ON DEW, &c. 223 



ascends ; and this conclusion is so far con- 

 firmed, by the observations of Mr. Pictet and 

 Mr. Six taken together, that the increase of 

 temperature is found to be greater in a given 

 space very near to the earth, than in an equal 

 space more remote from it. 



To return to the immediate object of this 

 article, the fact is certain, whatever may be 

 thought of my explanation of it, that, in every 

 clear and still night, the air near to the earth is 

 colder than that which is more distant from it, 

 to the height at least of 220 feet, this being the 

 greatest to which Mr. Six's experiments relate. 

 If then a hill be supposed to rise from a plain, 

 to the height of 220 feet, having upon its 

 summit a small flat surface covered with grass ; 

 and if the atmosphere, during a calm and serene 

 night, be admitted to be 10 warmer there, than 

 it is near the surface of the low ground, which 

 is a less difference, according to the observa- 

 tions of Mr. Six, than what sometimes occurs 

 in such circumstances, it is manifest, that, 

 should both the grass upon the hill, and that 

 upon the plain, acquire a cold of 10 by radia- 

 tion, the former will, notwithstanding, be 10 

 warmer than the latter. 



But the equality here supposed to be in the 

 cold acquired by grass, in two such situations, 



