TIME THAT WATER REMAINS UNDERGROUND. 587 



greater per kilometer than the gradient for the artesian water, as figured by 

 dividing the difference between the head and the point to which the water 

 would rise by the horizontal distance. Where the sandstone is perforated 

 by artesian wells near Lake Michigan the water rises with a pressure which 

 is a large fraction of the pressure due to head. 



If, for instance, the city of Chicago be taken, the nearest point at 

 which the Potsdam sandstone outcrops is about 150 kilometers, and the water 

 for the Chicago wells is probably mainly derived from points 150 to 250 

 kilometers from Chicago. The average elevation of the catchment area 

 above Chicago is 80 meters. The water of the early artesian wells at Chi- 

 cago, before they were so numerous as mutually to interfere when allowed 

 to flow, rose to an altitude of about 30 meters above the surface;" or, there 

 is a loss, in the entire distance of 150 kilometers or more, of only about 50 

 meters of pressure. This small loss of head, which may be attributed to 

 friction between the water and the sandstone, and within the water, is to 

 be distributed through this entire distance. It follows that in the capillary 

 openings of this sandstone the friction of the water is almost zero, else the 

 pressure at the wells would not so nearly equal the pressure due to head. 

 Of course the moment the wells are allowed to flow, and especially if there 

 be a number of contiguous wells which are allowed to flow, the pressure 

 rapidly drops. This is not due to friction through the major course of the 

 water, where the movement is slow, but is due to friction adjacent to the 

 wells, where the water is moving rapidly toward them. This is shown by 

 the fact that at a distance of from 100 to 200 meters from a well the pressure 

 is commonly as great while the well is flowing as it is at the mouth of the 

 well when the flow is stopped. 6 Indicating the same thing is the fact that 

 the flow of a well may be greatly increased by shattering the rock at the 

 bottom of the well, thus producing supercapillary openings into which the 

 water may issue from the capillary openings. If the area of these super- 

 capillary openings be many times that of the cross section of the well, the 

 chief source of friction is that of moving water in the well itself; for the 

 water has a chance to slowly ooze from the capillary into the supercapillary 

 tubes without rapid motion, and to make its way to the well tube through 



"Leverett, Frank, The water resources- of Illinois: Seventeenth Ann. Kept. U. S. Geol. Survey, 

 pt, 2, 1896, pp. 805-806, 811. 



&Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ann. Kept. 

 U. S. Geol. Survey, pt. 2, 1899, pp. 363-364. 



