LETTER IX. 



ON THE TEMPERATURE OE THE LOWEST STRATUM 

 OF THE ATMOSPHERE. 



The mean temperature of the lowest stratum of 

 air that rests on the ground is generally the same 

 as that of the soil, however great the variations 

 may be, which you observe from time to time. It 

 was formerly believed that this agreement occurred 

 only in temperate latitudes, and that in the torrid 

 zone the temperature of the ground was lower, 

 and, on the other hand, that towards the polar 

 circles, and beyond them it was higher than that 

 of the air. This assumption was grounded on 

 observations which had been made on springs. 

 But the inference has lost its force since we 

 learned that springs frequently receive their sup- 

 plies in great part from considerable distances, 

 thus bringing heat from the depths below, and 

 cold from the high lands above. It must then 

 at present be considered as certain, that there is 

 this relation between the temperatures of the air 

 and of the soil, that their yearly mean values are 

 the same. 



The direct determination of the mean tempera- 

 ture of the air is attended with greater difficulties 



