth—viz., fire and water in their results of volcanic action and erosion. The sea will tell us something more.
FOOTNOTES:
[31] From Artois, where the first European well of this kind existed.
XLVII. THE SEA AND THE SKY
THE SEA—SALT WATER—WAVES AND THEIR EFFECTS—UNDER WATER—THE FLOOR OF THE OCEAN.
From our childhood the sea has been the companion and playmate of thousands, the seashore their playground. Men have selected it for their professional training and livelihood. Authors write of it, poets apostrophize, scientists lecture upon it, and fathom it, bringing up from its depths many a new fact and illustration for those who cannot study it for themselves. There is nothing like it, nothing more majestic, more beautiful, more life-giving than the ocean—nothing so changeable nor so true.
From the days when we could toddle along the beach, picking up the shells, we have wondered at the ocean—What was beyond it? What did it conceal?
“What hidest thou in thy treasure-caves and cells, Thou hollow-sounding and mysterious main?”
Let us endeavour to find out.
The first thing that strikes us is the saltness of the sea. Sea water is salt. Why? One reason is because salts are carried into it by rivers, and besides, it is more beneficial as salt water. But let us look at the facts. We know that the earth contains many “salts,” as we can see by the saline springs. We have already given the chemical constitution of sea water, but it will be useful to repeat the proportions.
Water 964·74372 grains. Salt (Chloride of Sodium) 28·05948 ” Chloride of Potassium 0·76552 ” Chloride of Magnesium 3·66658 ” Bromide of Magnesium 0·02929 ” Sulphate of Magnesia 2·29578 ” Sulphate of Lime 0·40662 ” Carbonate of Lime (with traces of Iodine and Ammonia) 0·03301 ” ---------- 1000·00000 ” ----------
Some portions of the sea are not so salt as others, or, in other words, not so dense, and the saltness of the water prevents it being frozen so quickly as fresh water, which freezes at 32°. Salt water requires to be reduced to 28° before it freezes. Besides the various constituents mentioned above, sea water has been found to contain boron, bromine, strontia, etc., and even silver, for the copper of ships has been found to be impregnated with that metal.
If there is so much salt in the sea, it may be asked, why does it not continually become greatly saltier by additions. The reason is because tons of fresh water are continually pouring in, and though we can scarcely doubt that the sea is becoming gradually more salt as years pass away, the increase is very slight. On the other hand, evaporation is carrying water into the air and leaving the salt behind it. In seas like the Red Sea, where there is a great deal of evaporation and very little addition of fresh water in comparison, the water is extremely salt and bitter. The Baltic has little salt relatively to some parts of the Mediterranean.
Supposing that, as some allege, there are rocks of salt at the bottom of the sea, we must remember that springs of fresh water frequently bubble up to the surface of the ocean. This is a very curious phenomenon, and has been attested by Humboldt. He states that near Cuba these springs arise with considerable force, and the vessels trading on that coast get supplies of fresh water from these ocean springs. There is, or was, a similar uprising in the Gulf of Spezzia, and fresh water crustacea inhabit these localities. These occurrences prevent the sea from becoming too salt by evaporation. When salt water becomes tainted it is very offensive—much more so than fresh water. If, therefore, the ocean were not continually in movement, it would be very injurious.
So much for the water of the sea; let us now see what it does. We will glance at the surface ere we plunge into the depths.
In childhood, and even in after years, we most of us delight in watching the waves of the sea. What finer sight than that we can obtain on the bold Cornish coast with a westerly wind, when the great Atlantic waves come rolling in and dashing up to the tops of the Tintagel cliffs, wearing and grinding them away; hissing up the sands at New Quay, or thundering on the shores of “Bude and Boss”! Then the wind abates, the sea goes down, the billows become waves, the waves to wavelets grow, less and less, until there is a mere ripple on the surface which is never still. The mighty heaving of the ocean breast is the peculiarity of the sea.
Yet, again, as we stand to watch the waves, or run from them as they sweep in foam upon the sloping sand, we shall find that they increase or decrease in force, and the level of the water rises or sinks by degrees. The tide is flowing or ebbing as the case may be. So we know the surface has another—a current motion—besides the undulation of the water. The currents of the ocean are very valuable attributes, the Gulf Stream in particular bringing us warmth and, indeed, rain. There are three movements of the ocean—waves, currents, and tides.
The waves, perhaps, interest us most, as they come rolling in with irregular force, but all mightily impelled by the wind. We have all noticed the ripples on a puddle; the same action of the wind produces the grandeur of the waves of the ocean. The wave comes rolling in before the wind to break against the rocks or beach, and another forms to break in its place; the higher the waves the more quickly they appear to move. But when the wind has subsided the rolling, or “swell,” remains,—a long, lazy, undulating motion—a rocking to sleep of the billows of the sea. Without a ripple on the surface these huge rollers will glide towards the shore and break upon the shingle with a roaring sound which can be heard for miles, dragging the pebbles after them as they recede with a rattling like bones and marbles. The pebble ridge at Westward Ho! will illustrate this vividly at times, the sound being heard far inland like continuous thunder, and on a calm night, when there is no wind stirring, the roar of the ground swell is weird and mysterious in the gloom.
The height of waves is very varied. Observers say that forty-four feet is about the highest-known wave from hollow to crest. Waves of thirty-five feet have been often met with, and off the Irish coast and in the Atlantic sailors tell of waves “as big as houses.” But houses differ in size as do waves.
The rate which waves are estimated to travel varies with the wind-propelling force. The average hurricane wave travels at about forty-five miles an hour. But earthquake waves—those set in motion by subaqueous disturbance—have been known to travel at the rate of six hundred feet in a second for thousands of miles across the ocean. Such a one occurred after the earthquake which destroyed the town of Arica in August 1868, and the wave crossed the Pacific to Chetham Islands, 5,520 miles, in fifteen hours and twenty minutes. We have many of us seen the great tidal waves, or “bores,” which at certain seasons rush up our rivers—the Severn, for instance—with great violence, and at times forty feet high.
These tidal waves are also experienced in the Ganges, the Amazon, and at Bordeaux, as well as in China and elsewhere.
It may well be imagined that the tides also affect the land, and the theory of these ocean movements is a very interesting study. We have already referred to it under ASTRONOMY, for the Sun’s and Moon’s attraction is the main cause of the phenomenon, which is so familiar and yet so strange. But the consideration of the tides must be again entered upon here ere we proceed to view the effects of the sea upon the land, and how the physical geographical features alter.
Isaac Newton rightly attributed the cause of the tides to the attraction of the moon and sun. Spring tides occur when both luminaries are above the meridian, and the neap, or low tides, happen when the sun and moon are farthest apart. The highest tides are perceived after a new or full moon; the lowest, after she has passed the first or third quarter. In January the spring tide is highest of all, because the earth is nearest to the sun then, and his force of attraction, added to that of the moon, causes a very high tide. With the assistance of the accompanying diagrams we shall be able to make the tidal phenomena clear.
Suppose the moon to be at M, the point J (the sea) will be nearest to the moon and will be attracted, while the earth will exercise a retarding power to a certain extent. This attraction of the water from its usual level causes a kind of vacuum, into which the surrounding water flows and causes a high tide at H. At the opposite side the earth, not the water, is most attracted, and then the water rushes in to a certain extent to fill the vacancy left by the earth’s movement towards the moon. Another high tide is therefore caused at L, but not so high as the tide upon the opposite side, as the Moon is so much nearer the latter. The tide, then, is only the natural movement of the sea water to fill up the space the earth and other portions of the watery mass have vacated in obedience to lunar and solar attraction, which is, to a certain extent, counterbalanced by the attraction and resistance of the earth.
The neap tides are caused by the opposing forces of attraction of the sun and moon. The sun, as it were, pulls one way, the moon the other. The latter (being nearer) having twice the power of the former, causes a tide indeed, but it is a low one. The spring tide occurs when sun and moon together attract the water.
The effects of the rise of the tide are sometimes very disastrous, and when the wind assists the sea, and heaps up the water, the sight is grand in the extreme. On the coast of Schleswig, at Hallingen, the sea has washed away a whole cluster of islands, and now the waves cause tremendous inundations. About every six years, on the average, a great flood happens for such trifles as a high tide are of no account. In 1362 and in 1834 terrible destruction was wrought; the coffins and bodies were washed out of the graves. Piles of débris are then washed up, and sand and gravel accumulates for a time till again carried away.
