420 Notices of Memoirs — The Water- Supply of London. 



of the Thames at Hampton and of the Seine at Paris, both draining 

 areas composed partly of permeable and partly of impermeable strata, 

 is equal to about one-third of the rainfall. In a district where the 

 impermeable strata predominate, the total deliveries will be larger, 

 but they will follow close upon the rainfall ; whereas, as the per- 

 meable strata predominate, so will the rainfall be stored in the hills, 

 and its delivery be spread over a greater length of time. The 

 summer flow in a dry season consists almost entirely of the supplies 

 derived from deep-seated springs. Mr. J. T. Harrison, to whose 

 evidence we would refer for many interesting details on these points, 

 estimates this generally in the Thames district to be equal to one- 

 sixth of the rainfall. 



The importance of such a condition of things for the supply of 

 this large Metropolis cannot be over-estimated. It ensures that per- 

 manence and regularity which are necessarily among the most im- 

 portant elements in a Metropolitan water supply. With natural 

 subterranean reservoirs extending over above 2,000 square miles, a 

 storage reserve is provided comparatively independent of the seasons, 

 and maintained by the ordinary operations of nature, while no filtra- 

 tion can equal that effected through masses of sand, sandstone, earthy 

 limestones, or chalk, from 50 to 300 feet thick. The quantity of 

 mineral matter taken up is in most cases moderate, while the really 

 objectionable ingredient — the organic matter — is reduced to a mini- 

 mum. However different the results obtained in other cases, the 

 two under-mentioned eminent chemists agree in their conclusions on 

 this point, as will be seen by the following table, the quantities 

 having reference in the first case to 100,000 parts, and in the second 

 to a gallon of water : — 





Dr. Frankland. 



Prof.Wanklyn 



Sources of Waters Analysed. 



Organic 

 Carbon. 



Orsanic 

 Nitrogen- 



Nitrogen as 



Nitrates and 



Nitrites. 



Albuminoid 

 Ammonia 

 (representing 

 the Organic 

 matter con- 

 taining 

 Nitrogen). 



CaterhamWell (Chalk) 



•020 



•030 



•026 

 •256 

 •289 

 ■211 

 •260 



•006 



•010 



•012 

 •008 

 •004 

 •006 

 •024 



•027 



•045 



•422 

 •031 

 •022 

 •009 

 •192 



0^000 



Spring near Moor Park (Lower 



Greensand) 



Cold Harbour (Lower Greensand) 



Otter Spring (Chalk) 



Loch Katrine 



0-000 

 0130 



Welsh Waters 





Cumberland Lakes 



Thames Water at Hampton 



0-134 



At the same time the water is kept at a uniform low temperature, 

 and protected from light and air, conditions unfavorable to the exist- 

 ence of living organisms. Springs from such sources probably re- 

 present potable waters in their best state ; and amongst the favour- 

 able specimens of such waters may be instanced many Chalk springs, 

 the water from the Lower Chalk at Caterham, and some of the 

 springs of the Lower Greensands of Surrey. 



