FORMATION OF DEW. 249 



of trees, and especially grass, are the first to become 

 covered with dew; after them come stones, the 

 soil, and, last of all, metals, which we often see free 

 from dew, when the leaves are already covered with 

 large drops. Dew is formed most readily and 

 plentifully in cloudless, starry nights, because under 

 these conditions the cooling and radiation goes on 

 more quickly than when the sky is clouded. You 

 must long ago have remarked that the brightest 

 nights are proportionally the coolest; and that 

 when the sky is clear the contrast between the 

 temperature of day and night is greatest. Even 

 our hair and woollen garments are liable to become 

 bedewed under the sky of a cloudless night ; but 

 a simple roof of leaves, or an open tent, very effec- 

 tually checks radiation, and so lessens both the 

 cooling and the fall of dew. For the same reason 

 that side of leaves which is turned towards the 

 open sky, is always first and and most plentifully 

 wetted with dew. 



Gentle movements of the air are favourable to 

 the settlement of dew, because they remove the 

 cold layers of air, which have been already robbed 

 of their moisture, and replace them by fresh sup- 

 plies warmer and richer in water. But during 

 strong winds the formation of dew is interrupted, 

 since the quick exchange, of the cooled for the 

 warmer air, brings more warmth to the bodies that 

 are chilled by radiation, and thus checks or pre- 



