i89 



The physical conditions of forest land are that, owing to the 

 shelter from sun and wind, the atmosphere is generally colder 

 and damper than in the open country, and evaporation conse- 

 quently less. It is calculated that a hectare of forest land (2i 

 acres) gives off every day 37 cubic metres of oxygen and 37 

 metres of carbonic acid, leading to a great expenditure of heat ; 

 and that from every hectare of forest land sufficient heat is ab- 

 stracted to melt 316 cubic metres of ice. Ligneous plants also 

 withdraw from the ground and discharge as vapour more than 

 40,000 gallons of water per hectare per day, which causes a sen- 

 sible reduction of temperature. When clouds pass over a forest 

 they encounter a cool, damp atmosphere, the point of saturation 

 comes closer, and rain is caused. This condition of forest land 

 has been remarked on by aeronauts, who find that a balloon is 

 invariably affected, and drops when passing over forests. 



The advantages claimed for forests with regard to water supply 

 are that the trees act as regulators of the rainfall ; that the aver- 

 age quantity of rain falling on land covered with forests is greater 

 than in the open ground to the extent of about one-sixth ; that it 

 holds up the water for a time and discharges it later on when 

 water is most required in river basins, the rain being held back by 

 the leaves of the trees and coming to the ground more gradually ; 

 the rain that falls on the surface is also taken up by the layer of 

 dead leaves on the ground, which permits of a gradual percola- 

 tion to the sub-soil. Observations show that in summer the ground 

 of the forest is damper than that of the adjacent cleared land, 

 and snow remains for a much longer period in forest land before 

 melting than in cleared land. 



On the other hand, it has been contended by some of those who 

 have made a study of sylviculture that forests do not increase the 

 quantity of water flowing to the springs and rivers, but reduce it. 

 The numerous striking facts quoted do not bear out this conten- 

 tion, which is mainly based on the fact that the substratum water 

 stands at a lower level on forest land than in the adjacent cleared 

 ground. This fact is generally admitted to be the case at one 

 period of the year. As the result of many years' observations, it 

 has been found that the maximum level of underground water is 

 reached in May, that the water accumulates in the ground from 

 August to January ; and that the rivers are supplied by this re- 

 serve, and were it not for this accumulation many brooks and 

 river feeders would cease to fiow in summer. 



Several very striking examples are given by the authors of the 

 papers as to the deleterious effects of cutting down forests', espe- 

 cially in hilly districts. In the commune of La Bruguiere, the 

 forests on the slopes of the Black Mountain were cut down ; the 

 consequence of this removal of the trees was that a brook which 

 ran at the foot, and the water from which was used for driving 

 some fulling mills, became so dried up in summer as no longer to 

 be of any use, while in winter the sudden floods caused very great 

 damage in the valley. The forests were re-planted, and as the 



