DISTRIBUTION OF RAINS. 137 



The fifth and last modifying condition which I shall notice, is 

 the eflbct of the nature of the soil as cultivated or not cultivated, 

 upon rainfall. The influence of the nature of the soil upon pre- 

 cipitatioii, and hence upon the climate of a country, depends upon 

 its greater or less power to absorb and radiate heat, as also upon 

 its power to transmit over its surface the water which may fall 

 upon it, as rain, or be deposited as dew. 



It would hardly seem to be in the power of man so to modify 

 the action of physical forces as to determine the amount of heat and 

 moisture for any considerable portion of the globe. It is undoubted- 

 ly true, however, that by changing the conditions under which the 

 forces of nature act, he may modify the temperature, and hence the 

 distribution of moisture over limited areas. 



The effect of denuding land of trees and other vegetation is to 

 expose it to the full action of the rays of the sun, by which its 

 surface, in summer, acquires a temperature higher than that of the 

 surrounding cultivated land or forest, and this warmth imparted 

 to the stratum of atmosphere above, gives rise to ascensional cur- 

 rents of heated air. Now, when vapor-bearing winds mingle with 

 this heated air, their temperature is elevated, condensation cannot 

 follow, precipitation of rain cannot take place. Let the same 

 winds pass over a forest; the surrounding air cooled by evapora- 

 tion reduces their temperature, and immediate precipitation is the 

 •result. 



Coulter has well said : " The ocean, winds and woods, may be 

 regarded as the several parts of a grand distillatory apparatus. 

 The sea is the boiler in which vapor is raised by solar heat ; the 

 winds are the guiding tubes which carry the vapor with them to 

 the forest, where a lower temperature prevails. This naturally 

 condenses the vapor, and showers of rain are thus distilled from 

 the cloud masses which float in the atmosphere, by the woods 

 beneath them." 



The destructive effects of removing trees from large tracts of 

 country and the beneficent results attending the replanting of trees, 

 are matters of historical record. From Marsh's "Man and Nature," 

 we learn that the terrible droughts which desolated the Cape 

 Verde Islands, by which 30,000 persons perished, are attributed 

 to the destruction of the forests ; that the planting of 20,000,000 

 of trees by Mehemet Ali in Lower Egypt, has secured frequent 

 rains throughout a region of country seldom thus visited before. 



To enter upon a full discussion of this intricate but interesting 



