224 ESSAY 



can seldom exist. For, according to an ob- 

 servation made by Aristotle*, and since fre- 

 quently repeated, the air of high places is much 

 more agitated, than that upon low ground. 

 The frequent renewal, therefore, from this 

 cause, of the air in contact with the grass on 

 the hill, will prevent it from ever becoming 

 much colder than the general mass of the atmo- 

 sphere, at the same height. Consequently, any 

 diminution in this way of the 10 of cold, for- 

 merly supposed to occur there from radiation, 

 must be added to the difference of temperature 

 in the grass in the two situations. 



What has hitherto been said refers only to 

 the occurrences on the very summit of the hill. 

 With respect to its sides, these can be only a 

 little colder than the atmosphere upon a level 

 with them, even in its calmest state. For, in 

 fche first place, they do not enjoy the full aspect 

 of the sky; and, in the second, the air, whicli 

 is cooled by contact with them, will, from its 

 increased gravity, slide down their declivity, 

 and thus make room for the application of new 

 and warm parcels to the same surface. The 

 motion too, thus excited in the air, near to the 

 sides of the hill, must occasion a motion in that 



* Meteor, lib. 1. c. x. 