Travellers to France will notice the “dunes,” or sandhills of Calais, as the train winds its way to Boulogne. We find that whenever the shore is flat the shingle and sand are blown inwards and form “dunes,” and the sand is distributed far inland, checking all vegetation, and altering the features of the country. The wearing away of rocks by the water, the continual undermining of them by the waves, and sometimes the disengagement of great blocks weighing many tons—all these effects of the sea tend to alter the appearance of the land. We may observe the denudation in many places along the coast—the caves, holes, and tunnels eaten out by the water. In Norway the “Fiords” are very remarkable. They were formed by the upheaval of the land, and tell us of the glaciers which once filled them up. Thus by ice and water the solid land is ground down and eaten away hourly, daily, and for countless centuries, changing the place of the hard rock into a standing water, and the flintstone into a springing well.
We must now plunge beneath the waves, never fearing the rough surface; we shall find all smooth and quiet at the bottom of the sea.
THE BOTTOM OF THE SEA.
What can we tell about the bottom of the sea to which no man has ever reached living, and from which we have no information? We can lay our telegraph or telephone lines beneath the waves, and far from the restless waves in those quiet depths where no billows can reach. What treasures must lie hidden at the bottom of the sea! The treasures, the gold and silver, the merchandize, the wealth of centuries. The sailor lies sleeping there
... “Serene and safe From tempest and from billow; The storms that high above him chafe Ne’er rock his peaceful pillow”!
What can we hope to find at the bottom of the sea we cannot reach? Yes, but we can reach it. By sounding with Brooke’s lead (a cannon ball, as shown in the illustration), we can arrive at a certain knowledge of the composition of the ocean bed. The right-hand figure of the two is the lead when being lowered, and while it is sinking the cord remains tight. So soon as it touches the bottom the weight of the cannon ball divides the line, and the tube is easily drawn up again. It has been well greased, and so in the cavity of the rod some shells and sand are found adhering. These fragments tell us the composition of the bottom of the sea.
Here we find tiny shells, just as we find them in chalk, the same formation as that which piled up the cliffs which have risen from, or been discovered, by the sea. By other ingenious contrivances water can be fetched up from the bottom of the ocean, and the temperature can be gauged all along the sounding line. The expedition of the Challenger brought many interesting facts to light. Far down in these solitudes are marine animals,—crustacea, star-fish, seaweeds, and shells,—all of which are carried up by the dredge worked by a steam engine; for the resistance is very great, and the weight supported at the depth of two miles must be considerable, and is equal to four atmospheres. A thermometer has come up crushed even in its iron case, and so the creatures which inhabit and find means to live at the bottom of the sea must be specially fitted by Nature for the locality.
The configuration of the ocean bed has given rise to many different opinions. It has been maintained that there are mountains and valleys, hills and dales underneath the water, all clothed with marine vegetation, equal in height and depth to the terrestrial hills and vales. Again it has been declared that the ocean bed is level; but we find raised portions, which we call islands, which may be the tops of mountains, or portions of the mainland separated from their parent continent by an inroad of the sea, as are our islands of Great Britain.
The sea-bed, however, is very irregular. We find deep and steep valleys, and high hills, but the picturesque peaks caused by the action of air, frost, and water on earth are not, of course, represented under water. Between the Irish coast and Newfoundland we are told the bed is level for nearly four hundred miles. There is a deep declivity before we reach this plain. The centre of the Atlantic is a plain, and on it the most volcanic islands rise, such as Ascension and the Azores. Between England and Greenland there was at one time a land communication, as we have remarked under GEOLOGY, and there are submarine terraces now. [An immense river once ran through Western Europe somewhere about where our islands are.]
Under the Atlantic we have remains of foraminifera and other tiny animals, with red clay and volcanic remains which must have been of submarine origin. The Pacific shows us tops of mountains as islands (Hawaii Isles), and an enormous range must be hidden beneath the waters. What a change in the physical geography of the earth a slight sinking of the water of the ocean would make; England and the Continent would be united, and many sea-mountains (islands) discovered. The greatest ocean depth is four miles and a half, but in many places a few hundred feet less depth than at present would reveal many changes in the land.
Every year since the world has gained its present form the streams and rivers have been pouring water, and carrying mud, stones, and gravel ceaselessly into the ocean. In addition to this, the surface water washes the stones away, animals (corals) build up islands from the depths, and take up space in the ocean. We know that if we put our hand in a basin full of water we displace a quantity of the fluid; so we might imagine that, the sea being already full, every island formed would tend to an overflow of the sea, and the land would be thereby buried. That the sea does encroach upon the earth we know, but it also recedes. Here is the balance of Nature.
Rivers pour in water and material. The sun absorbs the water and prevents overflow; tiny animals make shells from the material. All the causes we have mentioned tend to permit the encroachment of the waters, but volcanic action and even earthquakes act also to neutralize this tendency by upheaving hills and mountains, which prevent the invasion of the sea by its elevation or by land depression. We have seen in our chapters upon GEOLOGY how the ocean beds have been upheaved, and remains of marine animals are daily found upon our highest hills. Thus the forces which sometimes cause such destruction in the earth are the means whereby the waters are kept in their places. But for volcanic action the land might all disappear by denudation and continual wear and tear, and be deposited at the bottom of the sea!
If it were not for currents, of which many defined ones exist in the ocean, and the never-ceasing flow and ebb of the tides, the sea would soon lose its purity and clearness. Though the water is salt and becoming salter, animalculæ and all kinds of plant-animals would still increase and multiply; so the decay of animal and vegetable matter would quickly render the ocean a source of pestilence and death to mankind, and be most injurious to animal life generally. But the movement is so ceaseless, and the various fish and mammalia (whales, for instance), by preying upon each other, as other animals on earth do, keep up the balance of production, and the organic matter deposited in the sea is also cleared away.
That the constant currents of the sea prevent the formation and growth of seaweed is clearly shown by the great “Sargasso Sea,” or tract of weed (Fucus natans), called the Gulf-weed. This great tract embraces thousands of square miles, and is situated in the very middle of the Atlantic Ocean, where there are but few currents; but surrounding it is the Gulf-Stream, an enormous current of water running at a regular rate of four or five miles an hour. This Gulf-Stream is supposed to be caused by the same laws and influences which determine the trade-winds—namely, a constant rarefaction of the water at the tropical parts of the earth, and a corresponding condensation at the Arctic portions, for warm water is much lighter than cold, and when the waters of the tropical regions become lighter, the heavier waters of the cold regions pressing down more forcibly tend to raise them above their proper level; they therefore flow towards those very parts which have sunk down by their contraction, and a constant current takes place; this current is the Gulf-Stream. It runs from the Gulf of Mexico northwards towards Newfoundland, turning by Iceland towards the British Isles, by France and Spain, onwards to the coasts of Africa and South America, the West Indies, and again to the Gulf of Mexico, although the return current does not go by the name of Gulf-Stream. This great stream of water, warmed by the tropical sun, serves the same two purposes fulfilled by the trade-winds—namely, a circulation and distribution of the superfluous heat of the equatorial regions, warming the northern countries; and cooling, by the return of under-currents, those in the tropics. The fogs of Newfoundland are caused by the great current of warm water entering the cold region and carrying with them surface-currents of moist air, which the cold condenses into fog, just as the breath is visible in a cold atmosphere. England owes its moist and mild climate to the same cause. The depth of the Gulf-Stream itself is very little. It is a mere layer of warm water. (See Sir George Nares’ reports of the Challenger expedition.)
In the foregoing pages you have now seen, and, we hope, gained, some information concerning the sea sufficient, at any rate, to induce you to enter more deeply into the subject than we can at present do. We have learnt how the sea water is composed, and what goes on on the surface. We have discussed waves, and referred to tides and currents, the wearing away and the renewal of land by the sea; we have dived beneath the surface, and found something to interest us at the bottom of the ocean. As we come up again we are surprised to find islands or reefs where none existed when we went down. What has caused this sudden appearance? They may have been slowly raised to the surface by coral insects, or suddenly by volcanic action. Let us consider the coral, which plays a very important part in our Physical Geography, before we proceed to the volcanic island.[32]
The low-lying islands are those formed by the skeletons of the coral insects, and the Coralline Islands are some of the most wonderful productions of nature. They are only found in warm climates, between the twenty-eighth degrees of north and south latitude, and limestone pure and simple is the chief component of the coral reef, as it is of the mountains erupted from the depths of the sea. “The detritus of corals, echinodermata shells, reticularia, and other living creatures,” says a writer on this subject, “deposit not only the salts of lime extracted from the ocean, but their own dead bodies to form the hard substance of the rock.”
