380 ELEMENTARY BIOLOGY 



418. Water power. As our industrial civilization depends 

 more and more upon the use of machinery, we are pressed 

 to find sources of energy for driving the machines. The con- 

 sumption of coal has increased so rapidly that the exhaustion 



FIG. 198. An eroded slope in western North Carolina 



On slopes from which the vegetation has been removed the rains and melting snows 



produce destructive effects of great practical importance. (From photograph by United 



States Bureau of Forestry) 



of the earth's supply is threatened. Water power seems to 

 be the only source of energy that is constantly renewing itself 

 at a sufficiently rapid rate. But to maintain the service of 

 waterfalls we must be sure of the steadiness of the water 

 supply, and this in turn depends upon the forest. 1 



1 When we burn coal as fuel we are of course again dependent upon the 

 forest (though not the forest of our own times), since all coal consists of the 

 modified remains of ancient vegetations. 



