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CHAPTER III. 



Formation of Clouds and Dew. 



" The clouds consign their treasures to the fields, 

 And, softly shaking on the dimpled pool 

 Prelusive drops, let all their moisture flow 

 In large effusion o'er the freshen'd world." 



Thomson. 



THE relations of the atmosphere to water are very important 

 and numerous. The various winds as they sweep over large 

 tracts of country, or the ocean, become charged with moisture, 

 which they bear with them to the higher regions, to be there em- 

 ployed in the formation of clouds, or rain. The formation and 

 dissolution of clouds, produces all the varied train of meteoro- 

 logical phenomena. The humidity suspended in the atmosphere 

 is derived by exhalation partly from the land, but ultimately from 

 the vast expanse of the ocean. The moisture, deposited by the 

 air is in the form of minute globules, which remain suspended, or 

 subsicfe slowly, constituting a cloud. When it comes near us, 

 whether it hovers on the tops of the hills, or spreads over the 

 valleys, it receives the name of &fog; when deposited from the 

 air in a clear night, upon the surface of the ground, or bodies ex- 

 posed to the air, it is called dew. 



In order to explain more clearly the formation of clouds and 

 also the deposition of dew, let us consider in what manner the 

 capacity of air for moisture will be affected either by heat or cold. 

 By capacity, we mean power of stowing away so as to render in- 

 visible. As a general rule, we find the capacity of air for mois- 

 ture increased by heating, and dimished by cooling, it being ca- 

 pable of taking up and holding in an invisible state, a greater 

 quantity of water when heated, than it can retain when cooled. 

 Although the capacity of air for moisture is increased by heating, 

 yet it is not proportional to the heat, but increases in a faster ratio. 



