172 ESSAY 



the greatest cold. They were all, however, in- 

 ferior in this respect to bodies of the first class. 



Solid bodies, having a surface exposed to the 

 sky, of at least 25 inches square, formed a third 

 class, on which such experiments were made. 

 The particular substances of this description, 

 subjected to trial, were glass, brick, cork, oak- 

 wood, and wax ; all of which were, likewise, 

 found inferior to the filamentous substances. 

 From these last experiments it follows, that 

 when a glass bulb of a thermometer is applied 

 at night to a body exposed to a clear sky, the 

 temperature exhibited by the instrument will 

 not be accurately that of the body in question, 

 except the disposition of the latter to become 

 cold, in such a situation, be the same as that of 

 glass. An example of this- fact has been given 

 in this Essay*. 



My principal experiments, however, of this 

 kind were made with snow. 



On the 25th of January 1813, the ground 

 being then covered with snow about an inch 

 deep, I went to my usual place of experiment 

 in the country ; but, during 8 hours that I at- 

 tended to my thermometers, the whole sky was 

 constantly overcast with clouds. The atmo- 

 sphere was, for the greater part of that time, 

 very still, and a thermometer on the snow was 

 * Page 164. 



