70 J. HOPKINSON — THE CHADWELL SPEING 



above, when held in the pores of unfissured chalk, can only 

 gradually take the place of the water which flows out. Hence 

 springs in the Chalk are what are called "permanent"; they do 

 not depend upon the rain which falls at the time — they do not 

 ebb and flow with the rainfall. It takes a long time for the rain 

 to saturate the Chalk, and a long time for the water to dx-ain out 

 of it into the springs and rivers. 



It therefore appears that the Chadwell Spring could not be 

 flowing freely one day and absorbing water copiously the next ; 

 that when it became evident that water was flowing into the basin, 

 it must have ceased to flow out of it for some time ; and that the 

 extent to which the water fell when the River Lea was stopped 

 from flowing into it is a measure of the length of time which had 

 elapsed since the spring ceased to flow. The reason why the 

 water fell so many feet immediately the supply from the Lea was 

 stopped, is that the plane of saturation in the Chalk had fallen to 

 that extent, probably having been sinking for some months, first 

 to the level of the outlet of the spring, and then to a level several 

 feet below it, the normal. level while the water was thus gradually 

 sinking having been maintained in the basin of the spring by the 

 water of the Lea flowing into it through the higher portion of the 

 JSew River and the cut connecting it with the spring. I speak of 

 it in the usual way as one spring although it is a collection of 

 springs. I have seen the water bubbling up from about a dozen 

 holes at the bottom of the basin. 



I believe that it is as a rule about six or seven weeks before the 

 spring fully responds to the rainfall, that is to say, that the greatest 

 flow of the spring is about that time after the rain falls which has 

 the greatest effect upon its volume ; that it is nearly as long 

 before dry weather affects it ; and that if our reservoir of saturated 

 chalk were in its natural state a fall of several feet in its level 

 would be the result of a dry period many months beforehand. But 

 it is not in its natural state ; the large amount of water pumped 

 up from great depths unnaturally exhausts it and causes fluctuations 

 in the rainfall to be more rapidly and acutely felt, or rather the 

 plane of saturation sinks more rapidly than it would otherwise 

 do when the rain is in defect and rises more slowly when it is in 

 excess. As, however, our springs are chiefly dependent upon the 

 winter rainfall, it is unnecessary to attempt to determine the 

 amount of this acceleration or retardation, our lack of underground 

 water not being due to the drought of the summer in which it 

 shows itself, but to dry weather some time beforehand. This is 

 not the case with the surface-water which oozes out of the gravel 

 above the Chalk, causing turbidity of the water in the basin of 

 the Chadwell Spring shortly after rain. 



Experiments continued for many years at Nash Mills near 

 Hemel Hempstead, and at Lea Bridge, show that the average 

 percolation through three feet of soil with grass growing on the 

 surface is between seven and eight times as much in the six winter 

 months, October to March, as it is in the six summer months, April 



