172 THE WEATHER, AND WEATHER PROPHETS. 



and his selection of the kind of vegetation with which it 

 shall be habitually clothed, place to a great extent 

 within his power. It is chiefly in his clearance or allow- 

 ance of arborescent vegetation, and in his artificial 

 drainage of the soil over extensive districts for agricul- 

 tural purposes, that his influence on these relations is 

 perceptible. The total rainfall, and (which is perhaps 

 as regards weather and climate of even more import- 

 ance) the frequency of showers on an extensive well- 

 wooded tract, or one entirely covered by forests, ought, 

 on every theoretical view of the causes which determine 

 rain, to be greater than on the same tract denuded of 

 trees. The foliage of trees defends the soil beneath and 

 around them from the sun's direct rays, and disperses 

 their heat in the air, to be carried away by winds, and 

 thus prevents the ground from becoming heated in the 

 summer ; while, on the other hand, a heated surface-soil 

 reacts by its radiation on the clouds as they pass over it, 

 and thus prevents many a refreshing shower, which they 

 would otherwise deposit, or disperses them altogether. 

 So again of drainage : by carrying away rapidly the 

 surface-water down to the rivulet, and so hurrying it 

 away to the ocean, it not only cuts off a great deal of 

 the supply of local evaporation, which is a mateiial 

 element in the amount of rainfall, but by causing the 

 suiface to dry more rapidly under the sun's influence, 

 it allows it also to become more heated ; and so to con- 

 spire with the action of the denudation of trees to prevent 

 rain. Evidence is not wanting to corroborate this d 

 priori view of the matter. The rainfall over large regions 



