Vol. 6$.~] THE MOTION OF SUB-SURFACE WATER. 97 



been determined with some considerable degree of accuracy in the 

 field ; and assuming, as Hoadley's experiments demonstrate, that the 

 outlines of cones of depletion in the same strata are approximately 

 similar, but vary in magnitude, as more or less parallel curves 

 according to the duration and intensity of the pumping ; then it 

 follows that, by adjusting the vertical scale of the diagram so that, 

 in place of a difference of 1 lb. of pressure per square inch, it 

 gives the pressure in lbs. per square inch corresponding to the 

 difference of water-level at rest and when pumping, then a corre- 

 sponding adjustment of the horizontal scale will at once afford data 

 as to the variation of pressure with distance in the field, provided 

 that the strata be uniform and of structure and texture similar to 

 the structure and texture of the experimental test-piece : the diagram 

 of loss of pressure then really becomes a scaled section of the cone 

 of depletion. This method of graphic representation can be further 

 applied, to afford an estimate of the percentage interference of two 

 contiguous wells. 



IV. Field-Observations. 



Although numerous geologists and hydrologists have at various 

 times conducted more or less extensive hydrological surveys, the 

 question of the controlling factors of sub-surface flow has not 

 received the full measure of attention that so important a subject 

 deserves, and most of the papers which have been written deal 

 rather with the shallow sub-surface waters of the gravels than with 

 those of the older formations. 



Of the former division, two very interesting contributions are 

 those in which Haas (26) points out the extreme irregularity of 

 level of the water in the gravels of Eastern Schleswig-Holstein, 

 owing to the restricted areas and very irregular disposition of the 

 gravels in the Boulder-Clay of that district ; and in which Salbach 

 (27) demonstrated, by trial-pumpings in the gravels of the "Rivers 

 Elster, ISaale, and Elbe, that there was not only a considerable 

 variation of level, but also of chemical composition of waters 

 derived from bores, in close proximity to one another, let into the 

 bed and banks of the stream respectively. In the latter division, 

 Chalon (28), Paramelle (29), Lueger (30), and other writers have 

 contributed not a few data on the influence of surface-configura- 

 tion and of the disposition of the impervious substratum forming 

 the base of the water-charged rocks. These writers in general 

 state that the levels of the underground waters vary on the whole 

 directly with the surface-configuration, and that the lines of sub- 

 surface flow are mainly coincident with those of the surface- 

 drainage. But the theorem is not so obviously apparent : for, 

 although the areas of greatest surface-elevation of a permeable 

 rock may, and most probably will be, areas of high sub-surface 

 water-level, still it does not universally follow that the drainage- 

 lines of both systems will also be coincident ; as the agencies 

 which give rise to the formation of surface drainage-lines are not 



Q. J. (i. S. No. 249. k 



