1873.] D. Waldie— 0« the Mnchhj Water of tlie ILigJi. 211 



clear, and in the required quantity, owing to the largely increased demand, 

 that they were pronounced a failure by the Superintending-Engineer, who 

 advocated a trial of the coarser sand combined with the Regulating Cup> 

 w.ith, as he said, assurance of success, and in the view of adapting this ar- 

 rangement to the whole eight filters if found satisfactory. 



No more complete condemnation of the arrangements I had recom- 

 mended could well have been made. Reasons were given for it, of which I 

 need only state the principal. The Palta sand was too fine, and consequent- 

 ly rapidly became choked up ; such sand was not used in England. It was 

 a principle laid down by Engineers that the muddy particles should not 

 penetrate more than an inch or two below the surface of the sand ; this was 

 not the case here, as they penetrated deep into the sand, and made it foul 

 throughout. The fine sand retarded the flow of water too much, and speedily 

 became choked up by the deposition of mud ; the proper retardation and 

 regulation of the flow should be effected from below, either by a greater depth 

 of coarser materials, or by the use of the Regulating Cup. 



My replies to these reasons were that the fine sand was not too fine for 

 the water which was to be filtered, and that if not used in England, neither 

 was such water filtered in England. This mud of the Hugli water du- 

 ring the rainy season could not be prevented from penetrating deep into 

 the sand, at least if the water were to pass at such rate as would be practi- 

 cally of use ; that the choking up of the sand to a certain extent, instead of 

 being an objection, was essential to its proper action as a filter for this water, 

 and the object should be not to prevent but to regulate it, — and that 

 it could be prevented neither by a greater thickness of coarse material nor 

 by regulating cups. And, further, that the difficulty and consequent great 

 amount of labour and trouble in filtering the water during the rainy season 

 was caused, not by the particular arrangement or nature of the filtering ma- 

 terials but by the nature of the water itself; and, consequently, that the 

 proposed remedy was entirel}" delusive, and would certainly fail. 



Though no formal opposition, so far as I am aware, has ever been made 

 to my statements or opinion respecting the quality of the water, neither has 

 the conclusion been formally admitted as correct, and it has been overlooked 

 or neglected in all reasoning on the subject, at least as a sufficient explana- 

 tion of the difficulty. The principles of sand-filtration were appealed to, and 

 these were explained to depend chiefly on the attractive power of the coarser 

 particles of sand for the finer particles of the mud suspended in the water ; 

 and this was represented as the most important part of the process, — " such 

 is, in fact, filtration," — apparently almost to the exclusion of what was called 

 mere straining. This explanation I hold to be totally erroneous. The most 

 important part of the process is straining, the prevention of the passage of 

 particles through narrow crevices between the grains of sand ; next is depo- 



