WATER-BEARma STRATA. 329 



ascertained, from the mean of a variety of experiments, to be 

 equal to three feet of water, supposing this to accumulate 

 on the surface of the ground, allowing nothing for evapora- 

 tion and nothing for soaking in. 



Now from thirty to forty per cent., or somewhat more than 

 one-third, of the rainfall is found to percolate through the 

 strata to go to wells and springs, the remainder being left 

 to shed off and to evaporate. 



The surface of England and Wales is equal to 58,320 square 

 miles, and taking one-third of the three feet, or one foot, we get 

 a quantity of water sinking into the bowels of the earth equal 

 to sixteen hundred and twentj^-five thousand eight hundred 

 and sixty -eight million two hundred and eighty - eight 

 thousand cube feet, or 10,161,682,800,000 gallons. To be 

 able to grasp this quantity better, I have compared it to the 

 Vj^rnwy Lake, just completed, for supplying Liverpool (which 

 contains 13,300,000,000 gallons), this vast subterranean 

 reservoir being therefore nearly eight hundred times larger 

 than that lake. 



It must be borne in mind that the percolation taken is the 

 average percolation over the area of England and Wales. 

 This varies somewhat according to the latitude and longi- 

 tude or geographical position of the area, viz., in England, 

 whether it is in the north, south, east, or west, or geologi- 

 cally, whether in Chalk, Sandstone, Pennant, Millstone Grit, 

 or otherwise, and according to the changes of temperature 

 due to the periods of the year, which necessarily results in 

 more or less evaporation. 



The average rainfall on the Mendip Hills is equal to forty- 

 four inches ; out of this an allowance for evaporation (if 

 calculated for j^ield) would have to be made of fifteen to 

 sixteen inches. 



Li the sandstones, with which I am more conversant, in 



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