222 FIELD GEOLOGY. 



hole being bored in any part of the upper dish the water 

 will rise through that hole until it stands at the same 

 level. If the upper dish were filled with clay or any 

 other impervious material, and a hole were bored 

 through, the water, of course, would rise in the hole in 

 a similar manner. 



Here are all the phenomena of deep-seated springs 

 and artesian wells. For what occurs on a small scale 

 in the sand confined between the dishes occurs in 

 nature in pervious strata confined between those that 

 are impervious, and the water rising in the hole re- 

 presents exactly an artesian well. The water-bearing 

 bed may be not absolutely continuous, but the same 

 results will follow so far as the continuity be unbroken; 

 or there may be outlets at a lower level than the out- 

 crop, when perennial springs are the result. These 

 springs affect the main supply in the same proportion 

 only as the discharge from them bears to the amount of 

 rainfall received at the outcrop. 



When a boring is made at a place situated at a level 

 lower than that of the outcrop of the water-bearing 

 stratum, the water rises above the surface ; this is the 

 case at several deep wells in the valley of the Lea, sunk 

 down to beds of sand and pebbles which come to the 

 surface at a higher level several miles to the north- 

 ward. 



It follows that when all the conditions of dip, permea- 

 bility, and continuity are known, it becomes a matter 

 not of speculation but of certainty to estimate the depth 

 at which water will be found, and the height to which 

 it will rise in the well. 



It may be repeated that the water-supply of a district 



