68 MODERN FOREST ECONOMY. 



fall of the drops of rain temporarily retained by the 

 foliaceous surface of the forests. Of these two masses of 

 water the first is evidently withdrawn from the contingent 

 of the inundation ; and the second does not reach the 

 ground until after a detention more or less prolonged, the 

 effect of which is to prolong the du ratio a of the flood. 



' The quantity withdrawn by evaporation from the leaves 

 is undetermined. Many expariments have been made 

 with varying results, ranging from 15 to 79 per cent., in 

 the vicinity of Paris.* The conditions under which 

 different results have been obtained, to which the 

 differences are attributable, have not been ascertained. 

 A difference in the kind of tree; in temperature at the 

 time ; in the violence of the storm, are only a few of the 

 first suggested conditions. The average will probably 

 be found to be much nearer the lesser number than the 

 greater : if we suppose it to be 20 per cent., or one-fifth of 

 the whole rainfall, on a given spot during the continuance 

 of the rainfall, this would occasion an enormous diminu- 

 tion of the flood. And such a diminution, in many cases, 

 might prevent the occurence of an inundation.' 



But beyond this there is the prolongation of the flood 

 occasioned by the retention for a time, more or less pro- 

 tracted, of the rainfall in the forest soil in so far as this 

 may be in excess of what would have been the case had 

 the ground been bare. M. de Gorsse remarks, in continua- 

 tion of what I have already cited : ' As for the water 

 which falls upon the ground, it immediately divides itself 

 into three distinct portions, one which is absorbed by the 

 layer of dead leaves, and by mosses, lichens, and herbs of 

 all kinds, which carpet the surface of the forest ; another 

 which infiltrates into the ground, in proportion to the 

 degree of its permeability, to go to be stored up in the 

 lower reservoirs giving birth to springs ; and lastly, the 

 excess of both, representing the superficial sheet of water, 

 which, flowing across the thousand obstacles of the forest, 



* See Suite aL'etude sw les Torrents desAlpes, de M. Cezanne, p. 96. 



