196 FORMATION 



and full of cankers, and rusty, and in some places 

 breeding fungi, unless they are natural inhabitants 

 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 atmo- 

 sphere 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 sur- 

 faces 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 instance, 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 com- 

 posed of water, and, according to their densi- 

 ties, 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 



