208 ELEMENTS OF GENERAL SCIENCE 



soil keeps the capillary spaces broken, and water does not pass 

 upward through the soil rapidly. If the soil is deeply plowed 

 at the beginning of the season and thoroughly cultivated on 

 the surface throughout the season, there is (1) a reservoir for 

 catching water and (2) a device for preventing its rapid 

 evaporation. 



230. Natural drainage of the soil water. Water may be 

 drained from soil by surface drainage or by underground 

 drainage, and either may take place naturally or artificially. 

 When rains or snows leave more water on the soil than the 

 soil will absorb, it flows away toward regions of lower levels. 

 When very heavy rains fall in a short time or when snow 

 that lies upon frozen or rocky soil thaws rapidly, the run-off 

 may be very great. When rain falls slowly, the soil will take 

 up quantities so large that, if it had fallen in a brief time, floods 

 would have been produced. Surface drainage naturally results 

 in making streams of water, which flow together, finally form- 

 ing a large stream which may by means of its tributaries 

 drain many hundreds of thousands of square miles. A river 

 system is a natural system of soil drainage. 



Under the surface the water also flows toward a lower 

 level, and these subterranean currents may sometimes open 

 to the surface as springs or natural wells, or open directly 

 into the watercourses and thereafter augment the surface 

 streams. The level at which water stands (fig. 101) beneath 

 the surface (the water table, as it is called) varies in its depth 

 below the surface in different regions. The deep clay and rock 

 layers of the earth are often undulating, and the water table 

 conforms to these undulations. For example, in digging a 

 well for domestic supply of water one farmer reached the 

 water table and secured an abundant supply at a depth of 

 16 feet. Another farmer less than half a mile away had to 

 dig down 48 feet before he secured water. Underground water 

 may exist in layers of rock or clay. Artesian wells are usu- 

 ally made by boring into deep layers of water, but they are 



