SUPPLY OF WATER. 47 



and the floods, taken at one-twelfth of the year, will add a further 



26-70 is-3^ 

 quantity, ^-^- = I 1 1 inches. 



Thus, on the whole, we collect 6-46+1*38-1- 1*1 1 8-95 inches 

 out of 30 inches as the proportion brought by the Thames from 

 the upper drainage to Oxford. A like proportion may be assumed 

 for the Cherwell. 



Half the quantity is spring-water filtered through oolitic rocks, 

 and on this account considerably charged with carbonate of lime, 

 and thrown out by clay beds containing bisulphide of iron, or 

 sulphates of lime, from which probably arise the small quantities 

 of salts containing sulphuric acid which are observed. 



The dry-summer flow at Surbiton, near Kingston, has been 

 estimated by Bateman at 380 million gallons daily ; by Simpson 

 at 400 millions ; say 65 million cubic feet, four times as much 

 as the flow at Oxford. The drainage area may be taken at 3600 

 square miles, or six times that of the upper drainage of the Thames. 



65,000,000 , . . ., . , 



~f~ =18,000 cubic feet per square mile in a day. 



By the observations of Mr. Stacey at Wolvercot and Wytham, 

 the upper drainage of the Thames yields in a day of dry summer 

 weather 24,000 cubic feet per square mile. 



The perennial supply, therefore, of the whole drainage of the 

 Thames, taken at the lowest, seems to be about three-fourths of 

 that of an equal surface of the upper drainage. As the rainfall 

 in the lower part of the Thames Basin is less than in the upper 

 part by about one-fourth, we might have expected the perennial 

 springs to be less productive, though it is to be remembered that 

 very large surfaces of dry chalk contribute to this supply, and that 

 a large extent of gravelly and sandy surface absorbs the rain and 

 stores it below the surface. 



Rainfall. The flow of water in the Thames, a matter of more 

 than local interest, is maintained by a rainfall which varies from 

 34 inches among the high western hills to 24 inches on the lower 

 eastern declivities. It is to the south and south-westerly winds, 

 which strike the Cotswolds while still damp from the Atlantic, 

 that we owe the larger quantity of our rain : as these winds pass 

 to the eastward, and there in a somewhat dryer state encounter 

 lower hills, the measured fall is reduced. On the eastern coast 



