ARTESIAN WELLS 



55 



rock is more impervious than one which is full of water, and the 

 MI list rue tun- of any bed which might serve as a reservoir is usually 

 full of water. 



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 

 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 a well is 100 miles from the outcrop of the water- 

 hearing stratum, and 200 feet below it, the water will rise something 

 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 drawing on the same 

 reservoir, on the degree of imperviousness of the confining bed above, 

 etc. Flowing wells, many of them relatively shallow, are frequently 

 obtained from unconsolidated drift. 



Map work. See Plates XC to XCIV of Professional Paper 60, U. S. Geological 

 Survey, and Exercise IV, The Interpretation of Topographic Maps, a laboratory 

 manual by Salisbury & Trowbridge. 



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