VARYING DENSITY OF ATMOSPHEKE. 



Are we then to infer, that the height of the atmosphere is 

 really not more than 5 miles ? We have a thousand evidences to 

 the contrary. The height of the summit of the mountain called 

 Dhwalagiri, one of the Himalaya chain, has been ascertained 

 to be 28000 feet, and clouds are seen suspended in the air far 

 above it. The atmosphere therefore extends to a height far above 

 26000 feet. 



14. This height of 5 miles is that which would limit the atmos- 

 phere, if air were such a fluid as water, so that stratum might be 

 heaped upon stratum to any height, without producing any com- 

 pression in the lower strata by the effect of the weight of the 

 superior strata. Air, however, is not such a fluid. It is, as has 

 already been shown,* compressible without limit, and not only 

 compressible but expansible. The air around us which composes 

 the lowest stratum of the atmosphere, is compressed by the entire 

 weight of the series of strata of air which are above it, and this 

 weight, as has been already shown, amounts to 15 Ib. upon a 

 square inch of surface. Now, if any portion of this air be sub- 

 jected to double that pressure, it will be contracted into half its 

 bulk, and will consequently have twice its density ; and if, on the 

 other hand, it be relieved of half the pressure, it will expand into 

 twice its bulk, and will consequently have only half the density. 

 In a word, the state of air as to density will depend upon the 

 pressure to which it is subjected. If that pressure be augmented 

 or diminished, the density of the air will be augmented or 

 diminished in exactly the same proportion. 



15. Air being therefore elastic, and consequently indefinitely 

 compressible and expansible, it follows that, as we ascend in the 

 atmosphere from stratum to stratum, the density must be con- 

 tinually diminished, because the quantity of air above each 

 successive stratum, being continually less, the weight pressing on 

 the strata is continually less, and consequently the density must 

 be proportionally less. 



Hence it is apparent that the actual height of the atmosphere 

 must be vastly greater than five miles. If the atmosphere, in 

 ascending, were imagined to be resolved into a number of layers, 

 each of which would contain the same weight of air, these layers 

 would increase in thickness in ascending. Thus, if the lowest layer 

 were 10 feet thick, a layer at such a height that half the entire 

 atmosphere was below it, would be 20 feet thick, because being 

 subject to only half the pressure it would have only half the density, 

 and would therefore occupy twice the bulk . In like manner, a layer 

 at such a height as would leave three-fourths of the atmosphere 



* Yol. ii., p. 5. 



105 



