ON DEW, &c. 227 



furnishes it, below what its heat had been in 

 the preceding day, and as one part of this de- 

 pression, the general cooling of the atmosphere, 

 is much more considerable on the plain than on 

 the hill, moisture must necessarily be deposited 

 more copiously in the former than in the latter 

 place. If the greater agitation of the atmo- 

 sphere, and the less quantity of moisture, during 

 clear weather, in its higher region than in the 

 lower, be added, it may readily be inferred, 

 that dew shall sometimes be altogether wanting 

 on a hill, though abundant on a plain at its 

 foot, agreeably to what has been actually ob- 

 served by Mr. Jefferson. 



IX. The leaves of trees often remain dry 

 throughout the night, while those of grass are 

 covered with dew. As this is a similar fact to 

 the smallness of dew on hills, I shall in account- 

 ing for it do little more, than enumerate the 

 circumstances on which it depends. 



1. The atmosphere is several degrees warmer 

 near the upper parts of trees on dewy nights, 

 than close to the ground. 2. The air in the 

 higher situation is more agitated, than that in 

 the lower. 3. The air at a little distance from 

 the ground, from being nearer to one of its 

 sources of moisture, will on a calm evening 

 contain more of it, than that which surrounds 

 the leaves of elevated trees. 4. Only the leaves 



