330 WATER-BEAEING STEATA. 



the area west of Bristol, at Chelvey and the neighbourhood 

 adjoining Nailsea, I should allow from experience a percola- 

 tion of from forty-five to fifty per cent. In making this 

 estimate I have regard to the level at which the country 

 lies, and to the fact that the streams have not their origin 

 from natural spring heads, but are produced by the over- 

 flowings of waterlogged strata issuing in the form of boWom 

 springs in the river bed. 



In the watershed and valley of the Frome, the water- 

 logging of the Pennant Grit and Upper Coal-measures and 

 surrounding sandstone gives a very large percentage of 

 percolation equal to fifty per cent., which results during 

 heavy rainfalls in the water shedding off almost bodily into 

 the Frome, and so producing disastrous floods. 



The Pennant Grit between the Upper and Lower Coal- 

 measures in theNailsea basin, nine miles west of Bristol, is a 

 good illustration of the water-yielding properties of this for- 

 mation, where it has a thickness of about three hundred feet, 

 and yields from five to five and a half million gallons per da}-. 



In chalk, which is more compact, from fifteen to twenty- 

 five per cent, only percolates, and in the county of Hertford- 

 shire, from the New River Company's sources, it is estimated 

 that the natural percolation amounts to four inches out of 

 twenty-six inches of rainfall ; but pumping opens the hori- 

 zontal apertures in the chalk, owing to carbonate of lime 

 being dissolved and carried away by the water with the 

 action of pumping. 



The cycle of climate in England over which a series of 

 gaugings should be taken is about seventeen years, during 

 which time wet and dry years will occur, and also excessively 

 dry years, in which the rainfall is small, and consecutive dry 

 years occur, which affects the percolation of water, and is the 

 measure of the yield which can be relied upon in wells. 



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