198 



THE ATMOSPHERE. 



would enter the space in which they were confined, without driving them be- 

 fore it, or otherwise disturbing their arrangement. 



ELASTICITY AND COMPRESSIBILITY OF AIR. 



It will be evident, upon the slightest reflection, that the elasticity of air must 

 be equal to the force which is necessary to confine it within the space it oc- 

 cupies. Let us suppose that A B, fig. 4, is a cylinder, having a piston P fitting 



D 



air-tight at the top ; and let us imagine that this piston P is not acted upon by 

 any external force having a tendency to keep it in its place. If the cylinder 

 below the piston be filled with air, this air will have a tendency, by virtue of 

 its elasticity, to expand into a wider space, and this tendency will be mani- 

 fested by a pressure exerted by the air on all parts of the surfaces which con- 

 fine it. The piston P will therefore be subject to a force tending to displace 

 it and drive it from the cylinder, the amount of which will be the measure of 

 the elasticity of the air beneath it. Now, if this piston be not subject to the 

 action of a force directed inward, exactly equal in amount to the pressure thus 

 exerted by the elastic force of the air, it cannot maintain its position. If it be 

 subject to an inward force of less amount than the elastic pressure, then the 

 latter will prevail, and the piston be forced out. If it be subject to an inward 

 force greater in amount than the elastic pressure, then the former will prevail, 

 and the piston will be forced in, the air being compelled to retreat within a 

 more confined space. In no case, therefore, can the piston maintain its posi- 

 tion, except when it is subject to an inward pressure exactly equal to the elastic 

 force of the air enclosed in the cylinder. 



The property of elasticity renders it necessary that, in whatever state air 

 exist, it shall be restrained by adequate forces of some definite amount, and 

 which serve as antagonist principles to the unlimited power of dilatation which 

 the elastic property implies. In all cases which fall under common obser- 

 vation, air is either restrained by the resistance of solid surfaces, or it is pressed 

 by the incumbent weight of the mass of atmosphere placed above it. It may 

 be asked, however, whether it will not follow from this, that the extent of our 

 atmosphere is infinite : for that, as we ascend in it, the weight of the superior 

 mass of air must be gradually and unceasingly lessened, and therefore the 

 force which resists the expansive principle being removed by degrees, the fluid 

 will spread through dimensions which are subject to no limitation. Although 

 it is undoubtedly true that these considerations lead us justly to conclude that 

 our atmosphere extends to a great distance from the surface, and that tho 

 higher strata of it are attenuated to a degree which not only exceeds the pow- 

 ers of art to imitate, but even outstrips the powers of imagination to con- 

 ceive ; yet still the understanding can suggest a definite limit to this expansion. 

 Numerous physical analogies favor the conclusion that the divisibility of matter 



