226 ESSAY 



offered on the greater warmth of hills at night, 

 in a certain state of weather, are strictly ap- 

 plicable to those only, which are insulated, and 

 of inconsiderable lateral extent ; and it is upon 

 such chiefly, if not solely, that this phenomenon 

 has been observed. The superiority of the cold 

 of a low plain, from radiation, over that of a 

 wide expanse of hilly ground, will, for obvious 

 reasons, be less; and no superiority of this kind 

 will probably exist in the former situation, when 

 the high ground is not only extensive, but flat 

 on the top, forming what is called a table-land ; 

 unless indeed, which seems to be actually the 

 case, the air of such an elevated country should 

 be commonly more agitated, than that of lower 

 places equally level. 



An explanation may be now easily given of 

 an observation by Mr. Jefferson of Virginia *, 

 which, however, had also been made by Ari- 

 stotle t, and Plutarch J, that dew is much less 

 copious on hills, than it is upon plains. For 

 allowing, at first, the surface of the ground to 

 be in both situations equally colder than the air 

 which is near to it ; still, as the production of 

 dew must be in proportion to the whole de- 

 pression of the temperature of the air which 



* Notes on Virginia, p. 132. f Meteor. Lib. 1 . c. x. 

 J De Primo Frigido. 



