206 



was owing to its own radiation to a clear sky, 

 the temperature of the atmosphere may seem 

 to increase 2, or more, notwithstanding that it 

 has received no real addition. Besides; the 

 heat which is extricated by the condensation of 

 vapour, during the formation of a cloud, must 

 soon be dissipated ; whereas the effect of greatly 

 lessening, or preventing altogether, the appear- 

 ance of a superior cold on the earth to that 

 of the air, will be produced by a cloudy sky r 

 during the whole of a long night. 



Dense clouds, near the earth, must possess 

 the same heat as the lower atmosphere, and will 

 therefore send to the earth, as much, or nearly 

 as much heat as they receive from it by radia- 

 tion. But similarly dense clouds, if very high, 

 though they equally intercept the communica- 

 tion of the earth with the sky, yet being, from 

 their elevated situation, colder than the earth, 

 will radiate to it less heat than they receive from 

 it, and may, consequently, admit of bodies on 

 its surface becoming several degrees colder than 

 the air. In the first part of this Essay, an ex- 

 ample was given of a body on the ground be- 

 coming at night 5 colder than the air, though 

 the whole sky was thickly covered with high 

 clouds*. 



* Mr. Prevost of Geneva, in his work on Radiant Heat, 

 p. 382, has already in this way, conjecturally, accounted for 

 the effect of clouds, in diminishing, at night, the cold of the 



