414 Notices of Memoirs — The Water- Suppl'j/ of London. 



EoYAL Commission on the Water Supply for the Metropolis. 

 Eeport of the Commissioners. Presented to both Houses of 

 Parliament by command of Her Majesty. London : Eyre and 

 Spottiswoode. 1869. Fscap. pp. 128. 



THIS important Eeport is now issued, and from it we extract the 

 following : — 

 General BemarTcs on the Sources and Springs in the Thames Basin. 



(pp. XXXV. -xl.) 



It may now be well to review all the resources of the Thames 

 basin before we proceed to consider the important question of the 

 future water-supply of the Metropolis. 



The drainage of the Thames A^alley above Kingston extends over 

 3,676 square miles ; this area receives an average annual rainfall of 

 about 27-2 inches, and one-third of the quantity due to this rainfall 

 flows down the Thames at Hampton. 



This delivery is the result of very complex conditions. One-third 

 of the area consists of impermeable clays, and two-thirds (or about 

 2,450 square miles) of permeable Oolitic limestones, sands, and Chalk. 

 From the former, the rainfall, after allowing for evaporation and for 

 vegetation, flows off at once, and whenever in excess gives rise to 

 floods, whereas the rainfall on the latter is at once stored up, and its 

 ultimate delivery through springs to the streams and rivers is spread 

 over weeks or months. To this cause is owing the permanence of 

 flow of a river draining a permeable rock district, compared with the 

 irregular delivery of a river draining an impermeable district, and it 

 is a consideration of great importance in a question of water supply. 



All permeable formations tend necessarily, by the absorption of 

 rain, to become charged with water up to the level generally of the 

 streams and rivers of the district ; and further, owing to the resist- 

 ance experienced by the rain water in passing through the strata, 

 the water, wherever the ground rises above the level of the rivers, 

 accumulates therein in proportion to the length, breadth, and height 

 of the range of hills, so that instead of the line of underground water 

 level presenting a flat surface between two vallej'^s, it presents a 

 curved surface varying according to certain conditions. Taking the 

 lowest point of escape as determining the permanent level above 

 which all the water tends to run off in springs, the height of the 

 curve above this level gives the head of water on which the springs 

 depend for their supply. The rise of the water being from the cir- 

 cumference of the hills to the centre, the underground reservoirs 

 form more or less dome-shaped masses, the surface of which is con- 

 stantly fluctuating in proportion to the difference between the amount 

 of rain percolating through the strata and the quantity of water which 

 escapes by the springs. 



In the district we have to deal with, the crown of the curve often 

 rises 50 to 200 feet or more above the permanent spring levels, 



