64 Dr. Edward FranMand [Feb. 21, 



with 1*8 foot of sand on the filters, compares favourably with the 

 Chelsea Company, the sand on whose filters is more than twice that 

 depth. Placed in the order of thickness of sand on their filters, the 

 following table shows that the metropolitan companies range as 

 follows : — Chelsea, Lambeth, West Middlesex, South wark. East 

 London, Grand Junction and New River. Placed iD the order of 

 efficient bacterial filtration, they range as follows : — Chelsea and 

 West Middlesex equal. New River, Lambeth, East London, South- 

 wark and Grand Junction. 



Thiceness of Sand on Filters. 



Chelsea 4'0 feet. 



Lambeth 2-8 ,. 



West Middlesex 2-G „ 



South wark 2-5 „ 



East London 2*0 „ 



Grand Juuction 1'^ „ 



New Kiver 1*8 „ 



Order of Bacterial Efficiency. 



("Chelsea. 



\West Middlesex. 



New Eiver. 



Lambeth. 



East London. 

 South wark. 

 Grand Junction 



4. When there is such an accumulation of deposit on the surface 

 of the 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 re- 

 moved from the surface. After this operation, there is sometimes an 

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

 noticed that the increase is greater in shallow than in deep filters, 

 and with high than with low rates of filtration ; and there is no 

 doubt that the efi'ect of scraping is considerably magnified when 

 coarser descriptions of sand are employed, as is the case in 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 Raio Biver Water 

 upon that of the Filtered Effluent. 



I have found that the number of bacteria in a given volume of 

 filtered water is often, though not invariably, influenced 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. 



