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run too rapidly into the sea, or the particles in question do not previously settle in some lake through which the rivers pass, the mud is deposited on the sides of their mouths, forming low grounds, by which the shores are prolonged and eneroach upon the sea; and when the waves, by casting up sand upon them, assist in their increase, whole provinces are created, capable, from their rich soil, of yielding, in the highest degree, to the support of man, and of being made the seats of wealth and civilization. It has been concluded, with reason, that the greater part of Lower Egypt owes its formation to the alluvial matter brought down by the Nile, aided by the sand cast up by the sea. The Delta of the Rhone is undergoing a similar augmentation, and it would appear that the arms of that river have, in the course of 1800 years, become longer by three leagues ; and that many places which were once situated on the brink of the sea, or of large pools, are now several miles distant from the water. In Holland and Italy, the Rhine and the Po, since they have been banked up by dykes, raise their beds and push forward their mouths into the sea with great rapidity. Such, indeed, has been the increase of new land formed by the latter, that the city of Adria, which there is no doubt was, at a very remote date, situated on the coast of the Adriatic, is now more than fifteen miles distant from the nearest part of it. At the same time, the river has, in consequence of embankments made to confine it, been so much raised in the level of its bottom that the surface of its waters is higher than the roofs of the houses in Ferrara ; and the Adige and the Po are higher than the whole tract of country lying between them. The same cause produces the alterations perceived to be taking place in many of those lakes which are traversed by rivers. The matter brought down by the rivers easily settles in the still waters of the lakes, and the necessary result is, that the basins of the latter are gradually undergoing a diminution. Lake Erie, one of the vast bodies of water in North America, is every year becoming shallower from the influx of pebbles and earth, and the constant accumulation of reeds and shells; and the diminution of the beautiful lake of
Geneva is also said to have been considerable within the memory of man.
The formation of new islands constitutes another distinct and interesting class among the changes to which the surface of the globe is subject. Those which have been raised up by volcanic agency are comparatively few; but those of coral, which owe their origin to marine insects, (of the class of zoophytes or plant animals,) are innumerable. Of the different coral tribes, the most abundant is that named the madrapore. It is most common in the tropical seas, and decreases in number and variety towards the poles; it surrounds, in vast rocks and reefs, many of the islands of the South Sea and Indian Ocean, and increases their size by its daily growth. The coasts of the islands of the West Indies, of those of the east of Africa, and the shores and shoals of the Red Sea, are encircled with rocks of coral. Several navigators have furnished us with accounts of the curious manner in which these formations take place; the following is extracted from Capt. Basil Hall's narrative of his voyage to the Loo-Choo islands :
“The examination of a coral reef, during the different stages of one tide, is particularly interesting. When the tide has left it for some time, it becomes dry, and appears to be a compact rock exceedingly hard and rugged; but as the tide rises, and the waves begin to wash over it, the coral-worms protrude themselves from holes which were before invisible. These animals are of a great variety of shapes and sizes, and in such prodigious numbers, that, in a short time, the whole surface of the rock appears to be alive and in motion. The most common of the worms at Loo-Choo is in the form of a star; with arms from four to six inches long, which are moved about with a rapid motion, in all directions, probably to catch food. Others are so sluggish, that they may be mistaken for pieces of the rock, and are generally of a dark colour, and from four to five inches long, and two to three round. When the coral is broken, about high-water, it is a solid hard stone; but if any part of it be detached at a spot which the tide réaches every day, it is found to be full of worms of
different lengths and colours; some being as fine as a thread, and several feet long, of a bright yellow, and sometimes of a blue colour; others resemble snails, and some are not unlike lobsters in shape, but soft, and not above two inches long. The growth of the coral appears to cease when the worm is no longer exposed to the washing of the sea. Thus, a reef rises in the form of a cauliflower, till its top has gained the level of the highest tides, above which the worm has no power to advance, and the reef, of course, no longer extends itself upwards. The other parts, in succession, reach the surface, and there stop, forming, in time, a level field with steep sides all round. The reef, however, continually increases, and being prevented from going higher, extends itself laterally in all directions. But this growth being as rapid at the upper edge as it is lower down, the steepness of the face of the reef is still preserved. These are the circumstances which render coral reef so dangerous in navigation ; for, in the first place, they are seldom seen above water; and in the next, their sides are so steep, that a ship's bows may strike against the rock, before any change of soundings has given warning of the danger."
