32 CLIMATE AND RESOURCES OF 



which are other layers of leaves in varying stages of decay, 

 the whole forming a mass which freely admits water, and 

 prevents its escape by surface-drainage. The shade of the 

 trees prevents the incidence of the direct rays of the sun ; 

 and the trees break the force of the wind and prevent the 

 surface of the ground being swept and dried by them. 



Thus forests preserve moisture in a country, firstly, by their 

 soil being in a condition to arrest and retain water; and 

 secondly, by the trees preventing the incidence of the sun's 

 rays on the surface, and checking the force of the winds j 

 thus the two chief causes of evaporation are absent. Some 

 slight amount of water is evaporated from the surface, but the 

 greater part gradually sinks into the subsoil, whence it is 

 taken up by the roots of trees, to be again given off by their 

 leaves in vapour, or finds its way by percolation, nature's own 

 system of subsoil drainage, to the nearest watercourses. The 

 ground under thick forests, where the leaves have been allowed 

 to accumulate and decay on the surface, is always moist, 

 showing that the amount of water lost by evaporation from the 

 leaves of the trees and the surface of the ground does not 

 equal the amount of yearly precipitation, and, in consequence, 

 we find that springs and watercourses often originate in 

 wooded lands and convey away some of the surplus water. 

 This seems to have been the state which existed in most 

 countries before the wants of an increasing population led to 

 a clearance of forests for agricultural purposes. 



The artificial causes of surface- drainage must have been 

 prominently called into play from the time that man com- 

 menced to till the earth, as every operation of husbandry, by 

 loosening and exposing the surface of the soil to the action of 

 the sun, the wind, and the rain, would diminish the cohesion 

 of its particles, and bring it into a state more readily acted on 

 by them. 



