198 ESSAY 



substances by lateral objects. 3. The heat com- 

 municated to the same substances by the air. 

 4. The heat which is evolved, during the con- 

 densation of the watery vapour of the atmo- 

 sphere into dew. 



The extent of the effect of all these checks 

 upon the production of cold, by the nightly 

 radiation of heat from bodies on the surface of 

 the earth, cannot, in the present state of our 

 knowledge, be properly estimated ; but facts 

 show that, notwithstanding their operation, the 

 cold originating in this source must be often 

 very considerable. 



1. Mr. Wilson once observed a difference of 

 16, from this cause, between the temperatures 

 of snow and of air. In taking the latter tem- 

 perature, however, he employed a naked ther- 

 mometer, on which account, in consequence of 

 what has already been mentioned by me, about 

 2 are to be added to the 16 noted by him, in 

 order to obtain the real difference between the 

 heat of the snow and the air at that time*. 



2. If Mr. Wilson, as was formerly said, had 



* As bright metals, when suspended in the air, and exposed 

 to a clear sky on a calm night, become colder than the sur- 

 rounding atmosphere, a thermometer covered with metalled 

 paper, and placed in the circumstances which have been just 

 mentioned, will mark a temperature less than that of the air 

 near to it. But, as the difference must be small, and as I 

 know of no way to estimate it accurately, I have hitherto 

 always neglected to consider it. 



