THE WATER SUPPLY OF LONDON. 177 



Lambeth, West Middlesex, Southwark, East London, 

 Grand Junction, and New River. Placed in the order of 

 efficient filtration they range as follows : Chelsea and West 

 Middlesex equal, New River, Lambeth, East London, 

 Southwark, and Grand Junction. 



4. When there is such an accumulation of deposit on 

 the surface of a sand filter that, for practical purposes, 

 sufficient water cannot be made to pass through it, the 

 surface of the filter has to be scraped ; that is to say, the 

 mud and about half an inch of the sand are removed from 

 the surface. After this operation, there is sometimes an 

 increase in the number of bacteria in the filtered water, and 

 it was noticed that the increase was greater in shallow than 

 in deep filters and with high than with low rates of filtration; 

 and there is no doubt that the effect of scraping is con- 

 siderably magnified when coarser descriptions of sand are 

 employed, as in the case of the filters of the London Water 

 Companies. I should like, therefore, to impress upon the 

 engineers of these Companies the desirability of using finer 

 sands than are at present employed. 



INFLUENCE OF THE BACTERIAL CONDITION OF THE RAW 

 RIVER WATER UPON THAT OF THE FILTERED EF- 

 FLUENT. 



I have found that the number of bacteria in a given 

 volume of filtered water is to a considerable extent in- 

 fluenced by the number contained in the raw water supplying 

 the filter ; and from this point of view, therefore, the bacterial 

 condition of the raw river water used in the Metropolis is of 

 no inconsiderable importance. 



Since May, 1892, I have made monthly determina- 

 tions of the number of microbes capable of developing 

 on a gelatine plate in a given volume of raw Thames water 

 collected at the intakes of the Metropolitan Water Companies 

 at Hampton ; and the number has varied during this time 

 between 631 and 56,630 per cubic centimetre, the highest 

 numbers having, as a rule, been found in winter or when 



