72 ECONOMICS OF FORESTRY. 



first, the mechanical obstruction which the foliage 

 offers reduces the amount of the water which 

 reaches the soil and lengthens the time during 

 which it can do so ; the foliage, together with the 

 loose Htter of the forest floor, also reduces the 

 compacting effect of the raindrops and the drying 

 effect of sun and wind, and keeps the soil granular, 

 so that the water can easily percolate ; then the 

 mechanical obstruction which the litter, underbrush, 

 and trunks, and possibly here and there moss, offer 

 to the rapid surface drainage of waters, lengthens 

 the time during which this percolation may take 

 place; and thirdly, the network of deeply pene- 

 trating roots, live and decayed, offers additional 

 channels for a change of surface drainage into sub- 

 drainage. In addition, it is claimed that, owing to 

 the influence on temperature and moisture condi- 

 tions of the air, together with reduced evaporation, 

 more water becomes available to the soil, and cer- 

 tainly the fact that the water, by ready percolation, 

 is withdrawn from the dissipative effects of sun and 

 wind must tend in this direction. 



The sponge theory so often proclaimed by lay 

 writers is rather a misconception of physical laws 

 and of the behavior of a sponge, although a moss- 

 cover — which is by no means the usual cover of 

 a forest soil — may be of great value in preventing 

 rapid surface drainage. This is attested by Robert 

 Gerwig, the builder of the St. Gotthard railway : — 



" One German square mile of moss-cover," he 



