ON DEW, &c. 257 



third, the difference was 6. An analogous 

 fact is mentioned by Gersten, who says, that an 

 horizontal surface is more abundantly dewed, 

 than one which is perpendicular to the ground. 

 IV. The covering of snow, which countries 

 in high latitudes enjoy during the winter, has 

 been very commonly thought to be beneficial to 

 vegetable substances on the surface of the earth, 

 as far as their temperature is concerned, solely 

 by protecting them from the cold of the atmo- 

 sphere. But were this supposition just, the 

 advantage of the covering would be greatly cir- 

 cumscribed ; since the upper parts of trees and 

 of tall shrubs are still exposed to the influence 

 of the air. Another reason, however, is fur- 

 nished for its usefulness, by what has been said 

 in this Essay ; which is, that it prevents the oc- 

 currence of the cold, which bodies on the earth 

 acquire, in addition to that of the atmosphere, 

 by the radiation of their heat to the heavens 

 during still and clear nights. The cause, in- 

 deed, of this additional cold, does not constantly 

 operate ; but its presence, during only a few 

 hours, might effectually destroy plants, which 

 now pass unhurt through the winter. Again ; 

 as things are, while low vegetable productions 

 are prevented, by their covering of snow, from 

 becoming colder than the atmosphere in con- 

 sequence of their own radiation, the parts of 



