THE WATER SUPPLY OF LONDON. 175 



in the near future, now that special attention is directed to 

 bacterial filtration, it will not be approached within 50 per 

 cent. This hope is based not only upon my own observations, 

 but also upon the exhaustive and exceedingly important 

 investigations carried out at the Lawrence Experiment 

 Station by the State Board of Health of Massachussetts, 

 under the direction of Mr. George W. Fuller, the official 

 biologist to the Board. 



More than six years have already been spent in the prose- 

 cution of these American experiments, and many thousands 

 of samples of water have been submitted to bacterial cultiva- 

 tion. The Massachussetts experimental filters are worked at 

 rates up to 3,000,000 gallons per acre daily, which renders 

 the results available 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 unfiltered 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 different depths 

 of the same sand, and with intermittent and continuous 

 filtration. 



The actual efficiency of these filters was also tested by 

 the application of the bacillus of typhoid fever. Very large 

 numbers of these bacilli and of other species were applied 

 in single doses to the several filters at different times, and 

 the effluent was examined four times daily for several days 

 afterwards. The results so obtained give a thoroughly 

 trustworthy test of the degree of bacterial purification 

 effected by each of the experimental filters, and these are 

 the data which have been largely used by the Mas- 

 sachussetts 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. 



