212" ESSAY 



This subject is illustrated by the following 

 experiment. On a dewy evening, I depressed 

 into soft garden mould a drinking glass, having 

 a thick flat bottom, until its brim was upon a 

 level with the surrounding earth, and at the 

 same time placed a similar vessel, with its 

 cavity also towards the sky, on the surface of 

 the mould. In the morning, the inside of the 

 depressed glass was entirely dry, while that 

 of the other was dewed. I then applied the 

 bulb of a small thermometer to the inside of 

 the bottom of each vessel, on which I found the 

 heat of that part of the depressed one to be 56, 

 but of the same part of that which stood on 

 the mould only 49 J. At this time the tem- 

 perature of the air was 53. The cause, there- 

 fore, was evident, both of the wetness of the 

 first vessel, arid of the dryness of the second. 



From this source also is to be derived the 

 reason, why the prominent parts of various 

 bodies were observed by Mr. Wilson to be 

 crusted with hoarfrost, while their more re- 

 tired and massy parts were free from it *. 



V. Bodies, exposed in a clear night to the 

 sky, must radiate as much heat to it during the 

 prevalence of wind, as they would do if the air 

 were altogether still. But in the former case, 



* Paper in Phil. Trans. 178O. 



