ATMOSPHERE is that invisible elastic fluid which surrounds the earth to an un known height, and encloses it on all sides. This fluid is essential to the existence of all animal and vegetable life, and even to the constitution of all kinds of matter whatever, without which they would not be what they are : for by it we literally may be said to live, move, and have our being : by insinuating itself into all the pores of bodies, it becomes the great spring of almost all the mutations to which the chemist and philosopher are witnesses in the changes of bodies. Without the atmosphere no animal could exist ; vege tation would cease, and there would be neither rain nor refreshing dews to mois ten the face of the ground; and though the sun and stars might be seen as bright specks, yet there would be little enjoy ment of light, could we ourselves exist without it. Nature, indeed, and the con stitutions and principles of matter, would be totally changed, if this fluid were want ing.
The mechanical force of the atmosphere is of great importance in the affairs of men, who employ it in the motion of their ships, in turning their mills, and in a thousand other ways connected with the arts of life. It was not till the time of Lord Bacon, who taught his countrymen how to investigate natural phenomena, that the atmosphere began to he investi gated with any degree of precision. Ga lileo introduced the study by pointing out its weight ; a subject that was soon after investigated more completely by Torricelli and others. Its density and elasticity were ascertained by Mr. Boyle and the academicians at Florence. Mari otte measured its dilatibility ; Hooke, Newton, Boyle, and Derham, skewed its relation to light, to sound, and to electri city. Sir I. Newton explained the effect produced upon it by moisture, from which Halley attempted to explain the changes in its weight indicated by the barometer.
The atmosphere, we have said, enve lops the whole surface of the earth, and if they were both at rest, then the figure of the atmosphere would be globular, be cause all the parts of the surface of a fluid in a state of rest must be equally removed from its centre. But as the earth and the
surrounding parts of the atmosphere re volve uniformly together about their axis, the different parts of both have a centri fugal force, the tendency of which is more considerable, and that of the centripetal less, as the parts are more remote from the axis, and hence the figure of the at mosphere must become an oblate sphe roid, since the parts that correspond to the equator are farther removed from the axis than the parts which correspond to the poles. The figure of the atmosphere must also, on another account, represent a flattened spheroid, namely, because the sun strikes more directly the air which encompasses the equator, and is compre hended between the two tropics, than that which pertains to the polar regions : hence it follows, that the mass of air, or part of the atmosphere adjoining to the poles, being less heated, cannot expand so much nor reach so high. Neverthe less, as the same force which contributes to elevate the air diminishes its gravity and pressure on the surface of the earth, highter columns of it about the equatorial parts, other circumstances being the same, may not be heavier than those about the poles. Mr. Kirwan observes, that in the natural state of the atmos phere, that is, when the barometer would, every where at the level of the sea, stand at 30 inches, the weight of the atmos phere at the surface of the sea must be equal all over the globe ; and in order to produce this equality, as the weight pro ceeds from its density and height, it must be lowest where the density is greatest, and highest where the density is least ; that is, highest at the equator and lowest at the poles, with the intermediate grada tions. On this and other accounts, in the highest regions of the atmosphere, the denser equatorial air not being supported by the collateral tropical columns, gradu ally flows over and rolls down to the north and south ; these superior tides have been supposed to consist of hydro gen gas, inasmuch as it is much lighter than any other, and is generated in great plenty between the tropics ; it is also supposed to furnish the matter of the aurora borealis and australis.