ON DEW, &c. 239 



atmosphere*. Gersten, moreover, describes 

 several appearances himself, which refute his 

 opinion. He mentions, for example, that the 

 higher parts of shrubs are more dewed than the 

 lower j that metallic plates, placed horizontally 

 in the air, are as much dewed on their superior, 

 as on their inferior surfaces ; and that convex 

 and cylindrical bodies, suspended in the air, the 

 latter having a position parallel to the horizon, 

 are dewed only on their upper parts. 



The principal reason given by Dufay for the 

 rising of dew is, that it appears more early on 

 bodies near to the earth, than on those which 

 are at a, greater height. But this fact readily 

 admits of an explanation on other grounds, that 

 have already been mentioned. 1. The lower 

 air, on a clear and calm evening, is colder than 

 the upper, and will, therefore, be sooner in a 

 condition to deposit a part of its moisture. 



2. It is less liable to agitation than the upper. 



3. It contains more moisture than the upper, 

 from receiving the last which has risen from 

 the earth, in addition to what it had previously 



* I have, in like manner, observed, on a cloudy night, a 

 piece of glass, laid over an earthen pan containing water and 

 placed upon the ground, to be wet on its lower side, while 

 the upper was dry j the glass being, in this situation, suf- 

 ficiently cold to condense the vapour of water heated by the 

 earth, but not enough so to condense the watery vapour of 

 the atmosphere, 



