192 FORMATION OF DEW. 



composed, and has an efflorescence of salts of lime 

 on it ; and it will be found that the buds of the trees 

 are black, and full of cankers, and rusty, and in some 

 places breeding fungi, unless they are natural in- 

 habitants of moist atmospheres. The flags in the 

 pavement, and even the granite in the streets, bear 

 marks of this humid and corrosive nature ; and an 

 atmosphere which produces those effects cannot be 

 the most salubrious for human beings. So much 

 for the earth fogs. 



Dew, it has been said, is produced much in the 

 same way as these fogs, and the only difference is 

 that the dew is produced only at or on the surfaces 

 of the objects upon which it appears, and is really a 

 product of the atmosphere, though it does not fall 

 through it ; while the fog is, at least in the first in- 

 stance, a product of the surface over which the air 

 is; though, after it has cooled the air down to a 

 certain temperature, it may, and often does, bring 

 about that state of things which produces dew. 

 There are instances, however, in which the fog does 

 not bring the temperature of the air down to the 

 dew-point, and these are usually called " dry fogs," 

 though they are composed of water, and, according 

 to their densities, contain as much water as those 

 fogs which are accompanied by dew. Dry fogs are 

 day-fogs rather than night-fogs, as, of course, the 

 surface of the earth does not cool so fast when it is 

 merely veiled from the sun by a fog as when the 

 sun is down. 



Simple as the process of the formation of dew is, 

 there have been some mistakes and disputes about 

 it. Some have written and spoken about " rising" 

 dew, and others about " falling" dew. But the dew, 

 as dew, that is, as visible drops of water, neither 

 rises nor falls, but is formed on the surfaces ; and as 

 the air has access to all surfaces except the interior 

 surfaces of air-tight vessels, the dew may form on 

 the side of a substance or under it, just the same as 



