ATMOSPHERE (Gr. atmos, vapor, ipliaira, sphere) is the name applied to the gaseous envelope which surrounds the earth. The existence of an A. is to us a matter of vital importance. We owe to its influence the possibility of animal and vegetable life, the modifying and retaining of solar heat, the transmission of sound, the grai'lual shading of day into night, the disintegration of rocks, and the occurrence of weather phenomena. In consequence of the action of gravity, the A. assumes the form of a spheroidal stratum concentric with the earth, and presses heavily on its surface. It exhibits, in common with all fluid bodies, the usual characteristics of hydrostatic pressure, but its internal condition differs from that of a liquid inasmuch as its particles repel] each other, and can only be held in proximity by external force. From this circumstance, it follows that the volume of any portion of air varies much more under the influence of external pressure than that of an equal volume of water; hence, the stratum of air nearest the earth is denser than strata in the upper regions, where, from their being subjected to the weight of a smaller mass of superincumbent air, the repulsive force of the particles has freer play.
That air possesses. weight, is illustrated by the following simple experiment: If a hollow glass globe of 5 or 6 in. in diameter be weighed first, when 'filled with air, and then, after the air has been extracted from it by means of the air-pump, it will, when thus exhausted, weigh sensibly less than it did before, and the difference of the two results will represent the weight of the quantity a air which has been withdrawn. It has been determined by Biot and Arago that 100 cubic in. of dry air, when the barometer is at 30 in., and the thermometer at 60° Fai.. weigh 31.074 grains. The law of Archim edes (see ARCHIMEDES, PRINCIPLE os'), that a body immersed in a fluid loses a part of its weight equal to the weight of the volume of fluid displaced by it, finds its application in the A. as well as in water. If a glass globe filled with air and closed be suspended at the extremity of the beam of a delicate balance, and be kept in equilibrium by a brass weight at the other extremity, and if the whole be then placed under the receiver of an air-pump, and the air extracted, the equilibrium previously existing in air will be dis turbed, and the larger body will become the heavier. The reason of this is, that when
first weighed, they each,lose as much of, their own weight as that of the respective vol umes of'air displaced bY diem, and fire therefore, made Imoyant, thought in different de grees, the ball with the larger volume having the greater buoyancy. In a vacuum, they are deprived of this buoyancy, and the larger body, suffering the greater loss, becomes sensibly heavier than the other. In like manner, a balloon filled with heated air or hydrogen gas is lighter than the volume of air displaced by it. It is therefore forced upwards till it reaches a stratum of such density that the weight of the volume of air there displaced by it equals the weight of the balloon itself. In this stratum it will remain poised, or move horizontally with the currents to which it may be exposed.
In endeavoring to determine the form, of the atmospheric envelope, is necessary to bear in mind that, according to the law of tluid-pressure, in order to produce a state of equilibrium at the level of the sea, the pressure of the A. must be equal at that level over the whole of the earth's surface. Gravity acts with less force on the air at the equator than on that at the poles, in consequence of the spheroidal form of the earth. It has there, in addition, to contend with the centrifugal force, which entirely fails at the poles, and which has a tendency to lighten the air by acting contrary to that of gravity. Hence we infer, that in order to produce the same pressure at the level of the sea, the atmospheric height at the equator must be greater than that at the poles, and that the A. must therefore possess the form of an oblate spheroid, whose oblateness is considerably greater than that of the •earth itself. The greater heat at the tropical regions must also have the effect of increasing the oblateness.