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CILAPTER IV.

The elasticity of the air, and the phenomena it explains.

The atmosphere is that ocean of air which surrounds our globe on all sides, and in which we live and breathe. We are plunged into the bottom of this vast aërial sea, as the fishes are plunged into the depths of the ocean. Before we were brought into the world, we were furnished with a diaphragm and lungs, with carti.. lages, ribs, and muscles, to enable us to draw in this vital fluid. The first rush of the air into the lungs, and the cries which accompany it, announce life and sensation. More than a hundred muscles are employed in drawing in and expelling this aërial fluid ; and this operation is continued, without intermission, till death. In this element we pass the whole of our existence, from the cradle to the

grave; it surrounds us wherever we go, whether on sea or land, and almost all our enjoyments depend on its benign agencies. This element, however, is impalpable to our senses.

By its transparency, it escapes our ocular inspection; by its thinness, it eludes our grasp ; it cannot be perceived by our smell or taste, nor even by our organs of hearing, unless when it is in a state of tremor and agitation.

But we are fully assured, in numerous instances, that the powers of nature may be in complete existence, though they are imperceptible to every organ of sensation ; and hence we ought to guard against an error common both to the vulgar and to philosophers, that "the things which we cannot see, have no real existence.” The atmosphere, though invisible, is one of the most important and essential constituents of our terrestrial habitation. We could live for a few days without food, or drink, or sleep; we could pass weeks and months without the light of the sun, or the glimmering of a star; but if we are deprived only for a few minutes of the vital air, the lungs refuse to play, the heart ceases to beat, the blood stagnates in the artéries and veins; we faint, we sicken, and die. The powers of the animal machine are broken; the thoughts and perceptions vanish ; the dust returns to its kindred dust, and the spirit returns to God who

it. We shall now chiefly attend to the illustration of the elasticity of the atmosphere. By the elasticity of the air, is meant that property by which it contracts itself into less space, when an additional pressure is laid upon it, and by which it recovers its former dimensions when the pressure is removed. When I take a piece of whalebone, or a watch-spring, and bring the two ends together, as soon as the force thus employed is removed, the spring returns to its former position. In such cases, we say that the body is elastic. When I take a small quantity of wool into my hand, and compress it, upon opening my hand, it recovers its former bulk, by the natural spring of its fibres; and hence we conclude that this substance possesses a certain degree of elasticity. In like manner, if I take a bladder and fill it with air, and apply a force to the sides of it, so as to compress it into a smaller space, when the force is removed it immediately expands, and fills the same space as before, which clearly proves that the air contained in the bladder is of an elastic nature.

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In consequence of this elastic property, the air always endeavours to expand itself, and to occupy more space. This is proved by taking a bladder, containing only a small quantity of air, tying its neck close, so as to prevent the escape of the air, and then placing it under the receiver of an air-pump. So long as the bladder is exposed to the pressure of the atmosphere, it will remain in the same state ; but, when the air is exhausted from the receiver, and the external pressure removed, the side of the bladder, which was flabby and lax, stretches itself out, swells, and becomes tight, being raised by the elastic power. And, if the air be again let into the receiver, the bladder returns to its former shape. By a similar experiment it is shown, that the expansive power of the small quantity of air in the bladder is capable of raising leaden weights of a considerable size. In consequence of this strong elastic power of the air, a person, by blowing into a pipe connected with several bladders, has been able sensibly to raise a mill-stone, which was placed upon the bladders; which demonstrates the very strong expansive power of a very small quantity of air.

On the same principle, were a bladder, containing a very small quantity of air, taken to the higher regions of the atmosphere, it would gradually expand the higher it was carried, in consequence of the pressure of the atmosphere being gradually diminished, till, at length, it would burst the bladder, by the expansive force with which it is endued. In like manner, heat increases the elasticity of air. If a bladder, containing a small quantity of air, be placed before a strong fire, the small portion of air it contains will expand, till the bladder appears quite full, and ready to burst. There is another striking experiment which demonstrates this elastic force of the air. When a thin bottle with flat sides is firmly corked, so as to prevent the included air from escaping, is placed under the receiver of an air-pump, and the air exhausted, the spring of the air within it will dilate with so much violence, as to break the bottle to pieces. In like manner, were the pressure of the external air completely removed from our bodies, and the escape of the internal air prevented, the elastic force of the air within us would immediately tear the lungs and other vessels to pieces, force the blood through the arteries and veins, and put an end to all the functions of the animal machine. If an animal, as a cat, mouse, or bird, be put under a receiver, and the air exhausted, the animal will be at first oppressed as with a great weight, then grow convulsed, their bodies will swell, and if they are allowed to remain only for a few minutes, they inevitably die. Were we to take a shrivelled apple, and put it under the receiver of the air-pump, and exhaust the air, the skin will gradually swell as the pressure of the air diminishes, the wrinkles will be filled up, and the apple will appear as if freshgathered. When the air is let in, it returns again to its former withered state. The effect now stated, is owing to the elasticity of the air in the inside of the apple, which expands when the atmospheric pressure is removed.

From a variety of experiments it is demonstrated, that the spring of the air is equal to its weight, and produces the same effects as its pressure; for, action being equal to re-action, the force which the elasticity of the air exerts, in endeavouring to expand itself, is equal to the force with which it is compressed, just as it is in the spring of a watch, which exerts no force, but in proportion as it is wound up. If a quantity of air, therefore, is included in a vessel, and is of the same density with the surrounding air, its pressure against the sides of the vessel is equal to that produced by the external atmosphere. Hence it is that we can break a square glass bottle, either by the

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