238 ESSAY 



The only argument used by the French aca- 

 demicians, in support of their opinion, is, if I 

 understand it rightly, that as much dew is ob- 

 served under an inverted glass-bell, as in any 

 other situation. But admitting, for a moment, 

 this to be true, they would not thus prove, 

 that the ground is the only source of that 

 fluid. 



Gersten was led to think, that dew rises from 

 the earth, by often finding grass, and low shrubs, 

 moistened with it, while trees were dry. Re- 

 specting this fact, I shall add nothing to what 

 I have lately said upon it. But his chief argu- 

 ment is derived from another fact related in the 

 first part of this Essay, which is, that a plate of 

 metal, laid upon bare earth on a dewy night, 

 will remain dry on its upper surface, while it 

 becomes moist on the lower. This also is easily 

 explicable by what has already been mentioned 

 by me. For the lower side of the metal, in con- 

 sequence of the upper being in contact with the 

 air and being exposed to a clear sky, is colder 

 than the earth a little below the surface, and 

 therefore condenses the vapour, which strikes 

 against its bottom ; while the upper side, from 

 being frequently warmer, and never more than 

 a little colder than the air, is for the most part 

 unable to condense the watery vapour of the 