The coral insect is a zoophyte (Anthozoa), which, as may be seen from the illustrations, assumes curious and elegant forms, and the coral it produces is a limy or calcareous deposit, which is fixed upon a rocky base. As years go on these accretions become greater and greater, and at length rise above the water. When a little distance below it, the reefs form dangerous and frequently unsuspected barriers, upon which ships are wrecked. The red coral is dredged up from the Mediterranean, where there are extensive coral fisheries. This coral is found deep in the water, and never rises to the surface. Formerly there were coral reefs in the European seas, but the changes of temperature stopped their production. The “atolls,” or circular coral reefs with an opening at one side, have been described by Professor Darwin. “Who,” says the great naturalist, “would not be struck with wonder and admiration on catching sight for the first time of this vast ring of coral rock, often many miles in diameter? Sometimes a low green island is seen beyond it, with a shore of dazzling whiteness; outside is the foaming surf of the ocean, and within it a broad expanse of tranquil water, of pale green colour and exquisite purity.” These “atolls” mark the situation of sunken islands, and the extension of them and the barrier reefs would seem to indicate a slow but decided sinking of the bottom of the Indian and other oceans; but the “reefs” tell us that the land to which they are attached has not become depressed, and may have become elevated. We may then conclude that a continual rising and depression of the land is taking place in various oceans, indicating a sinking of the ocean bed in one locality and the result of volcanic activity in another, for no active volcanoes are found in the regions of depression.
We must now leave the sea and come to land again, to consider volcanoes and volcanic action there.
VOLCANOES AND EARTHQUAKES.
The various phenomena of volcanoes form a subject very difficult to be explained, as it is impossible to ascertain positively the cause of volcanic action. Whether the earth is interiorly a mass of molten rock and fire, or whether the heat is created by the intense contractile force and movement of rocks, and their motion thus developed into heat aided by chemical combination, we cannot absolutely determine. The theory restricting volcanic phenomena to the upper crust of the earth, by supposing the local accumulations of hot liquid masses of rock, which are forcibly emptied by the expansion of vapours, may perhaps be found the true one.
The majority of the volcanoes are found near, or at no very great distance from, the sea. We may therefore expect to find that water has something to do with the eruptions as it has in the case of the Geysers. But this hypothesis will scarcely hold good in every case, though volcanoes of later ages are limited to regions very different from those in which volcanic action used to be. For instance, in America we have only volcanoes on the Pacific side, and the Andes furnish several. Mexico, Central America, and California possess many volcanoes, and as far north as Alaska we find Mount Elias. There are plenty of extinct volcanoes in Europe, but the Mediterranean produces the active vents; and about the Red Sea and the Caspian, and even in the central chain of Asia, there are volcanoes far from water. The Hawaii isles, on the other hand, are all volcanic, and Australasia furnishes us with remarkable specimens; so altogether the testimony tends to prove that where volcanic remains are apparent the sea had at one time been, or now is, near at hand.
Burning mountains have been familiar to us from our childhood in pictures, and by stirring narrative of destruction wrought by them. The volcano is generally a mountain rising to a cone, but Vesuvius presented quite the appearance of a hollow basin at the top, before it suddenly broke forth and buried Herculaneum in ashes. Von Buck visited it in 1799, and declares it had at one time risen, like an island, from the sea. There are about two hundred and seventy volcanoes at present in activity; four in Europe; eleven in Iceland and Jan Meyen’s land; in Asia, ninety-three; in Africa, twenty-six; forty-six in North America and the Aleutian Isles; twenty-seven in Central America and the Antilles; in South America, thirty-one; and twenty-four islands with volcanic tendencies largely developed. There may be many more “resting.”
Volcanoes, then, are openings or vents which communicate with the melted rock within the earth, and the conical form of volcanoes is owing to the deposits of volcanic matter as it falls from the opening called the crater. If we let a small spade full of mould run through our hands, or from the spade, it will form a small cone, the heavier particles sliding to the base at a certain slope. Thus the volcano builds its own hill, and inside the crater we find cones from which smoke and steam issue. These cones within the cone are the points of issue of vapour and smoke, miniature volcanoes making up a whole.
The signs of eruptions are much the same, and usually occur a couple of days before the actual outbreak takes place. First smoke is perceived, perhaps, and the escape of various noxious gases accompanied by earthquakes occur. Now the eruption may commence and blow away the summit of the mountain, as in the case of the commencement of the catastrophe of A.D. 79, when the whole side of Vesuvius was torn away, and continuous showers of ashes fell for days and nights, burying everything, while the hot lava poured down the sides. Stones and ashes with vapours are hurled into the air. Clouds of steam are formed, and vivid electrical discharges take place in these clouds, while water dashes down, carrying stones (“volcanic bombs”), and reflected lurid flames from within are cast on the steaming clouds, which look like fiery columns. Then the lava issues in a white, hot, steady, irresistible stream, covering everything, and burning up all vegetation.
New volcanoes are continually in process of formation, and at Santorin for hundreds of years volcanic action has been busy in forming islands. These violent efforts of Nature frequently give rise to earthquakes, which are the most destructive of natural convulsions. The records of late occurrences are fresh in the minds of all readers, and need not be specified. The slow subsidence and gradual upheaval of the land is still going on, but we are frequently startled by the account of a rupture of the ground or the destruction of a portion of a city.
The motion of the earthquake is generally in a direct line, and undulating. Sometimes what are termed vertical shocks arise and destroy solidly-built edifices. Mountains have been overturned by earthquake shocks, and trees have been twisted round. Sometimes the ground yawns into enormous fissures. The sea is tossed into great waves and encroaches upon the land, and when the sea recedes the recession of the water is followed by a more terrible invading wave sweeping all before it. Earth tremblings often occur far away from volcanoes, and without any visible connection with volcanic action. There are many aspects of land and water which the student of geography will remember, but which need not be separately treated of. We must, however, refer to plains, plateaus, and lakes. The mountains also play a most important part in Physical Geography and in “Climatology,” as they collect the vapours for rain, and make the valleys fertile, and thick with vegetation. We have spoken of the mountains under GEOLOGY, and the various formations and strata will be found enumerated there, but now we have to do with the mountain chains in their physical aspect as regards their shape and appearance on the globe.
Any elevation rising from a base more than 1,000 feet may fairly be termed a mountain, and solitary mountains are usually volcanic, because eruptive rock does not produce chains of mountains. The origin of mountains is probably due to the contraction and compression of the crust of the earth—not merely the surface, but the whole thickness between us and the supposed molten interior. Mountains did not exist from everlasting, for the very good reason that they are (in most cases) composed of stratified rocks. Stratified rocks are sedimentary rocks, and must have been deposited below water, and hardened long before they were thrust up by pressure. Moreover, we find (as has already been explained) shells and remains of marine animals on the higher summits, which prove to demonstration that these mountains are composed of rocks which were laid down under the sea.
Professor Dana was one of the first geologists to advance the theory that contraction and lateral displacement are the causes of the elevation of mountains. A very good illustration of this theory was made by Chamontier, who covered an india-rubber balloon with a thick layer of wax, and when it had hardened sufficiently he pricked a hole in the bladder, which immediately contracted, and the wax at once rose up into tiny similitudes of mountains, showing in a sufficiently clear manner that such protuberances may be produced by the pressure of the earth’s contraction, and in such a mass as our earth the elevations would naturally be very great.
Professor Geikie has shown how, by a very simple experiment, the contortion of mountain strata is effected by pressure. A number of cloths or towels placed flat on a table represent the sedimentary rocks. Place a board with a weight on the top, and the towels will remain flattened. But by holding two boards at the sides and pressing them together (the weighted board still remaining), we shall find the towels crumpled and upheaved like the Jurassic strata shown in the illustration (fig. 712). Professor Heim calls the central masses wrinkles of the earth’s crust. So the Alps were pressed up or heaved into the air, the weather—rain, frost, snow, and sunshine—imparting the infinite variety of “Horn,” “Needle,” and “Peak,” so expressively applied in Alpine nomenclature;—the Matterhorn, Wetterhorn, Weisshorn; the Pic du Midi, Aiguille de Dru, Aiguille Verte, and many other mountains in well-trodden Switzerland will occur to the reader at once.
