ON DEW, &c. 



heat radiated by the sun must, on this day, 

 have been transmitted in considerable quantity 

 through the thickest clouds ; since not only 

 was the earth's surface warmer than the air, 

 but a small body, covered with a substance not 

 readily admitting the entrance of radiant heat, 

 was colder than a similar body which was unco- 

 vered. In like manner, I observed at noon, on 

 the 2nd of January, 1814, during the prevalence 

 of a dense fog, a thermometer placed upon 

 swandown, which was lying upon grass thickly 

 incrusted with hoarfrost, to be 2 warmer than 

 the air, and 1 warmer than the grass*. 



In a calm and serene night, however, when 

 consequently little impediment exists to the 

 escape, by radiation, of the earth's heat to the 

 heavens, and when no heat can be radiated by 

 the sun to the place of observation, an immense 

 degree of cold would occur on the ground, if 

 the following circumstances did not combine 

 to lessen it. 1 . The incapacity of all bodies to 

 prevent, entirely, the passing of heat, by con- 

 duction, from the earth to substances placed 

 upon them. Q. The heat radiated to these 



* Another fact of the same kind, which occurred at the 

 same time, is that, although the temperature of the air was 

 3O, the hoarfrost on trees rapidly decreased, the solid matter 

 of the trees intercepting radiant heat, which had penetrated 

 through 4he fog from the sun, and converting it into heat of 

 temperature. 



