21S D. W.\\d'ie~-On the Muddi/ Water of the Iluc/li. [No. 4, 



peiided mud can l)e so very fine as tliose of the water of the Hugli during 

 tlie rainy season. Tliere are unfortunately no analyses, hut as they come 

 from Alpine regions they must be often diluted with much pure water from 

 melted snow. 



The whole thickness of the bed of filtering materials is only .8 metre 

 or about 2 ieet 8 inches, of which the upper layer is .3 metre or about 12 

 inches, consisting of very fine sand (Sable tres Jin de Montredon), below 

 which are layers of middling and coarse sand, gravel and broken stones. It 

 is stated that the filters might work more than eight or ten days, but if kept 

 going longer they would be more difficult to clean. This cleaning is effect- 

 ed by passing the water backwards and upwards through the sand, the 

 impure water being carried off from the surface by channels for the purpose. 

 I have to observe that here we have filter beds much thinner than those at 

 Palta, very fine sand and upward charging, all points that have been consi- 

 dered objectionable for the Palta filters. The cleaning by upward charging 

 requires considerable velocity of current and a continuance of it for four or five 

 hours of time. I have calculated from the data given that it would require 

 about 14 or 15 feet of perpendicular height of water, that is, about as much 

 water as one of the filter tanks, emptied of its filtering materials, would 

 hold two and a half times. No account is given as to how it is done, but 

 I concluded that the level of the canal from which the water is supplied to 

 the filters must be sufficiently high for the purpose.* 



I can also bring confirmatory evidence of another kind from English 

 waters, evidence to shew why these waters are not attended with such diffi- 

 culties in their filtration. There are no circumstances to produce such mud- 

 dij waters as are to be found even on the European continent, no Alps and 

 glaciers to produce this muddy water even at its source, no mountain snows 

 to melt, and no large falls of rain concentrated in one period. I refer to a 

 paper by Dr. Franklandf on the water supply of the Metropolis during 

 the year 1865-66. In this paper there are several tables of the principal 

 constituents of the water of nine Water Companies for every month of the 

 year. These tables shew that the amount of saline constituents varies during 

 the year, but never to near such an extent as that of the Hugli : they also 

 shew that the earthy salts vary in their amount as indicated by the hardness, 

 but never become reduced to nearly the same degree as those of the Hugli, 

 being at their lowest indeed nearly as much in amount as those of the Hugli 

 in December or January. The variation of course depends upon the rain- 

 fall, but this does not vary as respects either quantity or time in the same 



* I have since been informed by Dr. Tonnerre tLat the level of the canal is high 

 above the town. 



t Journ. Chemical Society, 1866, Vol. XIX, p. 239. 



