166 ESSAY 



did exist between the two latter, the thermo- 

 meter in the white paper case was commonly 

 lower than the other. 



The estimation of the heat, both of air and 

 of metals, on a dewy night, is liable to errors 

 from other causes. As these, however, are 

 trifling, I shall not mention them, but proceed 

 to state the results of my observations, upon 

 the temperature of metals exposed to the sky at 

 night, though unable to vouch for their entire 

 accuracy. 



Thin bright metallic plates, the least having 

 a surface of 25 square inches, and some of them 

 a surface of more that 100 such inches, were 

 several times observed, while lying on grass 

 which was attracting dew, to be 1 and 2, and 

 once 3, degrees warmer than the air 4 feet 

 above them. At other times, their temperature 

 was the same with that of the air. In both of 

 these cases their upper surfaces were always 

 free from dew. Metals thus situated were, 

 consequently, often much warmer than the 

 grass, which surrounded them. I made no ex- 

 periments on this point, during the nights, on 

 which occurred the greatest instances of cold 

 on grass, relatively to the temperature of the 

 air ; but I found, notwithstanding, during one 

 night, a metal on grass to be 10 warmer than 

 the exposed grass near to it. On two other 



