176 SCIENCE PROGRESS. 



2. The effect of size of sand grains upon bacterial 



purification. 



3. The effect of depth of material upon bacterial 



purification. 



4. The effect of scraping the filters upon bacterial 



purification. 

 Time does not permit of my giving the answers to these 

 questions in detail ; but they may be summarised as follows : — 



1. The rate of filtration between 500,000 and 3,000,000 

 gallons per acre per day exercises practically no effect on 

 the bacterial purity of the filtered water. It is worthy of 

 note that the rates of filtration practised by the several 

 Water Companies drawing their supplies from the Thames 

 and Lea are as follows: Chelsea Company, 1,830,000; 

 West Middlesex, 1,359,072 ; Southwark Company, 1,568, 160; 

 Grand Junction Company, 1,986,336; Lambeth Company, 

 1,477,688; New River Company, 1,881,792; and East 

 London Company, 1,393,920. Hence not one of the 

 London Companies filters at the rate of 2,000,000 gallons 

 per acre per day ; at which rate in the Massachussett's 

 filters 99*9 per cent, of the microbes present in the raw 

 water were removed. 



2. The effect of size of sand grains was found to be very 

 considerable ; and, in confirmation, I find that by the use of 

 a finer sand than that employed by the Chelsea Company, 

 the West Middlesex Company is able, with much less stor- 

 age, to attain an equal degree of bacterial efficiency. 



3. The depth of sand, between the limits of one and five 

 feet, exercises no practical effect on bacterial purity when 

 the rate of filtration is kept within the limits just specified. 

 And this result is quite borne out by my own experience 

 gained in the bacterioscopic examination of the filtered 

 waters of the seven Companies supplying the Metropolis 

 from rivers. Thus the New River Company, with i*8 feet 

 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 Metropolitan Companies range as follows : Chelsea, 