The slopes of mountains—though to the casual observer they may appear very much the same—are very different. We sometimes find a long, easy ascent, —more usually a steepish inclination, perhaps 20°;—in other places, such as on the Matterhorn, an almost perpendicular face. Forty-five degrees rise is very steep, and 53° is the limit of any great mountain’s slope. Cliffs and precipices there are, of course; witness the terrible fall from the Matterhorn to the glacier below—thousands of feet with one tremendous leap from the rock to the ice underneath; but mountain slopes are not precipices. As a rule, we find that one side of a mountain chain is steeper than the opposite one. It is harder to climb up from Italy to Switzerland than to ascend in the opposite direction. The Pyrenees are also steeper on the south side. The Scandinavian mountains likewise are steeper in the west. The Himalaya are steepest towards the sea, so are the Ghauts. We here find a difference between the slopes of the NEW and OLD WORLDS. In the former we have the less precipitous mountain slopes towards the east; in the old world they are towards the north, and an inspection of a physical map of the world leads us to the conclusion that the Atlantic and Pacific Oceans are the boundaries of entirely different degrees of slopes. The Pacific and Indian Oceans would appear to border the more precipitous mountain sides; the Atlantic and its connections those less steep.
As a rule, we have the most elevated portions of the earth, mountains, and high tablelands, in equatorial regions; and within the torrid zone every terrestrial climate is to be found, owing to the snows of the high mountains and the heat of the valleys, which are naturally closely connected with the upheaval of mountain ranges. We have already spoken of the never-ceasing influences of the air and water upon the rocks, and we need say little about valleys. There are valleys of dislocation, denudation, and undulation. The great valley of Western Asia, wherein lie the Caspian and Aral seas, seems to have been caused by the upheaval of the Caucasus and the Persian plateau.
PLAINS are very varied. We have European Heaths and Landes; American Savannahs, Prairies, and Pampas; Asian Steppes, and African Deserts. All of these possess certain features in common, more or less vegetation, and sometimes absolute sterility.
PLATEAUS, or Tablelands, are elevated plains frequently undulating in character. The Plateau of Bolivia is 13,000 feet high, and extends along by the Andes. The tableland of Quito is nearly 10,000 feet high, and borders on the giants Cotopaxi and Chimbarazo.
Rivers and lakes add not only to the wealth of nations by their usefulness, but, by the additional picturesqueness of their appearance, to the beauty of the landscape. The velocity of rivers would be very much increased if it were not for the strong resistance offered by the banks and the stones to the current, and by friction. The Rhine and the Rhone, if thus unimpeded, would flow at a rate considerably over one hundred miles an hour; and our own little stream (the Thames), instead of eddying peacefully and twirling gracefully by Medmenham or Cookham, would rush along at the speed of the train which so often crosses it on its way to the sea.
The slopes of river-beds, like the slopes of mountains, vary very considerably, and the inclination of a river varies at different places; in a distance of seven hundred miles the Amazon only falls twelve feet, and the current flows chiefly by impetus already acquired. A slope of one foot in two hundred precludes all navigation, and at still greater inclines rapids and cataracts are formed—the great falls wearing away the river-bed by degrees; so it is calculated that hundreds of years ago Niagara Fall was much farther down the river, and the cataract is slowly moving up stream. In time, as the rock wears away, the height will disappear as the celebrated “Falls,” and will become a rapid within a few miles of the lake.
Lakes are derived from river-drainage and springs. Some are very salt, owing to evaporation carrying away so much water, and leaving the accumulated mineral salts. These very salt lakes are likely to dry up, as the supply of water is not equal to the demands of evaporation. Floating islands appear and disappear on many lakes. Derwentwater is one instance. On the uses of lakes and rivers it would be superfluous to dwell. We are more concerned to examine their influence on climate, and in this sense we must also consider mountains. But we will now group all the phenomena of the air and water, and their effect upon climate, under “Meteorology” in the chapters next following.
FOOTNOTES:
[32] Those who wish to study the subject fully should read “Corals and Coral Reefs,” by the late Charles Darwin, and Dana’s “Coral Islands.”
XLVIII. PHYSICAL GEOGRAPHY. METEOROLOGY
THE ATMOSPHERE—WINDS AND AIR CURRENTS—WIND PRESSURE—STORMS—RAIN-CLOUDS—WATER-SPOUTS—ATMOSPHERICAL PHENOMENA.
Under this heading we shall find the atmosphere playing a very important part. The air is composed of oxygen and nitrogen with some carbonic acid gas and aqueous vapour. We have, under the Chemistry section, discussed these constituents which unite to make up the air or atmosphere in the following proportions:—
Oxygen 210·0 Nitrogen 775·0 Aqueous Vapour 14·2 Carbonic Acid 0·8 ------- 1000·0 =======
It is a fact that all over the world the same chemical result is found. Whether we bottle up the air in the valley, or, as Gay-Lussac did, go up to an elevation of 21,000 feet in a balloon, we shall find the air of the same chemical composition. In Europe, Asia, Africa, and America, it is all the same. The pressure is less as we ascend, and we cannot manage to breathe in very high altitudes so well as upon the ground for which we were fitted, but the air is the same.
The atmosphere, then, is not always equal in density, nor is it quite transparent. The light from sun and stars is, to a certain extent, lost, and it has been calculated that the sun’s rays lose one-fifth part of their brightness passing through the atmosphere. We all know what the air is. We breathe it, we feel it blowing, we witness its effects. Were it not composed as it is we should die or go mad; plants would not live, and the earth would become a desert. Air is everywhere—invisible; a so-called empty vessel is full of air because an animal will live in it till the atmosphere has become vitiated by the carbonic acid from the lungs. Yet air, or rather its watery portion, is visible when condensed.
Vapour is not perceptible. But how does it become so? We cannot see the air, how can we see a portion of it? We can answer this question by illustration. The steam from an engine is not visible on a very hot day. But when the day is damp and dull the vapour is condensed, and becomes visible; then air appears and is resolved into vapour again. This change was effected by heating water and then cooling it, when it came back to water again. This watery vapour is always present in the atmosphere. Heat, also, is present in the atmosphere, and the sun is the origin of that heat.
Heat, we know, is the effects of the rapid motion of small particles of matter, and is radiated from our bodies—so we feel cold; it reaches our bodies, and we feel warm. So air is heated or cooled by the sun, not in its absence, except when the earth and air have been so warmed during the day that the heat is given out by them long after sunset. We have read of the pressure of the atmosphere in the PHYSICS section, and that warm air is lighter than cold air, as shown in the ascension of the Montgolfier balloon. It is this variation of temperature of the atmosphere that gives birth to one great meteorological agent—viz., the WIND, which we will now consider.
WINDS AND AIR CURRENTS.
We can easily illustrate the cause of winds. Suppose we have a hot room and a cold one, and we suddenly open the door of communication between them, the heated air which has risen to the ceiling of one room will rush out through the upper part of the opening of the door, and the cooler current will flow in just above the floor. If we place a lighted candle in the upper and lower part of the opening we shall see the flame tending outwards from the heated room, and in an opposite direction from the cold room. In the centre of the open door there will be but slight disturbance. So it is in nature. The warm air ascends, cooler air rushes in to fill the space, and a storm or a breeze is created. The balance must be restored. The upper current probably moves one way, and the lower the other way. Thus clouds are said to be “coming up against the wind” when they are moving in an upper current, or in a different direction to that the wind is blowing just above the earth’s surface.
The wind moves, with varying velocity. We have a gentle breeze when the motion of the air is about five or six miles an hour, a good breeze at twenty-five miles an hour, a high wind at thirty-five, and a gale at fifty. Hurricanes travel at sixty and seventy miles an hour, and do enormous damage. Near the Equator we do not find much wind, and this fact has caused the name of the Region of Calms, or “The Doldrums” of sailors, to be bestowed upon that portion of the globe, but this belt of calm has no fixed position. It follows the sun’s course, and is the region of greatest heat, and, as it were, the centre of a concentric circle of currents. The hot air rises and goes away; air rushes in north and south, and causes what are called the North-East and South-East Trades, or Trade Winds, owing to their being so useful in commerce for ships, or to the old meaning of the word trade, a “regular course.” The calms of the Tropic of Cancer are called the “Horse Latitudes.”
