LETTEE X. 



ON THE TEMPERATURE OF THE HIGHER LAYERS 

 OE THE AIR. 



Our atmosphere is, as I have already shown 

 you, but very little warmed by the direct action 

 of the solar rays. But the lowest stratum of air 

 shares the temperature of the ground : by this 

 heat it is expanded, and becoming lighter, must 

 mount (just as you see taking place with the 

 smoke of a chimney, and the air surrounding a 

 candle-flame), to the higher regions ; allowing the 

 colder and heavier air to press into the position 

 which it leaves. These upward currents of air 

 convey the heat of the soil to considerable heights, 

 suffering, however, a gradual reduction of tem- 

 perature. This cooling, in clear still air, is 

 nearly proportional to the height, and for five 

 hundred feet of elevation amounts to about 1° 

 Cent. (l°-8 Eahr.) 



These gradual depressions of the temperature 

 of the air depend much less on an actual loss of 

 heat, since it loses its heat by radiation extremely 

 slowly, than on the lessening of the pressure : 

 This latter effect is due to the increase of height, 



