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direct pressure of the atmosphere, after the air has been extracted from it, or by removing the pressure of the atmosphere, and allowing the elasticity of the air within to exert its expansive force.

It is owing to the elasticity of the air that it is susceptible of dilatation and compression. To what degree air of the same density which it possesses at the surface of the earth is capable of being compressed, has not yet been fully ascertained. Dr. Halley informs us that he has seen it compressed, so as to be sixty times denser than in its natural state. Some have supposed that no bounds can be fixed to the condensation of air. But it some experiments made in London, and by the Academy of Florence, that no force whatever is able to reduce air into eight hundred times less space than that which it naturally possesses at the surface of the earth. It is owing to the power of being artificially condensed, that forcing-pumps produce their effects, and that an air-gun is enabled to discharge a ball to a considerable distance with great violence. The air is forced into a certain compartment of the gun by means of a syringe or condenser, which drives the air in, and suffers none of it to come back till it be sufficiently condensed. When the valve which confines the air is opened, the air by its elastic power rushes in behind the ball, and forces it out of the barrel with great violence. It would be better for mankind, however, that no such instruments were ever constructed. Science ought always to have for its object the construction of instruments and machines which have a tendency to promote the comforts of mankind, not those which may be employed by unprincipled men as weapons of destruction; and, therefore, the construction of this instrument is alluded to merely as an illustration of the powerful effect of the elasticity of the air. Would to God that guns, and cannons, and warlike instruments of all descriptions were for ever unknown among men; that swords were beaten into ploughshares, and spears into pruning-hooks; that nation might no longer lift

up sword against nation, but delight themselves in peace!

The dilatation or expansion of air, in virtue of its elastic force, is found to be very surprising. In several experiments made by the honourable Mr. Boyle, it dilated first into nine times its former space, then into thirty-one times, then into sixty, and then into one hundred and fifty. Afterwards, it was brought to dilate 8,000 times its space, then into 10,000 times; and, at last, into 13,679 times the space it originally occupied, and all this was effected by its own expansive force, without the help of fire, or the principle of heat. Hence it appears that the air we breathe near the surface of the earth is compressed by its own weight into at least the 13,000th part of the space it would occupy in vacuo. And, as it has been found that it may be compressed into a space sixty times less than that which it generally occupies, it follows, that the space which it will possess when most dilated, to that which it occupies when condensed, will be nearly as 820,000 to one! The amazing force of this elastic power of the air, were it properly directed, might be made to act as a strong mechanical power, and there can be little doubt that many of the terrific operations of nature—such as earthquakes, volcanoes, the rising of new islands from the bottom of the ocean, and the detachment of rocks and fragments of mountains amidst the ranges of the Alps, the Andes, and other mountainous regions—are to be ascribed, at least to the partial operation of this power, in combination with other physical agents.

It has been a subject of inquiry among philosophers, whether the elastic power of the air is capable of being diminished or destroyed. Mr. Boyle endeavoured to discover how long air would retain its spring, after having assumed the greatest degree of expansion his air-pump could give it, but he never observed any sensible diminution. Mr. Desaguliers says, that air, which had been inclosed half-a-year in a wind-gun, had lost none of its expansive power;

and Mr. Roberval asserts that he has preserved air in the same manner for sixteen years; and after that period, he observed that its projectile force was the same as if it had been newly condensed.

Various causes have been assigned by philosophers to account for the elasticity of the atmosphere. The general opinion which now prevails is, that it depends upon the latent caloric, or principle of heat, which it contains, and which enables it to retain its fluid form ; and that caloric is the most elastic body in nature. But this is only an explanation of elasticity by an assumption of elasticity. It removes the difficulty only one step farther on, and leaves us still in the dark as to the nature of elasticity, and the reason why caloric is endowed with an elastic power. In this, as well as in many other instances, we must rest contented in resolving it into the will of the Deity, that such a property should be possessed by atmospheric air in order to accomplish some wise and beneficent purposes in the economy of creation.

The elasticity of the air explains a variety of appearances in nature and art. For example, beer or ale, when bottled, contains in it a quantity of air, the elasticity of which is resisted by the pressure of the condensed air between the cork and the surface of the liquid. On removing the cork, the liquid and the air which it contains are relieved from this intense pressure. The liquid itself, not being elastic, is not affected by this; but the elastic force of the condensed air, which has been fixed in it, having no adequate resistance, immediately escapes, and rises in bubbles to the surface, and produces the frothy appearance consequent upon opening the bottle. On a similar prin

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If a man fall into the water, and is drowned, the carcase in a few days rises and floats on the surface. The privation of life, and the stagnation of the fluids, are soon followed by a putrid fermentation which decomposes the body. This fermentation disengages a great quantity of air, which is disseminated among the internal vessels, and as this air cannot escape, the body swells by its expansion, till it becomes specifically lighter than the water, and rises to its surface. But, as the putrefaction goes on, the parts give way, the air escapes, and the body being thus rendered specifically heavier than the water, sinks to

It is likewise by the elastic property of air that fishes are enabled to rise and sink in the water. They are furnished with an air-bladder, which they have the power of contracting or dilating at pleasure. When the fish compresses this bladder, its whole volume becomes less, and it sinks in the water; when the pressure is removed, the air in the bladder instantly expands, and it is enabled to rise to the surface. A variety of instances of a similar kind, illustrative of the elasticity of the air, might be, exhibited ; but instead of dwelling on these, we shall now proceed to another department of our subject.

rise no more.

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