Readers of the life of Columbus will remember how his crew were affrighted at the persistency of the wind which bore him across, for no sail requires shifting, nor is a sheet altered while the vessel is making way with the “Trades.” Were the earth covered with water, we should find the trade-winds blowing equally over the surface, but the varying temperature of the land diverts them. The rarefaction of the air in the Sahara causes a westerly wind to prevail, which blows towards the land, instead of the trade wind we might expect to find.
The MONSOONS, again, are caused in like manner, for the ordinary “trade” from the south-east is changed by the elevation of the heated air in Central Asia into a south-west wind, and so in the south, in consequence of the heated air from Australia, the north-west trade appears as a north-east monsoon, but is altered to a north-west wind. Nearly all the year round, therefore, we find the two winds, which are modifications of the “trades,” blowing in different directions and from different quarters. From November to March there is a north-east wind north of the Equator, and a north-west wind blows south of the Equator. From April to September a south-west wind blows at the north, and a south-east wind at the south of the line. The term monsoon signifies a “season,” and the changes of these winds give rise to tremendous storms causing great havoc.
SEA and LAND breezes are really little monsoons; they are caused by the heat of the sun in just the same way, but with miniature results. We all know the sea-breeze which comes in as the land gets hot during the day, for the land warms more quickly than the sea under equally existing circumstances. So again, in the evening, the land loses its heat more quickly, and then the cool air flows out again to take the place of the warmer sea air which is continuing to ascend. The intensity and regularity varies when the degrees of heat are most different between land and sea and in tropical regions; and the varied coast formation will of course affect the wind, but as a rule the fact may be accepted as plainly explained, sea-breeze in the morning, land-breeze at night, and amateur sailors in boats at our watering-places will do well to bear this in mind.
There are a great number of local winds deriving their names from their direction or influence. We may mention them briefly. The special terms for winds are—
The North Wind, or Tramontana. The North-East Wind, or Greco. The East Wind, or Levanter. The South-East Wind, or Sirocco. The South Wind, or Ostro. The South-West Wind, or Libeccio. The West Wind, or Ponente. The North-West Wind, or Maestro-Mistral.
The Mistral, or Maestrale, is well known at Nice as the north wind, while at Toulon it is a north-east wind. The other winds, such as the Sirocco, which in some places is a warm, damp wind, in Madeira is a hot wind, and likewise in Sicily, where it is equally warm and damp like steam. It has different names in various countries, such as Samiel in Turkey, and sometimes as Föhn in Switzerland, where it may, however, be a north wind—which, as all travellers know, is a dry and a hazy-weather breeze, yet sometimes moist. The Simoon is a very hot wind raising sand-storms in the deserts, and experience has shown it to be very prejudicial to life in consequence of the fine sand and the tremendous heat it carries with it. Egypt is subject to another hot wind, called the Khamsin, and the west coast of Africa is subject to the Harmattan, a dry, easterly wind. The cold, dry wind of the Himalaya is known as the Tereno. In South America there is the same wind, the Pampero blowing east and south-east. The Euroclydon, mentioned by St. Paul, is the modern bora over the Adriatic. Malta rejoices (or laments) in the Gregale, a north-east wind. There are several other terms, such as the Puna of Peru, a very drying wind; the Purgas in Labrador, the Tourmente in France, and Guxen in Switzerland. Then we have the Hurricane, from “Ouracan” of the Caribs; the Typhone, or Tae-fun of China, so called from the dreaded god Typhon of Egypt; and the Tornado—all very violent winds, and circling round, causing, so to speak, whirlwinds, by which trees are uprooted, and houses destroyed.
The measure of the velocity of wind is performed by anemometers, which record the velocity in feet per second, and the amount of pressure. The anemometer is a well-known apparatus, with its four arms terminating in “cups” and a “tablet” anemometer, which is more or less disturbed or deflected from the vertical line by each gust of wind, and thus the score of degrees is marked by an indicator, which is moved as the tablet is deflected. We annex a table of wind pressure and velocity—
PRESSURE OF THE WIND.
+------------------+------------+----------------------+ | Velocity. |Pressure per| | +---------+--------+ sq. foot. | | | Miles |Feet per| Rouse and | Description of wind. | |per hour.| second.| Smeaton. | | +---------+--------+------------+----------------------+ | 1 | 1·47 | 0·005 | Very gentle. | | 2 | 2·93 | 0·020 | | | 3 | 4·40 | 0·044 | Light airs. | | 4 | 5·87 | 0·079 | | | 5 | 7·33 | 0·123 | Light breeze. | | 10 | 14·67 | 0·492 | | | 15 | 22·00 | 1·107 | Brisk breeze. | | 20 | 29·34 | 1·968 | | | 25 | 39·67 | 3·075 | Strong breeze. | | 30 | 44·01 | 4·429 | | | 35 | 51·34 | 6·027 | High wind. | | 40 | 58·68 | 7·873 | | | 45 | 66·01 | 9·963 | Gale. | | 50 | 73·35 | 12·300 | | | 60 | 88·02 | 17·715 | Heavy Gale. | | 80 | 117·36 | 31·490 | Hurricane. | | 100 | 146·70 | 49·200 | Tornado. | +---------+--------+------------+----------------------+
The south-west wind is more constant than any other, and the west wind in our islands is more frequent than the east; tables have been compiled showing the average number of days upon which the winds blow from different quarters, but need not be quoted. Storms can generally be anticipated by the barometer, which falls very quickly for “wind.” The quarter whence the breeze may be expected is often indicated by the streamers of clouds, or “mare’s tails,” across the sky; though we must admit the opposite direction to that anticipated by casual observers may often prove the right one.
Hurricanes and tornadoes are really whirlwinds in motion. The rotatory movement of the air is from right to left in the northern hemisphere, and from left to right in the southern—that is, in the opposite and same directions respectively as the hands of a watch move. The whirlwinds are caused by two currents of air meeting at a certain angle, just as a whirlpool is the result of opposing currents of water.
The use of the wind in nature cannot be over-estimated. It is frequently destructive and terrible in its effects, but these comparatively trifling damages are as nothing when weighed against the advantages conferred upon mankind by the wind and the currents of the atmosphere. The north cold is tempered by the warm south wind. The pollen and the seeds of plants are borne on the wings of the wind, and the clouds are carried over the land to “drop fatness” upon our fields. The want of free circulation of air is very injurious. Witness the terrible affliction of goitre, so prevalent in the closely shut-in valleys such as the Rhone Valley, where cretinism or congenital idiotcy is distressingly prevalent.
VAPOUR AND CLOUDS.
Vapour, as we have heard, is invisible, and is produced by heat. As the visible steam (which is invisible as it issues from the safety valve at the actual aperture, and nearly invisible altogether on a hot day) is produced by combustion, so vapour is produced by the heat of the sun’s rays. But there are some observations to be made respecting these rays, which are the cause of vapour, and therefore of cloud, rain, dew, frost, ice, snow, and water all over the earth; and we must look at the circumstances closely.
Those who have followed us through this volume will remember that at the end of Chapter VIII. we remarked upon the spectrum, and made a few observations respecting the heat spectrum, and the velocity of light rays, which became too rapid to be observed, and then they developed heat—invisible heat—produced by non-luminous waves, which proceed from the sun as surely as visible rays or light. Professor Tyndall has written very pleasantly upon this subject, and, with his clear leading, any reader can study for himself.
We have now arrived at the conclusion that there are visible and invisible rays giving us respectively light and heat. These latter are the means whereby the ice is melted, and by which water is evaporated to vapour, and formed into CLOUDS when it is chilled or condensed. Here is another link in the beautiful chain constructed by Nature. We cannot penetrate far into any portion of the system of the universe without being struck with the wondrous harmony that exists between every portion of it. Thus heat and light, vapour, cloud, rain, dew, and ice are all intimately connected.
A cloud, then, is a visible body of vapour in the atmosphere, which is supported by an invisible body of vapour. It will remain thus invisible so long as the atmosphere is not saturated with moisture. The air can contain a great quantity of moisture without its being rendered visible, and so when the day is hot we see no steam from the locomotive. It is absorbed into the dry atmosphere. But when the day is “damp” we find that the air has nearly as much moisture as it can carry, and the steam is condensed, a portion falling in tiny drops like rain. This is proved every day in cold weather when ice is found in the windows—the cold air has condensed and frozen the water breathed out from our lungs, and snow has been known to fall in a ball-room when a cold current of air was admitted.
