ON DEW, &c. 215 



will occur more or less similar to that which 

 existed in some experiments formerly related 

 by me, in which a small portion of grass was 

 surrounded by a hollow cylinder. 



VIII. An observation closely connected with 

 the preceding, namely that, in clear and still 

 nights, frosts are less severe upon hills, than in 

 neighbouring plains*, has excited more atten- 

 tion, chiefly from its contradicting what is com^ 

 monly regarded an established fact, that the 

 cold of the atmosphere always increases with 

 the distance from the earth. This inferior cold 

 of hills is evidently a circumstance of the same 

 kind, with that ascertained 'by Mr. Pictet and 

 Mr. Six, respecting the increasing warmth, in 

 clear and calm nights at all seasons of the year, 

 of the different strata of the atmosphere, in pro- 

 portion as these are more elevated above the 

 earth. As the greater cold of the lower air is 

 the less complicated fact, I shall attempt to ex- 

 plain it in the first place. Mr. Pictet, indeed, 

 furnishes an explanation himself, by ascribing it 

 to the evaporation of moisture from the ground. 

 But to show that this is not just, it need only 

 be mentioned, that the appearance never occurs 

 in any considerable degree, except upon such 



* Theophrastus also remarks, that it freezes less on hills 

 than on plains, but without mentioning, that this happens 

 only on calm and serene nights. Lib. v. c< xx. 



