November 15, 1901.] 



SCIENCE 



751 



but a recent paper by Professor Slichter * 

 upon the motion of ground waters showed 

 me that this was an entirely unnecessary 

 assumption, and gave me the additional 

 data needed upon this point. This chart 

 (Fig. 1) is a horizontal diagram. A repre- 



FlG. 1. 



sents one well and B another well, separated 

 by a homogeneous porous medium. Into 

 the well B, I pour water. In the well A 

 there is no water at the outset ; and the 

 water flows from the well B to the well A 

 through the medium. What is the path of 

 the water ? Its flowage is represented by 

 the curved lines. Some of the water goes 

 in a nearly direct course. Another part 

 takes a somewhat curved course. Still 

 other parts of the water follow a very in- 

 direct course, represented by the longer 

 curved lines. All the available cross sec- 

 tion is utilized. If, for instance, this room 

 were filled with water, and water were 

 running in at one place in the front end of 

 the room and were escaping at one place in 



* ' Theoretical Investigation of the Motion of 

 Ground Waters,' by C. S. Slichter, Nineteenth Ann. 

 Eept. U. S. Geol. Surv., 1899, Pt. II., pp. 295-384. 



the rear end of the room in equal quantity, 

 would the water simply follow the direct 

 line between the two openings ? You know 

 perfectly well it would not. The entire 

 available cross section of the room would 

 be utilized, although the more direct course 

 would be utilized to a greater extent than 

 the more indirect course. This is intended 

 to be illustrated on the chart (Fig. 1) by the 

 lines representing the nearly direct courses 

 being close together, and the lines repre- 

 senting the indirect courses being farther 

 apart. 



This chart (Fig. 1) then represents the 

 horizontal circulation. If we pass to the 

 vertical circulation the flowage is repre- 

 sented by this chart (Fig. 2). The water 

 is being poured into the well B and passes 

 to the well A. The water follows the 

 course of the curved lines, so that with a 

 diflerence in head equal to the difference 

 in the level of the water in the two wells, a 

 considerable part of the water being poured 

 into B and passing through the homo- 

 geneous porous medium to A penetrates 

 a considerable depth, from which it rises 

 and enters the well A . Now what will be 

 the limit in nature of the downward search 

 of underground water ? We have already 

 given it. Manifestly the lowest limit of 

 effective circulation at any place is the 

 bottom of the zone of fracture at that place. 

 The zone of flowage below is practically 

 impervious. However, an impervious lim- 

 iting stratum may exist at depths far less 

 than the bottom of the zone of fracture. 

 An impervious limiting stratum, perhaps a 

 shale, may be found at a depth of 300 feet 

 or less, or at any depth intermediate be- 

 tween this and the bottom of the zone of 

 fracture for the strongest rocks. Where 

 there are one or more pervious strata which 

 are inclined, and above, below and between 

 which are impervious formations, there 

 may be two or more nearly independent 

 circulations. To illustrate, at Denver, the 



