1896.] on the Past, Present and Future Water Supply of London. 63 



three million gallons per acre daily, which renders the results avail- 

 able for application to public water supplies ; indeed, none of the water 

 delivered in London is filtered at so rapid a rate as this. It was 

 found that, at these rates, all the disease-producing germs which were 

 intentionally, and in large numbers, added to the un filtered water, 

 were substantially removed. The filters were so constructed and 

 arranged as to allow direct comparison of the bacterial purification of 

 water under different rates of filtration — with sand of different degrees 

 of fineness, with diff'erent depths of the same sand, and with intermit- 

 tent and continuous filtration. 



The actual efficiency of these filters was also tested by the appli- 

 cation of the bacillus of typhoid fever. During the earlier portions 

 of the year 1893 very large numbers of these bacilli and other sj^ecies 

 were applied in single doses to the several filters at different times, 

 and the effluent was examined four times daily for several days after- 

 wards. The results so obtained give a thoroughly trustworthy test of 

 the degree of bacterial purification effected by each of the experi- 

 mental filters, and these are the data which have been largely used by 

 the Massachusetts State Board of Health in deducing the rules which 

 they consider ought to be observed in water filtration. 



Among the subjects investigated by means of these experimental 

 filters were : — 



1. The effect upon bacterial purification of the rate of filtration, 



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. 



These important experiments and my own bacterioscopic examina- 

 tions of the London waters, continued for four years, lead to the 

 following conclusions : — 



1. The rate of filtration, between half a million and three million 

 gallons per acre per day, exercises, practically, no effect on the bacte- 

 rial purity of the filtered water. It is w^orthy 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 Com- 

 pany, 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 Com- 

 pany, 1,393,920. Hence not one of the London companies filters at 

 the rate of two million gallons per acre per day, at which rate in the 

 Massachusetts filters, 99 • 9 per cent, of the microbes present in the 

 raw water were removed. 



2. The effect of the size of sand grains is very considerable. 

 Thus, by the use of a finer sand than that employed by the Chelsea 

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

 storage, to attain an equal degree of bacterial efficiency. 



3. The depth of saud between the limits of one and five feet exer- 

 cises no practical effect on bacterial purity, when the rate of filtration 

 is kept within the limits just specified. Thus the New River Company, 



