242 ESSAY 



must yet be admitted, that some of the moisture, 

 which forms during clear and still weather, on 

 bodies situated upon or near its surface, is in 

 most cases to be attributed to this source ; 

 since, in my experiments, substances on the 

 raised board became much later moist than 

 others on the ground, though equally cold with 

 them. The quantity from this cause, however, 

 can never be great. For in the first place, until 

 the air be cooled by the substances attractive 

 of dew, with which it comes in contact, below 

 its point of repletion with moisture, it will be 

 always in a condition to take up that which has 

 been deposited upon grass, or other low bodies, 

 by warm vapour emitted by the earth ; just as 

 the moisture formed upon a mirror by our breath 

 is, in temperate weather, almost immediately 

 carried away by the surrounding air. Accord- 

 ingly ; I have sometimes, in serene and still 

 weather, observed dew to appear sparingly 

 upon grass in the shade, several hours before 

 sunset, and to continue in nearly the same 

 quantity till about sunset, when it would in- 

 crease considerably, at the time that the same 

 fluid began to show itself on the raised board. 

 In the second place; though bodies situated 

 on the ground, after they have been made suf- 

 ficiently cold, by radiation, to condense the 

 vapour of the atmosphere, will be able to retain 