People are sometimes apt to think that if the sun were very hot, glaciers, and such icy masses, would diminish; but we think after what has been said respecting the power of the sun’s rays to evaporate water, all will see that the contrary is the fact. Without sun-heat we should have no cloud, and as clouds give us rain and snow and ice and glacier, we must come quickly to the conclusion that glaciers and snow are the direct results of the heat of the sun. The “light” rays of the sun do not penetrate snow, and that is why our eyes are so affected in snowy regions. The poor Jeannette sufferers a short time since were blinded by reflected light, and dark spectacles are worn on all Alpine expeditions. The invisible rays, as we have said, dissolve the ice into rivers.
The atmosphere produces clouds by expansion of vapour, which chills or cools it, and it descends as rain. To prove that expansion cools air is easy by experiment, but if we have no apparatus we must make use of our mouths. In the body the breath is warm, as we can assure ourselves by opening our mouths wide and breathing upon our hands. But close the mouth and blow the same breath outwards through a very small aperture. It is in a slight degree compressed as it issues from the lips, and expanding again in the atmosphere feels colder. Air compressed into a machine and permitted to escape will form ice.
Water is present in clouds which assume very fantastic and beautiful forms. We know nothing more enjoyable than to sit watching the masses of cumuli on a fine afternoon. The grand masses built up like the Alps appear to be actual mountains, and yet we know they are but vapour floating in the air, and presently to meet with clouds of an opposite disposition, and produce a thunderstorm with torrents of rain. Those who will devote a few minutes every day to the steady examination of clouds, will not be disappointed. They give us all the grandeur of terrestrial scenery. Mountains, plains, white “fleecy seas,” upon which tiny cloudlets float, and low upon the imaginary yet apparent horizon, rise other clouds and mimic mountains far and farther away in never-ending distance.
A pretty, light, feathery cloud, with curling tips and fibres, is known as cirrus, and exists at a very great elevation. Gay-Lussac went up in a balloon 23,000 feet, and even at that height the cirri was far above him in space. We can readily understand that at such an extreme elevation they must be very cold, and they are supposed to consist of tiny particles of ice. Such clouds as these are very frequently observed at night, as cirro-cumulus around the moon, and a yellowish halo, apparent to all observers, is thought to be coloured by the icy particles of the lofty cirrus. The beautiful and varied phenomena of perihelia, etc., are due also to the snowy or icy flakes of the cirri and cirri-cumuli, caused by the refraction of light from the frozen particles. These cirri clouds are indicative of changeable weather as “Mares’ tail” skies, and long wisps of cloud, foretelling storm.
The cirro-cumulus is the true “mackerel” sky, and is formed by the cirri falling a little and breaking off into small pieces of cumulus, which is a summer (day) cloud generally, and appears in the beautifully massive and rounded forms so familiar. The stratus is, as its name implies, a cloudy layer formed like strata of rock. It is generally observable at night and in the winter. It often appears suddenly in the sky consequent upon diminished pressure or a rapid fall of temperature. It is low-lying cloud sometimes, and at night forms fogs.
The cirro-stratus is perceived in long parallel lines, and indicates rain; when made-up rows of little curved clouds it is a certain prophet of storm, and when viewed as haze is also indicative of rain or snow. “Mock-suns” and halos are often observed in the cirro-stratus.
The nimbus is the rain-cloud, or condition of a cloud in which rain falls from it. It is upon this rain-cloud we can perceive the rainbow, and on no other cloud, but otherwise only in the sky.
We have now seen the varieties of cloud and their common origin with fogs and mists, which differ from them only in the elevation at which they come into existence, according to the condition of the atmosphere.
The uses of clouds are many and varied. Their first and most apparent use seems to be the collection and distribution of rain upon the earth. But besides this, they shelter us from the too great heat of the sun, and check the evaporation at night. Supposing we had no clouds we should have no rain. If we had no rain the earth would dry up, and the globe would appear as the side of the moon appears—a waterless desert. The invisible vapour in the atmosphere will produce cloud, but the moon can have no atmosphere in that sense. Vapour will also absorb heat, and intercept the sun’s heat rays, acting much as clouds do in preventing radiation and great changes of temperature.[33]
All animals and plants depend upon moisture in the atmosphere as much as upon the varying degrees of warmth. A dry east wind effects us all prejudicially; warm, soft airs influence us again in other ways. Air will be found drier as a rule in continents than in islands or maritime districts, and this will account for the clearness of the sky in continental regions. Fogs and mists arise when the air is what is termed saturated with moisture, and colder than the earth or waters upon it. So the celebrated and dangerous fogbanks of Newfoundland arise from the warm water of the Gulf Stream, which is higher in temperature than the air already saturated. And the same effect is produced when a warm wind blows against a cold mountain; the air is cooled, and condenses in cloud.
The cooling of the breath by the exterior air is exemplified in winter when we can perceive the vapour issuing from our mouths as we speak.
RAIN, SNOW, AND DEW.
Rain is produced by the condensation of vapour. “Vesicular vapours, or minute globules of water filled with air,” compose the clouds, and at last these vesicles form drops, and get heavy enough to come to the ground. Perhaps they are not sufficiently heavy to do so, and then they are absorbed or resolved into vapour again before they can get so far, because the lower strata of air are not yet saturated, and can therefore contain more moisture.
On the other hand, we may experience rain from a cloudless sky. This is no very uncommon case, and occurs in consequence of the disturbance of the upper strata when warm and cold currents come into collision and condense the vapours.
Rain is very unequally distributed. We shall find that the region of calms, which we mentioned in a former page, is also the zone of the greatest amount of rain. The heated air rises and falls back again, there being little or no wind to carry it away. The rainy season, therefore, sets in when a place enters the zone of calms. Equatorial districts have two rainy seasons, as they enter twice a year into the region of calms, but most places have only a wet and dry season, while north and south of the calm region we find rainless districts, or zones tempered by the trade winds, which are dry winds.
But if we suppose—as indeed is the case in South America—that these dry winds happen to come in contact with a cool mountain, the moisture of the air is precipitated in rain. In Australia, on the contrary, we have portions of land actually burnt up for want of rain, because the mountain chain breaks the clouds, so to speak, on a limited corner of the island, while the interior is parched. The winds also coming over India from the Bay of Bengal discharge clouds and rain in the Himalayan slopes. So we perceive that the situation of mountain chains have much to do with the rain-fall, and of necessity, therefore, with the vegetation and fertility of the land. This is another noticeable link in the great chain of Nature.
Perhaps it may now be understood why westerly and south-westerly winds bring rain upon our islands, and why the counties such as Westmoreland and Cumberland and those in Wales receive more rain than any other part of the United Kingdom. Seathwaite, so well known to tourists in the lake district, has the proud position of the wettest place in these islands. We find that when the westerly wind sets in it has come across the warm Atlantic water and become laden with moisture, which, when chilled by the mountains, is precipitated as rain.
The amount of rain that falls in the United Kingdom is carefully measured by rain-gauges, some of which are extremely simple. The water is caught in a funnel-mouthed tube, and measured in a measuring glass every four-and-twenty hours. Thereby we can tell the annual rainfall in any given district, whether it be twenty inches or a hundred. One inch of rain actually means one hundred tons of water falling upon one acre of land. Therefore, if the annual report of rainfall (including all moisture) be twenty inches, we have an aggregate of 2,000 tons of water upon every acre of surface within the district. Twenty inches is a very low estimate. Some places have an annual rainfall of forty or fifty inches. In Cumberland we find 165 inches has been recorded! If we then multiply these last figures we get the enormous quantity of 16,500 tons of water upon every acre of land in the district in one year. It is reported from India that in the Khasia Hills the average is 610 inches, which must be the maximum rainfall in the world. At other places, in the north-west provinces, the fall is only seven inches. Sometimes in tropical rains we find fifteen inches of rain in a day, and that has been exceeded.
We can now judge of the enormous amount of moisture carried up by the sun and dispersed over the earth in rain, which swells our brooks and rivers, cleanses the air of its impurities, supplies our springs, carries with it into the sea lime from the rocks for the shells of marine animals, and then leaving its salts, is again evaporated to form clouds, which discharge the fresh water continually upon the earth in a never-ceasing rotation.
SNOW.
“We all know what SNOW is,” you will say, perhaps. Well, then, will any ordinary young reader tell me what he knows about snow? “It falls from the sky in white flakes,” says one. “It’s frozen rain,” remarks another. “Why, snow is snow,” says a third. “There’s nothing like it; it’s white rain-water frozen.”
