CANAL. 215 



The streams are supplied from swamps — from 

 springs — and from an union of both. Cultivation 

 dries up the streams which proceed from swamps, 

 and diminishes pro tanto those that are fed par- 

 tially from that source. Even those derived from 

 springs exclusively, may be deprived of their sup- 

 ply by various causes. Springs fail as well as 

 marshes, but not so often, and this may be owing 

 to the failure of rain, or to the clay which holds 

 the water, giving way for a less tenacious sub- 

 stance. On the other hand, the clearing of a 

 country sometimes exhibits waters formerly con- 

 cealed in the cavities of rocks, by filling up the 

 fissures with earth ; and in former times the 

 leaves of trees thickening on the surface of the 

 earth formed a compact bed, which exposed the 

 rain water collected in it to the power of evapo- 

 ration, but the removal of the leaves and the 

 opening of the earth by cultivation, enable the 

 rain to penetrate into the ground, and to collect 

 in copious and perennial springs below the influ- 

 ence of solar heat. 



