^ . Bravender — Watershed of the Thames. 423 



SOUTH OF THE THAMES. 752 



TheEay 651 J 



High worth district 9^ ) 123 



The Cole 48^) 



875 

 The rainfall over this 875 square miles may be taken at 28 inches 

 per annum, from which 12 inches should be deducted for evapor- 

 ation. The rainfall on a square mile, with this deduction, will be 

 1,376,710 cubic yards. But to arrive at the probable available 

 quantity a further deduction should be made for percolation into 

 porous strata, which may not afterwards be thrown out by springs, 

 and in consequence may not reach the Thames, but pass under it. 

 Some of this water loe know must be brought to the surface by 

 faults, as at Boxwell Spring ; but this can only be part of what 

 percolates through the beds of the upper Oolite to the surface of the 

 Fuller's earth. The water which is absorbed by the porous beds of 

 the Inferior Oolite is very large, amoimting to a third of the rainfall. 

 Mr. Bravender then proceeded to point out that the greatest loss by 

 percolation is in those streams which pass over the Inferior Oolite, 

 the area of which is about 106 square miles. The Churn, Coin, 

 Windrush, and Evenlode, lose greatly in passing over the loose and 

 rubbly rock of the Inferior Oolite. The average yearly loss is not 

 less than half the rainfall, after deducting for evaporation, leaving 

 eight inches of available water. The available depth over the other 

 strata in this watershed may be taken at twelve inches. The mean 

 would appear to be somewhere between nine and eleven inches. 

 Assuming it to be nine inches only, and the inhabitants of London 

 to be 3,250,000, then 272 square miles of watershed at this depth 

 will supply them with thirty gallons of water each per day. It will 

 thus appear that the watersheds of the Churn, Ampney and Marston 

 brooks, the Coin, and of the Ock, amounting to 292^ square miles, 

 will more than supply the entire population of London. The Churn, 

 the Coin, and the Windrush having an aggregate watershed of 301 

 square miles, are more than sufficient for the purpose ; and the 

 watersheds of the Chm-n, Coin, and Ock, containing 260 square 

 square miles, are nearly sufficient for the purpose. London is at 

 present said to be supplied with 643,000 cubic yards of water daily, 

 which is at the rate of more than thirty-three gallons per head for 

 three millions and a quarter of inhabitants. If this be correct, 

 London is more abundantly supplied with water than many other 

 towns. The author then gave a detailed account of a series of 

 observations of the actual quantity of water flowing down the river 

 in the twenty-four hours in August, 1864, near Castle Eaton, and he 

 found it to be in one instance thirteen millions of gallons. In the 

 September following the volume of water had decreased to seven 

 millions of gallons. On the 8th of October only one million gallons 

 descended in the twenty-four hours. In the morning of this day 

 (Mr. Bravender said), "I had ascertained no water was passing- 

 down the Thames at the bridge at Ciicklade ; there was no 