The last answer we received is the nearest of all. Snow is not snow, paradoxical as that sounds. Snow is Ice! Flakes of snow are ice-crystals—white, because reflecting light. In the section of MINERALOGY we mentioned crystals, which are certain definite shapes assumed by all substances, and we gave many examples of them. Just as alum crystallizes and rock crystal assumes varied and beautiful forms, so ice crystallizes into six-rayed stars.
It is to Professor Tyndall that the world is chiefly indebted for the descriptions of snow crystals and ice flowers. In his work upon “Heat as a Mode of Motion,” this charming writer shows us the structure of ice flowers. He describes a snow shower as a “shower of frozen flowers.” “When snow is produced in calm air,” he says, “the icy particles build themselves into stellar shapes, each star possessing six rays.” We annex some drawings of snow crystals, which are, indeed, wonderfully made. Hear Professor Tyndall once again:—
“Let us imagine the eye gifted with a microscopic power sufficient to enable us to see the molecules which compose those starry crystals: to observe the solid nucleus formed and floating in the air; to see it drawing towards it its allied atoms, and these arranging themselves as if they moved to music, and ended by rendering that music concrete.” This “six-rayed star” is typical of lake ice also.
Snow sometimes reaches us in a partly melted condition; under these circumstances it is called sleet, and snow being much lighter than rain (ice is lighter than water), it descends less directly, and represents about one-tenth the depth of the rain-fall. The use of snow in warming the earth is universally acknowledged, and as it is such a bad conductor, a man in a snow hut will soon become unpleasantly warm.
Ice is only water in another form, and snow is ice; and it is the air in the snow that gives it warming properties. These are all simple facts, which any one by observation and careful reading and study may soon ascertain for himself. We have another frozen fall of water from the clouds—viz., hail, which may possibly be the development of sleet.
HAIL is formed by the falling rain being frozen in its descent, or when different currents meet in the atmosphere. A hail-storm is accompanied with a rushing sound, as if the hail-stones were striking against each other. They are very destructive, and actual hail showers occur in summer more frequently than in winter, and a peculiarity noticeable with regard to hail is its infrequent occurrence during the night.
Records of destructive hail storms are plentiful. The hail assumes a great size, weighing sometimes as much as two ounces, and measuring several inches round. Thunder and lightning are very frequent accompaniments of hail showers.
DEW is moisture of the atmosphere deposited on a cool surface—another form of condensation, in fact. Cold water in a tumbler will produce a “dew” upon the outside of the glass when carried into a warm atmosphere. Such is the dew upon the grass. It is produced by the air depositing moisture as it becomes colder after a warm day when much vapour was absorbed. Warm air can hold more water than cold air, and, the saturation point being reached, the excess falls as dew at the dew (or saturation) point. We have previously remarked that one use of clouds was to prevent rapid radiation of heat which they keep below. Under these circumstances—viz., when a night is cloudy—we shall find much less dew upon the grass than when a night has been quite clear, because the heat has left the atmosphere for the higher regions, and has then been kept down by the clouds; but on a clear night the air has become cooled rapidly by radiation, and having arrived at saturation point, condensation takes place.
Dew does not fall, it is deposited; and may be more or less according to circumstances, for shelter impedes the radiation, and some objects radiate less heat than others. Hence some objects will be covered with dew and others scarcely wetted.
When the temperature of the air is very low,—down to freezing point,—the particles of moisture become frozen, and appear as hoar-frost upon the ground. Thus dew and hoar-frost are the same thing under different atmospheric conditions, as are water and ice and vapour.
We have now come round again almost to whence we started. We have seen the land and water, and the parts that water, in its various forms, plays upon the land, and its effects in the air as rain, etc. We have noticed the winds and air currents as well as the ocean and its currents. We know what becomes of rain and how it is produced, and how the sea works upon the shore, and how clouds benefit us. There are besides some less common phenomena which we will now proceed to examine.
FOOTNOTES:
[33] See “Molecular Physics” (Tyndall).
XLIX. PHYSICAL GEOGRAPHY. METEOROLOGY (continued)
ATMOSPHERIC PHENOMENA—THUNDER AND LIGHTNING—AURORA BOREALIS—THE RAINBOW—MOCK SUNS AND MOCK-MOONS—HALOS—FATA MORGANA—REFLECTION AND REFRACTION—MIRAGE—SPECTRE OF THE BROCKEN.
There are a great number of interesting, and to inhabitants of these islands uncommon,—perhaps we might say fortunately uncommon,—phenomena, which overtake the traveller in other countries. We have referred to whirlwinds and tornados, and will now mention two phenomena connected with these storms. There is the water-spout, for instance, and sand-pillars in the desert, which are whirled up by these winds in spiral columns of water and sand respectively. The tiny whirlwind at cross-roads, which picks up straws and leaves, is the common appearance of whirling or crossing currents of air.
Waterspouts, when they are permitted to come near a ship at sea, or when they break upon land, which is seldom, are very destructive. The waterspout is begun generally by the agitation of the sea, and the cloud above drops to meet the water, which at last unites with it, and then the column of whirling liquid, tremendously disturbed at the base, advances with the prevailing wind. Its course is frequently changed, and ships within its influence would be speedily wrecked. The only way to save the vessel is to fire a cannon ball through the column and break it.
A waterspout once devastated a district in the Hartz mountains of Saxony. “A long tube of vapour descended to the earth, and several times was drawn upward again; but at last it reached the ground, and travelled along at the rate of four-and-a-half miles in eight minutes, destroying everything in its way.”
On another occasion at Carcassonne in 1826, “a reddish column was seen descending to the ground, and a young man was caught up by it and dashed against a rock.” His death was instantaneous.
The cause of these whirling winds is supposed to be in the action of vertical currents of air which ascend heated, and return rapidly as cold air. The “waterspouts,” etc., are quickly formed. The tornado is a monster whirlwind like a waterspout in form, and advances at a tremendous rate—eastward as a rule. It moves in leaps and bounds, passing over some portions of the ground and descending again. The current of air is directed to the centre; the cyclone, as mentioned, has a spiral or rotatory movement.
Thunder and lightning have been, to some extent, described under the head of ELECTRICITY, but some observations may also be introduced here, as storms of that nature appertain to meteorology distinctly.
Electricity is always present in the atmosphere, and arises from evaporation and condensation as well as from plants. As the air becomes moist, the intensity of the so-called “fluid” increases, and more in winter than in summer. Clear skies are positively electric, and when large, heavy clouds are perceived in process of formation in a sky up to that time clear, a storm is almost certain to follow. These “thunder clouds,” in which a quantity of electricity exists, attract or repel each other respectively. The cloud attracts the opposite kind of electricity to that within it; and when at last a tremendous amount has been stored up in the cloud and in the air, or in another cloud, the different kinds seek each other, and lightning is the result, accompanied by a reverberation and commotion of the air strata, called thunder.
Lightning most frequently darts from cloud to cloud, but often strikes the ground, whereon and in which are good conductors, such as wet trees, metals, running water, etc. The “electric fluid” assumes different forms—“forked,” “sheet,” and “globular.” The second is perhaps the most familiar to us, and the third kind is the least known of all. There are many well-authenticated instances on record in which lightning with the form and appearance of fireballs has entered or struck houses and ships.
“Fulgarites” are vitreous tubes formed in sandy soils by the lightning in search of subterranean water-courses, for running water is a great conductor of electricity.[34] The fire-ball form of lightning has been known to enter a school-house where a number of children were, and to singe the garments of some, killing others. The ball passed out through a pane of glass, in which it bored a hole, breaking every other pane, however, in its transit. Another instance occurred in which the lightning ran about the floor of a room, and descending the stairs, exploded without doing any injury.
Lightning, like the electric current of the laboratory, will not always set fire even to inflammable objects. An electric spark can be passed through gunpowder without setting fire to it, and lightning will often shatter the object without firing it. Death by lightning is instantaneous, and in all probability quite painless; for we may argue from analogy, that as those who have been rendered insensible by lightning have had no remembrance of seeing the flash which strikes so instantaneously, nor of hearing thunder after it, it is instantaneous in its effects. Besides, the natural attitude is preserved, and the face is usually peaceful and limbs uncontorted after death by lightning.
There are some curious electrical phenomena, such as St. Elmo’s Fire, already noticed under ELECTRICITY; and in some parts of America, in very hot weather, such a light is perceived to issue from trees as the fire glides through the forest. Many instances are on record concerning the luminosity of pointed sticks, and even of the tails and manes of horses in certain conditions of the atmosphere, and of the universal power of electricity and its pervading influence in nature. The benefits conferred by thunderstorms in purifying the air, and in the production of ozone and nitric acid, are very great, and apart from the magnificent phenomena exhibited, are well worth our attention, though beyond our reach.
