220 ESSAY 



atmosphere to the colder earth. Mr. Leslie *, in- 

 deed, ascribes this effect to the descent of cold 

 air from the higher regions of the atmosphere ; 

 but if this were just, a less cold ought to be 

 found, on a clear and still night, in the lower 

 than in the higher strata, which is contrary to 

 the uniform results of numerous experiments 

 by Mr. Pictet and Mr. Six. Winds too, which 

 produce such a mixture, always lessen the noc- 

 turnal decrease of temperature in the lowermost 

 part of the atmosphere. 



Having thus shown, that air is capable, both 

 of absorbing heat, which is radiated into it, and 

 of radiating heat, which had before formed a 

 part of its temperature, I proceed to apply the 

 knowledge of these facts, to the explanation of 

 the phenomenon observed by Mr. Pictet and 

 Mr. Six. 



This phenomenon occurs on those nights 

 only, which permit bodies, on the surface of 

 the earth, to become cold by radiating their 

 heat to the heavens. On other nights, when 

 .bodies, thus situated, were not colder than the 

 air, I have observed the atmosphere, within the 

 limits of 9 feet from the ground, the boundary 

 of my own experiments, to decrease a little in 



* On Heat and Moisture, p. 11, and 132. 



