42 INDIRECT UTILITY OF FORESTS. 



comparatively small portion sinks into the ground, so as to 

 become available for the feeding of springs. Of the rain 

 falling over a forest close on one-fourth is intercepted by the 

 crowns of the trees, and the other three-fourths fall upon a 

 layer of humus which possesses a great capacity of absorbing 

 water and of retaining it for a time. It has been shown, for 

 instance, that mosses of the genus Hypnuni which grow under 

 the shade of conifers can absorb up to five times their own 

 weight of water, and peat mosses of the genus Spliagniim up 

 to seven times; again, the leaf-mould to be found in a middle 

 aged well preserved beech wood can absorb and retain for a 

 time a consideralile quantity of water. Part of the water thus 

 absorbed penetrates into the ground, is taken up by the roots 

 of trees or becomes available for the feeding of springs, 

 while the rest gradually finds its way into the nearest stream. 

 In this manner, well preserved forests must have a decided 

 effect upon the sustained feeding of springs and the modera- 

 tion of sudden floods in rivers. When, however, the humus 

 has been saturated with water, and rain continues, the effect 

 of forests as regards inundations must cease, because the 

 additional water follows the law of gravity and finds its way 

 into the valleys. Hence the effects are of limited extent, a 

 matter which has frequently been overlooked in discussing 

 the subject. In order to moderate inundations to any appre- 

 ciable extent, it would be necessary to keep a very large 

 proportion of the catchment area under forest, and even such 

 a measure would only afford protection to a certain extent. 



b. Protection of the Soit. 



Water rushing down a bare slope possesses a great 

 mechanical power, by means of which it loosens the soil and 

 carries it down hill. In this way, landslips are often caused, 

 ravines are formed and fertile land, situated at the foot of 

 the ravines, may be covered with silt and rendered valueless. 

 Frequently, the debris collects in rivers and forms obstructions 

 which are followed by a diversion of the bed and erosion of 