Terrestrial magnetism, however, is still more puzzling in its action than is electricity, and the study of the needle, its destination, inclination, and intensity, which are marked upon charts, just as are the weather reports of the Times, is an interesting one. These magnetic maps are termed the charts of ISOCLINIC and ISODYNAMIC lines. The declination of the magnetic needle from the true north is its deviation from that point, and the “inclination” is its dip towards the horizon. The line of its direction being known as the magnetic meridian, its divergence from this line constitutes its declination. There are places where it does not deviate, and these, in direction north and south, are called lines of “no variation.” There are also places in the equatorial regions where the needle does not “dip.” The line connecting such places is termed the Magnetic Equator, and north or south of this the needle dips respectively to north or south in degrees coinciding with the distance from the equator.
The earth, then, acts as a magnet, and attracts the needle, but the magnetic poles are not identical with the terrestrial poles. The north magnetic pole was reached in 1831 by Sir James Ross, when the dip was only one minute less than 90°, and the south magnetic pole was very nearly reached also by him in 1840. The magnetic equator passes between these two points.
It is to magnetic atmospherical disturbance that the aurora is due. These northern (or southern) phenomena are extremely brilliant and diversified. In temperate regions the aurora does not present such grand forms as in the extreme north. There the spectacle is astonishingly beautiful. The sky at first clouds over, and mist is developed. Humboldt has eloquently described the aurora borealis, and the beautiful changes of light, the constant movement, flashes, etc., denoting a “magnetic” storm, as electrical discharges indicate an electric storm, although the area affected by the former is far more extensive than that of the latter, and there is no thunder accompanying the magnetic storm, with the production of which the electricity of the earth is unassociated. To the continuous flow of this electricity the aurora is due, and the flashes are only the electric current descending towards the earth. But the true reason of the phenomena may have to be yet discovered, for nothing absolutely certain is known as to the origin of the aurora.
Amongst the numerous effects of refraction and reflection of light the RAINBOW is most common and the most beautiful. If we hold a chandelier “drop” in the sunlight, we shall see a brilliant representation of the rainbow on the wall or on the carpet. The three colours—red, yellow, and blue—mingle or shade away into seven—red, orange, green, blue, yellow, indigo, and violet. These colours are all found in the rainbow.
The colour of the atmosphere—the usual blue tint of the sky—arises from the blue rays of the spectrum being reflected more than the rest by the aerial particles, and the less vapour the bluer the sky, because the vapour gives it a whitish or misty tint. At sunset and sunrise the sky is red or yellow, like gold, or of crimson hue. This is because the sun’s rays have so much farther to come to us at sunrise or sunset, as you will readily perceive if you draw a line from the sun to the sides and then to the top of the arc of the heavens. The blue rays are thus lost in space, while red and yellow, which travel so much faster than blue, are transmitted to the eye, not giving the air time to absorb them.
If you go under water and look at the sun it will appear very fiery indeed, and we may likewise imagine that fiery crimson rays, which betoken atmospherical disturbance, very often are due to the moisture through which they are transmitted. Wet and storm frequently succeed a crimson sunset, which betokens much moisture in the air. The sun is similarly seen through the steam issuing from an engine, and the colours vary according to the density of the steam in its stages of condensation.
Vapour, we know, is invisible and transparent, but when it has been condensed into rain-drops, and the sun is shining, if we stand with our backs to the sun we see what we call the rainbow, because a ray of light entering the drop is reflected, and as all rays are not of equal refrangibility, the light, which is composed of three simple rays, is divided and reflected into those and the complementary colours. When the sun is at the horizon, the rainbow, to an observer on the earth (but not on a mountain), will appear to be a semi-circle. The higher the sun rises the lower is the centre of the rainbow. So we can never see rainbows at noon in summer because the sun is too high. A second rainbow is not uncommon, the second reflection producing the colours in a different order. The colours in the “original” range from violet to red; in the “copy” they extend from red to violet. “Rainbows” are often visible in the spray of waterfalls and fountains.
HALOS are frequently observed surrounding the moon, and then we are apt to prognosticate rain.
“The nearer the wane The farther the rain,”
is an old couplet referring to the appearance of the moon, and is supposed to foreshadow the weather by the size of the halo, which is caused, as we know, by the existence of vesicular vapour in the atmosphere.
MOCK SUNS, or parhelia, and mock-moons, or paraselenæ, are continually observed in cold climates, where the tiny ice particles are so abundant in the air. These phenomena were recognized by the ancients, and halos round the sun can be observed by means of darkened glasses. We annex an illustration of a mock sun and moon seen on the continent of Europe. Readers of Mr. Whymper’s “Scrambles in the Alps” will remember the gorgeous, and to the guides mysterious, fog-bow or sun-bow seen as the survivors of the first and most fatal ascent of the Matterhorn in 1865 were tremblingly pursuing their descent over the upper rocks of that mountain.
The MIRAGE, or Fata Morgana, is a very curious but sufficiently common phenomena, and in the Asiatic and African plains it is frequently observed. When the weather is calm and the ground hot, the Egyptian landscape appears like a lake, and the houses look like islands in the midst of a widely-spreading expanse of water. This causes the mirage, which is the result of evaporation, while the different temperatures of the air strata cause an unequal reflection and refraction of light, which give rise to the mirage. Travellers are frequently deceived, but the camels will not quicken their usual pace until they scent water.
The Fata Morgana and the inverted images of ships seen at sea are not uncommon on European coasts. Between Sicily and Italy this effect is seen in the Sea of Reggio with fine effect. Palaces, towers, fertile plains, with cattle grazing on them, are seen, with many other terrestrial objects, upon the sea—the palaces of the Fairy Morgana. The inverted images of ships are frequently perceived as shown in the illustration (fig. 729), and many most extraordinary but perfectly authentic tales have been related concerning the reflection and refraction of persons and objects in the sky and on land, when no human beings nor any of the actual objects were within the range of vision.
It will be well to explain this phenomenon, and the diagrams will materially assist us in so doing, for the appearances are certainly startling when realized for the first time. The Spectre of the Brocken we see mimics our movements, and we can understand it. But when apparently solid buildings appear where no buildings have been erected,—when we see—as has been perceived—soldiers riding across a mountain by a path, or ledge, perfectly inaccessible to human beings even on foot, we hesitate, and think there is something uncanny in the sight. Let us now endeavour to explain the mirage.
Suppose that in the annexed diagram the space enclosed between the letters A, B, C, D, be a glass vessel full of water. The ship is below the horizon, the eye being situated at E—the glass vessel of course representing the atmosphere charged with moisture. The eye at E will perceive the top of the mast of the ship, S, and we may imagine a line drawn from E to S. Then put a (short focus) convex lens at a just above this (imaginary) line, and a concave one, b, just over it. Through the former an inverted ship will be seen, and an erect one through the latter at S′ and S″ respectively. We now have the effect in the air just as reproduced in nature by the difference in temperature in air strata, which cause it to act like a concave lens when the density of the water diminishes towards the centre, and like a convex lens when it is increased.
This can be proved by heating the air (by hot irons) above the glass vessel filled with oil, and the effects will be just the same as through the lenses. Dr. Wollaston obtained the mirage by using a clear syrup,—about one-third of the vessel full,—and filling it with water. The gradual mingling of these fluids will produce the phenomenon. The illustration in the margin (fig. 731) shows us the rays proceeding from the ship’s hull, and refracted into the line reaching the eye, above the line proceeding from the mast, so the ship appears hull uppermost; the rays cross at x. But if they did not cross before they reach the eye, the image would appear as at s´ p´ in an erect position.
The Spectre of the Brocken arises from a different cause. Such appearances are only shadows,—projected on thin clouds or dense vapours at sunrise, or when the sun’s rays are directed horizontally,—for of course vertical rays will throw the shadow on the ground on to the zenith. Balloons are also reflected thus, and much interest has been caused by the appearance of a twin balloon, until the aerial voyagers have discovered the cheat by seeing the shadowy aeronaut imitating their actions, and the second balloon has been discovered to be an airy nothing.
FOOTNOTES:
[34] “Fulgarites” are composed of melted quartz, and on Mount Ararat many have been found which give a character to the formation of the lesser Ararat.