ELEMENTS OF GENERAL SCIENCE 



7- 



-G 



-4 



-3 



-'2 



2- 



becomes relatively dry. In the woods no stream appears for 

 quite a time, and then it is small, but it lasts for a much 

 longer time than that in the open field ; when it stops flow- 

 ing, the soil of the woods is still very moist (fig. 147). The 

 trees and undergrowth retard the falling rain. Humus acts as 

 ___ __^_^_^^_^^__ ___ __ a sponge in holding large 



quantities of water, so that 

 streams start slowly ; and 

 when they do start, the 

 leaves, twigs, and bushes 

 retard the movement so 

 that the ability of the 

 water to remove soil and 

 to carry it is held in check. 

 Humus retains much of 

 the moisture after the 

 stream has stopped flow- 

 ing. Furthermore, the cov- 

 ering of trees tends to 

 reduce the loss of water 

 by evaporation. 



309. Loss from water 

 erosion. The damage done 

 to soils by erosion is very 

 great. The soil is carried 

 away, the fields are cut into 

 deep gullies, and often the 

 owner of the land realizes 

 too late that the best part of his soil has passed entirely beyond 

 his control (fig. 148). Obviously regions with hilly topography 

 are more subject to erosion than level fields. Such fields will 

 lose less soil if they are kept planted in grass, as pasture or 

 meadow, or in timber. In some places, where hilly land must 

 be cultivated, it is terraced around the hills in order to pre- 

 vent erosion or to catch any soil that may be moved by water. 



April 

 2-9 



April 

 14-17 



April 

 22-29 



I Forested 



Deforested 



FIG. 147. Effects of forests upon drainage 



Diagram to represent rim-off from two sim- 

 ilar areas in White Mountains, one of which 

 has been deforested. The vertical bars rep- 

 resent in inches the proportionate run-off. 

 Note that the forested area retains the larger 

 amount of water. From data by the United 

 States Geological Survey 



