PHENOMENA AND PKODUCTION OF DEW. 271 



upon the grass, vriH sint nearly 20 degrees below one suspended in the air 

 at a little distance above. 



All bodies hare not an equal capacity for radiating heat, 



stances is dew but some cool much more rapidly and perfectly than others. 



deposited most Hence it follows, that with the same exposure, some bodies 



will be densely covered with dew, while others will remain 



perfectly dry. 



Grass, the leaves of trees, wood, eta, radiate heat very freely : but polished 

 metals, smooth stones, and woolen cloth, part with their heat slowly: tha 

 former of these substances will therefore be completely drenched with dew, 

 while the latter, in the same situations, will be almost dry. 



The surfaces of rocks and barren lands are so compact and hard, that they 



can neither absorb nor radiate much heat ; and (as their temperature varies 



but slightly) very little dew deposits upon them. Cultivated soils, on the 



contrary (being loose and porous) very freely radiate by night the heat which 



they absorb by day; in consequence of which they are mucii cooled down, 



and plentifiilly condense the vapor of the air into dew. Such a condition 



of things is a remarkable evidence of design on the part of the Creator, since 



every plant and inch of land which needs the moisture of dew is adapted to 



collect it; but not a single drop is wasted where its refreshing moisture is not 



required. 



.... . 605. Dew is deposited most freelv upon a calm, clear night, 



What circnm- . , '. , • ,. - .7 



stances influ- smce under such cu-cumsiances heat radiates irora the earth 



ence the pro- j^^g^ freelv, and is lost in space. On a cloudv niffht, on the 



ductiou of dew 7 - ' *^ " , • j 



contrary, the deposition of dew is almost entirely interrupted, 



since the lower surfaces of the clouds turn back the rays of heat as they 

 radiate, or pass oflf from the earth, and prevent their dispersion into space ; 

 the surface of the earth is not, therefore, cooled down sufficiently to chill the 

 vapor of the air into dew. 



When the wmd blows briskly, also, little or no dew is formed, since warm 

 air is constantly brought mto contact with soUd bodies, and prevents their re- 

 duction in temperature. 



Can dew be ^^^w is alwajs fomied upon the surface of 

 properly said to ^^le material upon which it is found, and does 

 not fall from the atmosphere. 



Other things being equal, dew is most abundant in situations most e?cposed, 



because the radiation of heat is not arrested by houses, trees, etc. Little dew 



is ever observed in the streets of cities, because the objects are necessarily 



exposed to each other's radiation, and an interchange of heat takes place, 



which maintains them at a temperature uniform with the air. 



* , , Dew rarelv falls upon the surface of water, or upon ships 



Does dew form . " „,, „,..,. , 



upon the sur- m mid-ocean. The reason of this is, that whenever the 



face of wate r ? aqueous particles at the surface are cooled, they become heaner 



than those below them, and sink, while warmer and lighter particles rise to 



the top. These, in their turn, become heavier, and descend ; and this pro- 



