M^VXUAL OF AGrJCULTUliE. 257 



" When by any cause the temperature of the air is reduced, it& 

 particles (molecules), approach nearer each other, and so do those 

 of the vapours held suspended in the air ; and as steam becomes 

 visible when mixed with atmospheric air, so vapour becomes 

 visible when it suffers condensation by a reduction of tempera- 

 ture, and then becomes clouds. These differ much in altitude 

 and size." In this way we can visibly perceive the contained 

 watery vapour in our lung exhalations during frosty weather, as 

 also at such times as the atmosphere is already saturated with 

 moisture. 



On this natural provision of the aerial absorption of moisture 

 depends the entire system of circulation of w^ater from earth to 

 sea, and vice versa. From the immense tracts of the tropical 

 seas, and those of less heated zones, enormous volumes of water 

 are being continually uplifted into the thirsty air. And this 

 saturated air, becoming impelled in every direction by all the 

 winds that blow, meets with cold elevations and land surfaces, 

 or colder currents ; whereby, losing its excess of temperature 

 by means of radiation, its corresponding excess of moisture is 

 condensed, and descends as rain to promote vegetable life, to fill 

 our springs, rivers, and lakes, and take a leading part in effecting 

 the physical and chemical changes which are constantly occurring, 

 and altering the general contour of the world. 



The frequency rather than the amount of rainfall, indicates 

 the atmospheric humidity of any district. Generally speaking, 

 the number of days in a year on which there falls rain, increases 

 as we recede from the equator to the temperate zones ; and the 

 amount of rainfall decreases with distance from the equator with 

 increased elevation above sea-level, and with distance from the 

 sea : — 



From N. lat. 12° to 43'' there are 78 rainy days. 

 43 to 46 „ 103 „ 

 46 to 50 „ 134 „ 

 50 to 60 „ 161 ,, 



The rainfall of the tropics is estimated at 95, and of the tem- 

 perate zones at 34 inches annually. In some parts of the "West 

 Indies, as much as 600 inches have fallen in one year. 



In the comijaratively small area of Great Britain there is yet 

 large diversity of the amount of rainfall, humidity, and tempera- 

 ture of the atmosphere. This, besides being due to local physi- 

 cal causes, arises from others prevailing over a large portion of the 

 earth. 



In some districts sheltered by higher ground from unfavour- 

 able winds, the temperature is much hioher than it is in less 

 favoui-ed districts, and from the condensing action of the hills, 

 the liumidity is also greater. The prevailing westerly winds 

 come moisture-laden from the Atlantic into contact with the 



