PURIFICATION OF WATER FOR DRINKING PURPOSES 131 



From the above it appears then that the average 

 position during the three years of the 



thus ac^ain exhibiting: the most remarkable coincidence 

 between theory and practice. 



The importance of these results lies in their proving 

 that in the matter of sand-filtration we are no longer 

 working in the dark, but that we now know the factors 

 upon which the success of the process depends, and by 

 attention to which its efficiency may be maintained or 

 even increased. 



A searching investigation has recently been further 

 made by one of us into the efficiency of the process of 

 sand-filtration as practised by the several London water 

 companies, the performance of the individual filter-beds 

 having been, as far as possible, made the subject of ex- 

 amination. The results obtained are collected in the 

 following tables ^ : — 



Grand Junction Waterworks, Hampton^ June 25, 1892 

 (Percy Franldand) 



Description 



Unfiltered Water 



No. of Colonies 

 from 1 c.c. 

 of water 



First small storage reservoir nearest river . . 1,703 



Second small storage reservoir further from the river . 1,156 

 (The water does not come to rest in these small reser- 

 voirs, but passes through them to the filter-beds.) 

 Intake from Thames . . . , . . 1,991 



^ Evidence given by Percy Frankland before the Eoyal Commission 

 on Metropolitan Water-supply, November, 1892. 



K 2 



