298 ELEMENTS OF GENEKAL SCIENCE 



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

 floods would have been produced. Surface drainage natu- 

 rally results in making streams of water, which flow together, 

 finally forming 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 watercourses and thereafter augment the surface streams. 

 The level at which water stands beneath the surface the 



FIG. 143. The water table 



Diagram to represent the water table in a series of hills and valleys. Line A rep- 

 resents the position of the water table in wet weather ; line B, in dry weather ; 

 S, springs ; W, wells 



water table, as it is called (fig. 143) 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 dig- 

 ging a well for a 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 48 feet before he secured water. Underground water 

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

 usually made -by boring into deep layers of water, so as to 

 strike the water in the lower parts of the layer. The pres- 

 sure from the higher portions of the same layer will cause the 

 water to rise to the surface, and sometimes it is sufficient to 

 cause the water to rise many feet into the air. 



