GROUND-WATER 



221 



pervious, some lower one probably is. No layer of rock is more 

 impervious than one which is full of water, and the substructure 

 of any bod which might serve as a reservoir is usually full of water, 

 even when the rock, if free from water, would be porous. 



If the outcrop of the reservoir is notably above the site of the 

 well, and if it is kept full by frequent rains, the "head" will be 



Fig. 181. Artesian well at Woonsocket, S. D. (U. S. Geol. Surv.) 



strong, though the water at the well will not rise to the level of the 

 outcrop of the reservoir. Experience has shown that an allowance 

 of about one foot per mile of subterranean flow should be made. 

 Thus if the site of the well is 100 miles from the outcrop of the water- 

 bearing stratum, and 200 feet below it, the water will rise some- 

 thing like 100 feet above the surface at the well. This rule is, 

 however, not applicable everywhere. The failure of the water to 

 rise to the level of its head is due chiefly to the friction of flow 

 through the rock. The more porous the rock the less the friction. 

 The height of the flow is also influenced by the number of wells 



