DEW. 75 



consequence, the exterior air immediately in contact 

 with it. After a time, moisture begins to deposit. The 

 temperature at which this occurs is noted, and is the 

 dew point. 



173. DEW. The earth cools, as has 



Explain the . 



formation of been before stated, every clear night, by 

 radiation. The air in immediate contact 

 with it, becomes thereby so much cooler, that it cannot 

 retain all its water in the form of invisible vapor, and 

 the deposition of the surplus in the form of dew is the 

 consequence. 



174. Grass and foliage receive most dew 

 because they are good radiators, and losing 



the most their own heat most rapidly, cool down 

 the air sufficiently to cause a deposition of 

 its moisture. The soil itself, and stones, receive less, 

 or none at all, because they do not, by their own ra- 

 diation, become sufficiently cool to produce the same 

 effect. Dew does not form on a cloudy night, because 

 the clouds radiate heat to the earth and thus prevent 

 the requisite cooling. 



175. CAPACITY FOR VAPOR : EXPANSION 



How is it -r. . -, -i 



known thatthe NOT THE CA USE. It must not be Supposed 



increased ca- that the increased capacity of air for va- 



pacity of air . 



for moisture por, which results from heating, is owing 

 * to its ex P a nsion. Air does indeed expand 

 about one-twentieth between 50 and 100, 

 but its capacity for moisture is quadrupled by the same 

 rise of temperature. 



