AND EVArOllATION. 55 



The conclusion is irresistible that if a great quantity of water 

 ■were abstracted from the Chalk at Watford, Ilickmansworth, or 

 Harefield, the same thinj;; would happen to the Ver as has doubtless 

 liappenod to the Colno, the sprin<;s in its bed would be converted 

 into swallow-holes, and the same would happen to the Gade (with 

 its tributary the Bulbourne) and the Chess. 



"When a great quantity of water is pumped from a well in the 

 Chalk, an inverted cone of exhaustion is formed, its apex being at 

 the point of abstraction or surface of the water in the well after 

 the water has been lowered by pumping, and its base (inverted) 

 at the plane of saturation of the Chalk. 



This is not a true cone, for its outline, in section, will not be 



straight, but curved something like this — ^\ y^ , the reason 



being that the effect of the abstraction of water, in partially drying 

 the chalk, will be greater near the well than at a distance from it. 

 The deeper the well, and the lower the point to which the water 

 is reduced, and the greater the distance to which lateral headings 

 are driven, the larger will this cone of exhaustion be, and the 

 greater will be the area injuriously affected by it. 



As water will percolate more rapidly through layers of flints 

 than through solid chalk, in sinking a well in the Upper Chalk, 

 water will naturally flow freely into it as each layer of flints 

 is reached, and slowly into it from the chalk between the layers. 

 It might therefore be possible to abstract a large quantity of water 

 fi'om a lower bed of flints without immediately drawing the water 

 out of a higher bed, but as the whole of the Upper Chalk is very 

 permeable, sooner or later the water in the higher bed of flints 

 must percolate through the chalk between the two layers and get 

 into the lower layer, if that be exhausted, or partially exhausted 

 of the water it contains. 



The whole of the Chalk being thus permeable, though varying 

 in the degree of permeability, a guarantee that a well will be 

 carried a certain depth into it and will be " stein ed" to that depth 

 and made quite watertight so as to prevent water from getting into 

 it from an adjoining stream flowing over the Chalk, or fi'om other 

 (shallower) wells in its vicinity, is of no more value than would be 

 a guarantee that water would only be pumped from the bottom 

 of a tank in order not to lower the surface of the water at or near 

 the top of it, except that in the case of the tank the water would 

 be immediately lowered, and in the case of the Chalk it would take 

 some time to percolate from the higher level to the lower. A more 

 precise parallel, obviously, would be found in a filter, and no one 

 would contend that water could be drawn from the bottom of 

 a filter without lowering the surface at the top of it, although 

 some time would be taken for the water to filter through. 



Therefore, experiments of short duration, made with the object 

 of testing the effect of pumping in one well upon the level of the 

 water in an adjoining well, are of no value ; it may be some time 

 before the effect shows, but the cone of exhaustion of one must 



