208 ESSAY 



difference at night, in very foggy weather, be- 

 tween the temperature of the surface of snow, 

 and that of the air. Several observations by 

 myself tend to confirm that of Mr. Wilson. An 

 instance, however, as was formerly said, oc- 

 curred to me of a difference at night of 9 

 between the temperatures of grass crusted over 

 with hoarfrost, and of air, during a very dense 

 fog. A fact, remarked by Mr. Leslie, respect- 

 ing fogs, serves to explain this apparent ano- 

 maly. For it was found by that philosopher*, 

 from experiments made with his photometer, 

 that in mists and low fogs the diminution of the 

 sun's heat is small, when compared with what 

 occurs, when the sky is obscured by a dense 

 body of clouds ; and it will, I presume, be 

 readily granted, that the same state of the at- 

 mosphere, which allows the heat of the sun to 

 pass copiously, will also give a ready transit to 

 heat radiated by the earth. Now there are 

 several reasons for believing, that the fog, dur- 

 ing which grass was 9 colder than the air, did 

 not ascend far above the ground. 1. The baro- 

 meter had been falling for some days before, 

 and it is a matter of common observation, that 

 great fogs seldom occur, except it be high. 

 2. On the day preceding the observation, the 



* On Heat and Moisture, p. 57. 



