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not everywhere rise to the same height when the clay is pierced, but the well will only drain that which presses with the greatest force in its immediate vicinity, without affecting that which is at a distance. If there is a free and extensive drain in any part of such a formation, then it is obvious that the water in that vicinity will rise no higher than the level of the drain. Thus the water in the well marked L on the cut, rises no higher than the Thames, bacause that river cuts through the London clay, and serves as a drain to the same water bearing stratum which supplies the other wells.

Wells situated in level countries, and in alluvial formations, generally require to be sunk only thirty or forty feet, and sometimes no more than twenty before water is found. These are not commonly supplied by springs, but merely by the draining of the water which exists within the circuit of a few yards, into a cavity. During severe droughts, many such wells fail, which shows that they are supplied only by the rain which percolates from the surface, and not by deeply seated springs.

But there are some extraordinary phenomena connected with springs which require a different explanation, if indeed they can be explained at all.

There is little difficulty with respect to those springs which rise in salt marshes, or which gush from the fissures of rocks under the sea. The sources of these are in the distant hills; or in the strata of the vicinity, situated higher than their outlets; and the presence of the sea or marsh it is plain, could not affect them, since the water from these do not penetrate their sources. This principle will also account for such springs as rise on small islands at little distances from the sea shore, where they could not have been collected from the rain falling there.

There are however springs which arise near the tops of hills, and which are so situated as to make it apparent that their sources could not exist in the same hills, nor in those in the immediate vicinity. The water with which such are supplied, must therefore, come from the higher hills or mountains, at a distance, and passing the intervening valley, rise by hydrostatic force to these outlets. Many rocks are so full of fissures, as to present no difficulty in supposing that considerable rivulets might run. among them, at great depths below the surface. Rocks also frequently contain large cavities, so that some rivers

sink down into them and disappear for miles, when they again issue from their hiding places, and continue their courses. In limestone districts it is well known that large cavities are of common occurrence. Perhaps, therefore, the manner in which water is conveyed to the springs, situated as above described, may be as follows. Water, from hills at a distance, and more elevated than the springs, descend through fissures, to a cavity in the valley, which cavity communicates with another fissure running to the spring. In this manner the hydrostatic pressure from the highest hill, would overcome that from the lower one, and the water would be perpetually transferred from one to the other.

Fig. 48.

The annexed cut, fig. 48, will make this obvious. The rills a, are supposed to unite and fall into the cavity below b, from which, the greater pressure from a, forces the water up the hill, through a fissure, to c, where the spring issues.

That water runs in considerable streams under the earth and among the fissures of rocks, is proved by its issuing in springs, sometimes in large quantities. Dr. Macculloch states, that a spring in Staffordshire, is computed to discharge more water annually, than all the falls in the surrounding country; and the same, even to a greater degree, is true of that of the Sorgne, in France.

A writer in Featherstonhaugh's Journal, for August, 1831, p. 65, refers to a great body of water which issues from the ground, ten miles from Harrisburgh, Virginia, and which is known under the name of "Big Spring." He says, “it should rather be called a river, so large is the body of water which rises suddenly from the foot of a limestone hill, and continues in a stream some yards in breadth, and half a foot deep, with force sufficient to turn two large mills immediately below."

There is a spring at Kingston, R. I., which arises from primitive rocks, and discharges such a quantity of water

that a grist mill has been driven by it for a great number of years, and more recently, a large cotton factory has been erected below the corn mill, which depended entirely on the water of this spring to turn its whole machinery.

From these, and such like facts, there can be but little doubt, that small streams are constantly running under ground among the crevices of the rocks, and that such springs are formed by a union of many of these tributaries, in a similar manner to which larger streams are formed on the surface of the earth, by the union of several smaller

ones.

CHANGE OF CLIMATE.

It will be the object of this section to show, that the temperature of the earth's surface, at some period anterior to the era of history, suffered a material, and probably a sudden change, and that in consequence, the climates of different countries have become colder than they were at some remote period.

This is a subject of great interest in geology, and although the idea of a universal change of climate was once strongly controverted, most writers, at the present day, consider that there is sufficient evidence, that the temperature of the earth's surface is much lower than formerly.

