Vol. 6 3 .] 



THE 3H0TI0X OF SUB-SURFACE WATER. 



91 



The now at all other points within the periphery of the cone 

 of depletion is also increased by an amount proportional to the 

 corresponding differences of level. At points on the margin of the 

 basal area of the cone of depletion, the difference of level (as also 

 the increase of velocity) is zero, or the velocity at such points is that 

 of the normal interstitial flow. 



It is not often that an opportunity offers to collect any exact in- 

 formation on the effect of great differences of natural pressure on the 

 sub-surface flow, although, if such data were collected, they would 

 be of considerable interest. Prof. Boyd Dawkins, however, informed 

 me, some time ago, of an instance recorded in the course of the 

 Xorthwich Brine -pumping Subsidence Compensation Enquiry : 

 during Christmas 1892, there was a 20-foot subsidence in a field at 

 Marston, probably due to the abstraction of the underlying salt, by 

 the flowing brine on its way to the pumps, and the caving-in of the 

 superincumbent rock ; almost simultaneously with the subsidence 

 a rise of the water-level occurred in the pump-wells at Messrs. 

 Brunner, Mond & Co/s Works at North wich, over 2 miles away from 

 the subsidence. In all probability the caving-in of the strata set 

 up a wave of compression which passed rapidly through the sur- 

 rounding rock, causing an instantaneous increase of velocity in 

 the interstitial water, and an increased delivery into the wells. 



Another interesting record is that of Messrs. S. Cortauld & Co/s 

 well at Booking (Essex), recorded in several issues of the British 

 , Association Reports. The water-level was only 12 inches above 

 the surface (130*7 feet O.D.), on April 21st, 1884, the day before a 

 severe earthquake ; but the next record, on April 28th, gives the 

 level as 31*5 inches above the surface. This increase continued 

 until July 7th, when the maximum level of 58*5 inches above the 

 surface was attained. Other water-levels of this well are shown 

 in the following Table (VI) :— 









Table VI 







Year. 



Hei 



Maximum. 



r/ht of vjater ah 

 Bate. 



ove surface, 

 Minimum. 



in inches. 

 Bate. 



Maximum annual 



difference of level, 



in inches. 



1883... 



19 



March 26th. 



8 



October 8th. 



11 



1884... 



58-5 



July 7th. 



9-5 



January 14th. 



49 



1885... 



46 April 7th. 



29 



December 14th. 



17 



1886... 



37*5 January 18th. 



265 



July otb. 



11 



If sufficient data of similar nature to those already discussed be 

 obtained for any strata, it is possible, as I have shown in another 

 paper (24), to obtain a fairly-accurate estimate of the yield of wells 

 and filter-galleries, the formula being based upon the data derived 

 from experimental investigations of flow and pressure and from 

 field-surveys. 



