244 OUTLINES OF NATURAL PHILOSOPHY. 



Air resists compression equally in all directions. No 

 limit can be assigned to the space which a given 

 quantity of air would occupy if all compression were 

 removed. 



338. As we ascend from the surface of the 

 earth, the density of the air necessarily diminishes : 

 For, each stratum of air is compressed only by the 

 weight of those above it ; the upper strata are 

 therefore less compressed, and of course less dense, 

 than those below them. 



339. Supposing the same temperature to be dif- 

 fused through the atmosphere, required the law 

 by which the density and elasticity of the air di- 

 minish as the height above the surface increa- 

 ses. 



Let D, D', D", be the densities of three contiguous 

 strata of air, each of a given thickness, so small, 

 that the density of the air in the whole space may be 

 regarded as constant. Let W be the weight of 

 the incumbent air pressing on the lowest of these 

 strata ; W that pressing on the next, and W" 

 that pressing on the third, and let m be a co-effi- 

 cient, such that D = m W ; then, because the 

 densities are as the compressions, D' = m W, and 



Therefore D D' = m (W W), and 

 D' D" - ?7i (W W"). 



2 But 



