ON DEW, &c. 243 



the moisture, which they acquire by condensing 

 the vapour of the earth ; yet, before this hap- 

 pens, the rising vapour must have been greatly 

 diminished, by the surface of the ground having 

 become much colder. These considerations, 

 added to the fact, that substances on the raised 

 board attracted rather more dew, throughout 

 the night, than similar substances lying on the 

 grass, warrant me to conclude, that on nights, 

 favourable to the production of dew, only a very 

 small part of what occurs is owing to vapour 

 rising from the earth ; though I am acquainted 

 with no means of determining the proportion 

 of this part to the whole. On the other hand, 

 however, in a cloudy night, all the dew that 

 appears upon grass may sometimes be attributed 

 to a condensation of the earth's vapour ; since 

 I have several times, in such nights, remarked 

 the raised board to be dry, while the grass was 

 moist. These nights were calm, and evapora- 

 tion from the grass consequently not copious. 

 When evaporation on cloudy nights was assisted 

 by wind, dew has never, as was mentioned in 

 the first Part of this Essay, been any where 

 observed by me*. 



* The interval between the first appearance of dew in the 

 afternoon on grass, in shaded places, and sunset, was formerly 

 said by me., on the authority, however, of only a few ob- 

 servations, to be considerably greater, than that between 



