1871J The Theory of Irrigation. 37 



above, and then proceed to show in what manner their evil 

 effects may best be remedied. 



It has been stated above that in the absence of human 

 interference the natural law of consumption and supply keeps 

 the forest growth, and the wild animals which live on its 

 products, in a normal state of equilibrium, — and the per- 

 petuity of neither is endangered until man interferes to 

 destroy the balance, and this he does, not wilfully, but in 

 order to contribute to his necessities. Thus, when the 

 means of subsistence began to fail on such ground as had 

 been left open by nature— and which must first have been 

 subjected by man for the supply of his necessities — and as 

 population increased, recourse was necessarily had to the 

 removal of a portion of the forest that stood in the way of 

 further extension of cultivation. A small quantity of wood 

 only being required for fuel and buildings, fire was most 

 probably resorted to in order to clear lands for agriculture, 

 which method, as is well known even at the present day, 

 renders the ground beneath especially suited for vegetation. 

 Such indiscriminate destruction of forests necessarily caused 

 a disturbance in the economy of nature by affecting the tem- 

 perature and humidity of the atmosphere, thus causing con- 

 siderable climatic changes ; by influencing the local distribu- 

 tion of rainfall ; and by its effect upon the flow of springs. 



" Forests," says Becquerel,* " act as frigorific causes in three 

 ways : — 1. They shelter the ground against solar irradiation 

 and maintain a greater humidity. 2. They produce a cuta- 

 neous transpiration by the leaves. And — 3. They multiply, 

 by the expansion of their branches, the surfaces which are 

 cooled by radiation. As these three causes act with greater 

 or less force, we must, in the study of the climatology of a 

 country, take into account the proportion between the area 

 of the forests and the surface which is bared of trees and 

 covered with herbs and grasses. We should be inclined 

 to believe a priori, according to the foregoing considerations, 

 that the clearing of woods, by raising the temperature and 

 increasing the dryness of the air, ought to react on climate. 

 The observations by Boussingault leave no doubt on this 

 point." 



With regard to the influence of forests upon humidity, 

 it must be remarked that the vegetable mould, resulting from 

 the decomposition of leaves and of wood, whilst it helps to 

 obstruct the evaporation from the mineral earth below, 

 absorbs the rains and melted snows that would otherwise 

 rapidly flow away. This moisture it subsequently parts 

 * " Des Climats et de PInfluenee qu' exercentles Sols Boises et Non-Boises." 



