THE WORK OF GROUND-WATER. 



243 



stratum next below the reservoir is not impervious, some lower one 

 probably is. No layer of rock is more impervious than one which is 

 full of water, and the substructure of any bed which might serve as a 

 reservoir is usually full of water, even if the rock be porous. 



If the outcrop of the reservoir be notably above the site of the well, 

 and if it be kept full by frequent rains, the ''head " will be strong, though 

 the water at the well will not rise to the level of the outcrop of the reser- 

 voir. 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 



Fig. 220. — Figure illustrating the principle of artesian wells in drift. 



be 100 miles from the outcrop of the water-bearing stratum, and 200 

 feet below it, the water will rise something like 100 feet about the sur- 

 face at the well. This rule is, however, not apphcable everywhere. The 

 failure of the water to rise to the level of its head is due to the adhesion 

 and the friction of flow through the rock. The more porous the rock 

 the less the reduction of head by friction. The height of the flow is also 

 influenced by the number of wells drawing on the same reservoir, on 

 the degree of imperviousness of the confining bed above, etc. 



Flowing wells, often relatively shallow, are frequently obtained 

 from unconsoKdated drift. Some such relations as suggested by Fig. 220 

 would afford the conditions for flowing wells in such a formation. 



