226 D. VTsLldio—Oii tJie 3Iiiddi/ Water of the HuglL [No. 4, 



I was not content with these small experiments but tried the process 

 by clarifying the water first by such small proportions of these precipitants, 

 settling one or two da3^s and then passing through a sand filter, and found 

 it to answer perfectly, the filtration going on easily and rapidly. The 

 apparatus was small, the precipitating vessel holding about 45 gallons, 

 the filter being a Zinc tube of about six inches diameter. There were two 

 filters, one with Palta and one with Magra sand : the Palta sand filtered best 

 as I used as small a quantity as possible of the precipitating substances. I 

 could not try it on a larger scale, as my premises are now no longer on the 

 bank of the river. But there cannot in my opinion be the smallest doubt 

 but that the process would answer admirably on the large scale. 



My former experiments, at least in my own judgment, proved that the 

 Hugli water during the rainy season could not be filtered without unusual 

 difficulty, and that arising from a peculiarity in the water which I connected 

 with the peculiar distribution of the rainfall in this country', though I could 

 not then explain the cause : — it was a matter of fact whether it could be 

 explained or not. Plans proposed to overcome this difficulty, supported by 

 experience of water filtration in England, I declared would be useless, 

 because the water was different and English experience therefore not 

 applicable. 



One special contrivance, which it was alleged would be effective for the 

 purpose, I had tried, and had given my opinion that it was worthless for the 

 purpose. The best plan for filtering the water of the rainy season as it 

 presents itself in nature, I concluded, would be by the use of the Palta sand, 

 properly managed, which includes a proper relation between the amount of 

 filtering surface and the quantity of water to be filtered. All of these 

 statements and opinions I still adhere to, as they were conclusions drawn 

 from the observation of facts, the highest and only true authority from which 

 scientific conclusions can be drawn. 



A new idea has supplied me with the means of explaining the nature 

 of the peculiarity, and that not by superseding but by confirming the 

 correctness of my previous conclusions that it was connected with the 

 tropical rainfall, and that was by producing extreme dilution of the water. 

 It also indicated a way to remedy the difficulty of settling and filtering the 

 water. The evidence has been given in the preceding pages, and is founded 

 also on the authority of experimental facts open to scrutiny and criticism. 

 The conclusions, it appears to me, may be of value not only with reference 

 to the Calcutta water supply, but to the purification of water in cases where 

 the circumstances are similar, occurring more generally in tropical countries, 

 but even occasionally in other localities. 



