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sary 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 také root upon it, and a soil begins to be formed; a cocoa-nut, or the drupe of a pandamus, 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 volcanoes 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 in

habitants 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 revo. lutions, 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 has encroached upon the land; while it is known that many harbours in 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.

LESSON VI.

THE ATMOSPHERE. 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 different ingredients, termed oxygen and nitrogen gas. Of one hundred measures of atmospheric air, twentyone are oxygen, and seventy-nine 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 fame 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 forty-five miles above the earth's surface, and presses on the earth with a force proportioned to its height and density. From experiments made with the barometer it has been ascertained, that it presses with a weight of about fifteen pounds on every square inch of the earth's surface; and, therefore, its pressure on the body of a midddle sized man, is equal to about thirty-two thousand pounds, or fourteen tons avoirdupois, a pressure which would be insupportable, and even fatal, were it not equal on every part, and counterbalanced by the spring of the air within us. The pressure of the whole atmosphere upon the earth is computed to be equivalent to that of a globe of lead, sixty-six miles in diameter; in other words, the whole mass of the air which surrounds the globe, compresses the earth with a force or power equal to that of five thousand millions of millions of tons. This amazing pressure is, however, essentially necessary for the preservation of the present constitution of our globe, and of the animated beings which dwell on its surface. It prevents the heat of the sun from converting water and all other fluids into vapour; and preserves the vessels of all organized beings in due tone and vigour. Were the atmospherical pressure entirely removed, the elastic fluids contained in the finer vessels of men and other

animals would inevitably burst them, and life would become extinct; and most of the substances on the face of the earth, particularly liquids, would be dissipated into vapour. Besides these, the atmosphere possesses a great variety of other admirable properties, of which the following may be mentioned. It is the vehicle of smells, by which we become acquainted with the qualities of the food which is set before us, and learn to avoid those places which are damp, unwholesome, and dangerous. It is the medium of sounds, by means of which knowledge is conveyed to our minds. Its undulations, like so many couriers, run for ever backwards and forwards, to convey our thoughts to others, and theirs to us, and to bring news of transactions which frequently occur at a considerable distance. A few strokes on a large bell, through the ministration of the air, will convey signals of distress, or of joy, in a quarter of a minute, to the population of a city containing a hundred thousand inhabitants. It transmits to our ears all the harmony of music, and expresses every passion of the soul; it swells the notes of the nightingale, and distributes alike to every ear the pleasures which arise from the harmonious sounds of a concert. It produces the blue colour of the sky, and is the cause of the morning and evening twilight, by its property of bending the rays of light, and reflecting them in all directions. It forms an essential requisite for carrying on all the processes of the vegetable kingdom, and serves for the production of clouds, rain, and dew, which nourish and fertilize the earth. In short it would be impossible to enumerate all the advantages we derive from this noble appendage to our world. Were the earth divested of its atmosphere, or were only two or three of its properties changed or destroyed, it would be left altogether unfit for the habitation of sentimental beings. Were it divested of its undulating quality, we should be deprived of all the advantages of speech and conversation ; of all the melody of the feathered songsters, and of all the

pleasures of music: and, like the deaf and dumb, we could have no power of communicating our thoughts but by visible signs. Were it deprived of its reflective powers, the sun would appear in one part of the sky of a dazzling brightness, while all around would appear as dark as midnight, and the stars would be visible at noonday. Were it deprived of its refractive powers, instead of the gradual approach of the day and the night, which we now experience, at sun-rise we should be transported, all at once, from midnight darkness to the splendour of noon-day; and at sun-set should make a sudden transition from the splendour of day to all the horrors of midnight, which would bewilder the traveller in his journey, and strike the creation with amazement. In fine, were the oxygen of the atmosphere completely extracted, destruction would seize on all tribes of the living world throughout every region of earth, air, and sea. .

LESSON VII.

THE WINDS. A Change in the temperature of a portion of air-an increase or a diminution of the quantity of water which it holds in a state of vapour-in short any circumstance which causes it either to contract or expand, destroys the equilibrium among the different parts of the atmosphere, and occasions a rush of air, that is, a wind, towards the spot where the balance has been destroyed. Winds may be divided into three classes ; those which blow constantly in the same direction; those which are periodical; and those which are variable. The permanent winds are those which blow constantly between, and a few degrees beyond, the tropics, and are called trade-winds. On the north of the equator, their direction is from the north-east, varying at times a point or

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