The comparative economy of conveyance of fluid in pipes has been but little observed, and has only recently perhaps been applied for the purpose of cleansing. The following is an instance of the application of the principle:—A contract was about to be entered into by the West Middlesex Water Company for hauling out from their reservoir at Kensington the deposit of eight or ten years’ silt, which had accumulated to the depth of three or four feet. The contractor offered to remove this quantity, which covered nearly an acre of surface, for the sum of 400_l._, in three or four weeks. The reservoir was emptied in order to be inspected by the engineer and directors before the contract was accepted. It occurred to one of the officers that the cleansing might be accomplished more readily by merely stirring up the silt, to mix it with the water; and then if a cut or outlet were made in the main-pipe used for conveying the water to London, that it might be washed out. He accordingly got thirty or forty men to work in stirring up the deposit, and accomplished the work at the cost of 40_l._ or 50_l._ and three or four days’ labour, instead of so many weeks; when the directors went to see the basin, to decide upon the contract, the reservoir was as free from any deposit as a house-floor. Since the discovery thus made, the silt has been regularly cleansed out into the common sewers. It is to be observed, in respect to the relative cheapness of the two modes, that the contractor would only have removed the silt to the nearest convenient place of deposit in the immediate vicinity of the reservoir, whereas, in the fluid state, it might be carried at the actual cost of conveying water, as far as it is at present conveyed, and sold with a profit, 12 or 14 miles, and raised to heights of 150 feet, at 2½_d._ per ton. By the application of capital and machinery, the cost of conveyance of substances in suspension in a fluid, even at the water companies’ prices, may be rendered thirty and even more than forty times as cheap as collection by hand labour and removal by cartage. In the metropolis, where the persons who water the roads may obtain water gratuitously from pumps, the water supplied by stand-pipes by some of the water companies at 1_l._ per 100 tons, is found to be twice as cheap as the mere labour of pumping the water into the cart. By proper hydraulic arrangements heavy solid substances may be swept away through the iron pipes. These means which science gives of cheapening the cost of the conveyance of refuse from houses, will be available also in extending and completing the cleansing of the towns, of removing the filth which oppresses the poorer districts, and rendering the whole of it available, in the best form, for future use as manure.