THE ATMOSPHERE. 273 



DENSITY OF THE ATMOSPHERE. 



Height above the sea in miles. Volume. 



1 



2.705 2 



5.41 4 



8.115 8 



10.82 16 



13.525 32 



16.23 64 



At a height of 2.705 miles (11,556 feet) the atmosphere is of 

 half density, by calculation, or 1 volume is expanded into 2, 

 and the barometer would stand at 15 inches ; the density is 

 again halved for every 2.7 miles additional elevation. From 

 calculations founded on the phenomena of refraction, the at- 

 mosphere is supposed to extend, in a state of sensible density, 

 to a height of nearly 45 miles. It is certainly limited, but 

 whether by the cold prevailing in its higher regions, which may 

 liquefy or even solidify the aerial particles, or from their ex- 

 pansibility having a natural limit (page 18), is uncertain. The 

 atmospheric pressure also varies at the same place, from the 

 effect of winds and other causes, which are not fully understood. 

 Hence, the use of the barometer as a weather glass ; for wet 

 and stormy weather is generally preceded by a fall of the 

 mercury in the barometer, and fair and calm weather by its 

 rise. 



The temperature of the atmosphere is greatest at the earth's 

 surface, and has been observed to diminish one degree for every 

 352 feet of ascent, in the lower strata. It is believed, however, 

 that the progressive diminution is less rapid at great distances 

 from the earth. But at a certain height, the region of per- 

 petual congelation is attained even in the warmest climates ; the 

 summits of the Andes, which rise 21,000 feet, being perpetually 

 covered with snow under the equator. The line of perpetual 

 congelation, which has been fixed at 1 5,207 feet at latitude, 

 descends progressively in higher latitudes, being 3,818 feet at 

 60, and only 1,016 feet at 75. The decrease of temperature 

 with elevation in the atmosphere is ascribed to two causes. 

 1. To the property which air has of becoming cold by expan- 

 sion, which arises from an increase of the latent heat of air, 

 like that of steam, with rarefaction (page 54.) The actual 



