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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 principle we may account for the following appearance. 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 rise no more. 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.
The height of the atmosphere; or, the elevation to which it extends beyond the surface of the earth.
The height of the atmosphere is considered, by many writers and lecturers on this subject, as a point fully determined, and is treated as familiarly as the height of the Andes or the Alps, or of Mount Etna or Mount Blanc. But the height of the atmosphere has never yet been fully ascertained, and, it is probable, will never be accurately determined. If, indeed, the air were of an equal density, from the surface of the earth to the top of the atmosphere, its height might be easily determined; for it is found by experiment, that the weight of a column of air extending to the top of the atmosphere, is equal to the weight of a column of water of the same base and 32 feet high. Supposing water to be 840 times heavier than air—multiply 840 by 32 feet, and the product will be 26,880 feet, or 5 miles and 160 yards for the height of the atmosphere, were its density at every elevation exactly the same as at the surface of the earth. But we know that the density of the air decreases and is more rarefied and expanded the higher we go; and, from other considerations we know that it extends far beyond the limit now stated; so that this calculation can afford us no accurate idea of the height to which the atmosphere extends.
Another method, therefore, of determining this point was devised by philosophers, which approaches nearer to the truth. It is found by observation, that the sun is about eighteen degrees below the horizon before twilight comes to an end in the evening. Now, twilight is caused by the rays of the sun being refracted and reflected from the higher parts of the atmosphere to the earth; otherwise, we should be involved in total darkness at the moment the sun descended below the horizon. From this circumstance, the height of the highest part of the atmosphere which is capable of refracting the rays of light may be determined.
Let Fab (fig. 5) represent the horizon of an observer at A; s D, a ray of light falling upon the atmosphere at D, and making an angle, s D B, of 18° with the horizon; the angle s D A will then be 162°. From the centre c, draw c D, and it will be perpendicular to the reflecting particles at D, and will likewise bisect the angle s D A. In the right-angled triangle c D A, the angle c D A is equal to 81c; or, if we allow for refraction, 81° 30', A c, the radius, or half-diameter of the earth, is nearly