138 SCIENTIFIC AGRICULTURE. 



there is a current of air, there will be no condensation of 

 watery vapor so as to form dew or frost: hence they are 

 seldom or never seen on a windy night. 



In some parts of the world, as in sections of South America 

 and Mexico, dews are so copious as to supply the place of 

 rains. The cold ascribed by many persons to the light of the 

 moon, is nothing but the consequence of nocturnal radiation. 

 JMists, fogs, and clouds, are only floating vesicles of watery 

 vapor, which obscure the transparency of the atmosphere; 

 they differ only in the degree of their density. " A fog, [says 

 a celebrated naturalist,] is a cloud in which one is, a cloud is 

 a fog in which one is not" Fogs are not common in hot 

 countries, they rise to a small height, and are prevented by 

 winds. In Peru dense fogs continue for half the year. Day 

 fogs are volcanic ashes and vapors diffused through the air by 

 wind. The appearances of clouds may be changed according 

 to their height, density, distance, and the angles at which the 

 sun's light strikes upon them, &c. They are moved about 

 and broken apart by winds, and assume various and beautiful 

 hues, according to the different colors of the sun's rays which 

 they reflect 



Clouds, then, are merely floating, distant fogs, and arc most 

 frequently formed over some body of water or wet soil, 

 sxow. 



Snow is congealed water, which descends from the upper 

 regions of the atmosphere. The precise conditions of atmos- 

 phere requisite for its formation, or the manner in which it 

 takes place, is not yet well understood. The most that is 

 known respecting it, is in relation to the form of its flakes: 

 these are stellate, and composed often of hexagonal prisms, 

 arranged at an angle of 60, from each of which others fre- 

 quently shoot out at the same angle. The whiteness of snow 

 is said to depend on the minuteness of its crystals. In some 

 cases snow presents no appearance of crystalization. 