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That the climate of the northern hemisphere has undergone an important change," says Mr. Lyell, "and that its mean annual temperature must once have resembled that now experienced within the tropics, was the opinion of some of the naturalists who first investigated the contents of ancient strata. Their conjecture became more probable, when the shells and corals of the secondary rocks were more carefully examined, for these organic remains were found to be intimately connected, by generic affinity, with species now living in warmer latitudes. At a later period, many reptiles, such as turtles, tortoises, and large saurian (lizard-like) animals, were discovered in the European strata, in great abundance, and they supplied new and powerful arguments from analogy, in support of the doctrine, that the heat of the climate had been great when

our secondary formations were deposited. Lastly, when the botanist turned his attention to the specific determination of fossil plants, the evidence acquired the fullest confirmation; for the flora of a country is peculiarly influenced by its temperature; and the ancient vegetation of the earth might more readily than the forms of animals, have afforded conflicting proofs, had the popular theory been without foundation.

"It is not merely reasoning from analogy, that we are led to infer a diminution of temperature, in the climate of Europe; there are direct proofs in confirmation of the same doctrine, in the only countries hitherto investigated by expert geologists, where we could expect to meet with direct proofs. It is not in England, or Northern France, but around the borders of the Mediterranean, from the South of Spain to Calabria, and in the islands of the Mediterranean, that we must look for conclusive evidence on this question; for it is not in strata, where the organic remains belong to extinct species, but where living species abound in a fossil state, that the theory of climate can be subjected to the experimentum crucis. In Sicily, Ischia, and Calabria, where the fossil testacea, of the more recent strata, belong almost entirely to species now known to inhabit the Mediterranean, the conchologist remarks, that individuals in the inland deposites, exceed in their average size the living analogues."-Lyell's Geology, vol. i. p. 92.

The shells thus existing in strata, and in the fossil state, differ in no respects from those now found in the adjoining sea, except in size; the ancient ones being much larger than those now living. Hence the conclusion, that because these animals do not attain the size the same species did anciently, the climate has deteriorated.

It has also been ascertained that some species of shells found in the fossil state, in Italy, are now living in the Indian Ocean, and that these correspond in size; whereas the same species existing at present in the Mediterranean, are comparatively dwarfs in size, having been stinted in their growth, for want of the heat which now exists in the Indian Ocean.

These circumstances go far to show, that the climate of Italy is not so hot as formerly, for it is well known, that these shells attain a size in some proportion to the heat of the climate in which they are found..

Another and perhaps stronger proof, is drawn from the vegetable remains, which are found in various strata, especially in those of coal. M. Adolphe Brogniart, in his "Treatise on the classification and distribution of fossil plants," has come to the following, among other conclusions on this subject. First. "That in the strata of coal and anthracite, the vegetables preserved are nearly all cryptogamous, or monocotyledonous plants, as ferns,t equisetums, and lycopodiums, &c., and that some of these tribes which no longer exist, except as fossils, grew to an immense size in Europe."

(Some of the Equisetums were ten or twenty feet high, and from six to twelve inches in diameter. These tribes in our climate at the present day, grow from one to three feet in height, and are ordinarily about the size of a pipestem. A specimen of this tribe from the borders of Canada, now before us, is more than two inches in diameter, a proof that the climate of North America, as well as that of Europe has changed. Plants of the fern kind, in some parts of Europe, attained the height of forty or fifty feet; and the aborescent club-mosses were sixty or seventy feet high. No plants of these tribes, at the present day, ever attain one fourth of these sizes.)

Second. "That in the higher strata, a great variety of fossil vegetables exist, which, for the most part, appear to belong to similar tribes of plants, if not in species, at least in genera, to vegetables which still inhabit the hottest regions of the earth; nor is it probable that they have been transported to the places where they are found in Europe, from such climates, since their most delicate parts are uninjured." It is therefore, reasonable to suppose, that since the growth of these vegetables, the climate of Europe has suffered a great change.

The Count Sternberg, author of a splendid work, the "Botanical and Geological Flora," of the Ancient world,

* Plants with one Cotyledon, as wheat, Indian corn, and the grasses.

+ Polypodies and Brakes.

Horsetails. The scouring rush is a species.

Ground-pine or Club-mosses. The ground-pine, employed in dressing churches for Christmas, is an example.

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