Vol. 6^.~] THE MOTION OF SUB-SURFACE WATER. 81 



the second part ; Seelheim (7) demonstrated that the flow increased 

 faster than the pressure ; and Hazen (8), from a study of experi- 

 mental filter-beds, stated that the flow was affected, not only by the 

 pressure and the thickness of the bed, but also by the relative sizes 

 of the constituent sand-grains of which the bed was made up. 



Ten Brink, Theim, Pettenkoffer, Wolff (9), and others have 

 deduced data on the rate of flow of sub-surface water, by observa- 

 tions on the time of flow of water charged with fluorescin, common 

 salt, and lithia, from one well to another ; and, more recently, 

 Slichter (10) has devised an ingenious method of recording electrically- 

 the time of passage of water charged with an electrolyte from one 

 well to another. 



Seelheim, King (11), Van Hise, and Moore (12) have investigated 

 the variation in the porosity of strata with superincumbent pressure 

 and faulting. 



Ansted, Prestwich, Boyd Dawkins, and others have supplied 

 much information on the storage-capacity of the Chalk ; and Sterry 

 Hunt, Wheatstone & Daniell (13) have familiarized us with the 

 porosities of a great variety of rocks. 



Baldwin Latham (14), Honda (15), Pantanelli (16), Weyde (17), 

 Lucas (18), and others have noted the periodic variation in the 

 levels of the ground-water, and its interdependence on rainfall 

 and variations of barometric pressure. 



Among the more physical and mathematical contributions are 

 Slichter's (10) mathematical analysis, giving results for the motion 

 of water in rocks not unlike that of electricity in conductors; the 

 researches of Osborne Reynolds (19), Cohen (20), Wollny (21), 

 Welitsch-Konsky (22), King (11), and others, on the variation in 

 the viscositjr of a fluid with pressure and temperature ; and of 

 Girard (23), on the effect of dissolved salts on the rate of flow of 

 fluids through capillaries. 



Finally, there have been numerous surveys, of greater or less 

 extent, which have materially contributed to a more intimate 

 knowledge of the geology and hydrology of particular districts ; 

 among the best of these are the Reports of the 2nd and 3rd 

 Royal Commissions on London Water of 1892 and 1897, and the 

 monographs on W^ater-Supply of the Geological Surveys of Great- 

 Britain and the United States of America. 



II. Experimental Data on Porosity and the Flow of 

 Interstitial Water. 



I do not propose to attempt a discussion of all the many factors 

 which have an important bearing upon the quantity and the motion of 

 underground water, but to confine myself to the consideration of the 

 effect of porosity, pressure, surface-configuration, and stratigraphical 

 sequence upon the storage and motion of this water. 



At the outset, one experiences a difficulty in assigning an exact 

 definition to the term porosity ; for the measure of the porosity of 

 a rock is largely affected by the viscosity and temperature of the 



Q. J. G. S. No. 219. g 



