214 ESSAY 



by passing over another part of the board. The 

 reason then of the lee side of the raised board 

 being often colder than the windward is ob- 

 vious. 



VII. There is a remark by Theophrastus *, 

 which has been confirmed by other writers, that 

 the hurtful effects of cold occur chiefly in hollow 

 places. If this be restricted to what happens 

 on serene and calm nights, and it does not, I 

 believe, hold true in any other circumstances, 

 two reasons from different sources are to be 

 assigned for it. The first is, that the air being 

 stiller in such a situation, than in any other, the 

 cold, from radiation, in the bodies which it 

 contains, will be less diminished by renewed 

 applications of warmer air ; the second, that 

 from the longer continuance of the same air in 

 contact with the ground, in depressed places 

 than in others, less dew will be deposited, and 

 therefore less heat extricated during its forma- 

 tion. It will be seen in the last part of this 

 Essay, that, in the East Indies-, depressions in 

 the earth are artificially made, for the purpose 

 of increasing the cold, which appears in serene 

 nights. On this subject, however, it is to be 

 observed, that if the depressed or hollow places 

 be deep, in proportion to their horizontal ex- 

 tent, a contrary effect must follow ; as a case 



* Lib. v. c. xvi. 



