ON DEW, &c. 



which comes in contact with cold grass, will 

 not, when the surface is level, immediately quit 

 it, more especially, as this air has become spe- 

 cifically heavier than the higher, from a diminu- 

 tion of its heat, but will proceed horizontally, 

 and be applied successively to different parts of 

 the same surface. The air, therefore, which 

 makes this progress, must at length have no 

 moisture to be precipitated, unless the cold of 

 the 'grass which it touches should increase. 

 Hence in great measure is to be explained, why 

 on such nights, as have been just mentioned, 

 more dew was acquired by substances placed 

 on the raised board, than by others of the same 

 kind on the grass, though it began to form 

 much sooner in the latter than in the former 

 situation, those on the raised board having re- 

 ceived air, which had previously deposited less 

 of its moisture. 



A reason is now also afforded, why a slight 

 agitation of the atmosphere, when very preg- 

 nant with moisture, should increase the quan- 

 tity of dew ; since fresh parcels' of air will 

 hence be more frequently brought into contact 

 with the cold surface of the earth, than if the 

 atmosphere were entirely calm. 



V. Dew, in agreement with the immediate 

 cause which has been assigned by me for its 

 production, can never be formed, in temperate 