Another navigator gives the following succinct account of the manner in which, after being raised up,
the coral islands gradually acquire a soil and vegetation “ To be constantly covered with water seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark; but the coral, sand, and other broken remnants thrown up by the sea, adhere to the rock, and form a solid mass with it, as high as the common tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their adhesive property, and remaining in a loose state, form what is usually called a Key, upon the top of the reef. The new bank is not long in being visited by sea-birds; salt plants take root upon it, and a soil begins to be formed ; a cocoa-nut, or the drupe of a pandanus, is thrown on shore; land birds visit it, and deposit the seeds of shrubs and trees; every high tide, and still more every gale, adds something to
the bank; the form of an island is gradually assumed; and last of all comes man to take possession.".
The other chief agents in changing the surface of the earth are volcanos and earthquakes. The changes occasioned by the eruptions of the former are very considerable near the seat of action, but they operate over a less extensive field than either of those which have been already mentioned. The principal effect of the issue of subterranean fires is the elevation of the surface of the surrounding country; and the size of the mountains themselves must have been prodigiously increased by the matter thrown up during successive eruptions. Earthquakes appear to be brought about by the same causes as volcanic eruptions : but their action is much more tremendous than that of the latter. They are frequently accompanied by loud subterraneous noises, and are sometimes so violent, that the ground heaves up, and undulates like an agitated sea. They are felt, almost at the same instant, over a most astonishing extent; though happily, compared with this extent, their destructive ravages are confined within a small range. In those parts, which appear to be near the centre of their action, the most calamitous effects sometimes occur: whole cities are destroyed, and their inhabitants buried beneath the ruins; springs are stopped, and others gush out in new places; fissures are made in the earth; and enormous masses of rock and other materials sink down, or are detached from the mountains.
Such are the principal changes, which the surface of the globe is now undergoing. But great as they are, they could not have brought about those grand revolutions, which formerly visited the earth, and in which such multitudes of the animal race were consigned to destruction. The whole of them are insufficient to alter, in any perceptible degree, the level of the sea, still less to have occasioned an overwhelming of the land by that element. Some philosophers have endeavoured to prove that a gradual and general lowering of the level of the sea takes place, and have appealed to certain observations, which, if correct, tend to establish
the fact of a diminution of the waters along the northern shores of the Baltic. But it must not be forgotten, that though in some places the ocean has retired, or sunk in level, in others it hạs encroached upon the land; while it is known that many harbours of the Mediterranean have preserved exactly the same level since the time of the ancients. It is plain, therefore, that all variations upon the coasts of the ocean are merely of a local kind, and that if the different accounts are balanced, we must arrive at the conclusion, that the general volume of the ocean, and perhaps even its superficial extent, suffer neither increase nor diminution.
Library of Useful Knowledge.
The atmosphere is one of the most essential appendages to the globe we inhabit, and exhibits a most striking proof of Divine skill and omnipotence. It is now ascertained to be a compound substance, formed of two very different ingredients, termed oxygen and nitrogen gas. Of 100 measures of atmospheric air, 21 are oxygen, and 79 nitrogen. The one, namely, oxygen, is the principle of combustion. It is absolutely necessary for the support of animal life, and is the most powerful and energetic agent in nature. The other (nitrogen) is altogether incapable of supporting either Name or animal life. But the term atmosphere is also applied to the whole mass of fluids, consisting of air, vapours, electric fluid, and other matters which surround the earth to a certain height. . This mass of fluid matter gravitates to the earth, revolves with it in its diurnal rotation, and is carried along with it in its course round the sun every year. It has been computed to extend about 45' miles above the earth's surface, and it presses on the earth with a force proportioned to its height and density. From experiments made with the barometer it has been ascertained, that