Sources of IT'ater-supply.—The water-supply of small towns is fre quently obtained from wells. Owing-, however, to constant contamination of the soil with excrement, and other sources of pollution, especially where composts are in vogue, in the course of time the water of wells fed by sur face-drainage in such situations collies to be little else than diluted sewage, and, consequently, a prolific source of sickness and death. With the in crease and concentration of population, this method of water-supply must be abandoned.
Sfiring--water.—Flowing springs, where they may be relied upon for a constant supply of water, may be hollowed out so as to provide a cham ber of considerable area in which the water may accumulate, and from which it may be distributed in suitable pipes to the points of consumption. Where large bodies of water, snch as inland lakes, are accessible, they afford very desirable sources of supply (Glasgow, Chicago). In India such bodies of water are extensively employed for this purpose.
Rizrr-watcr: Purification.—The rivers, however, afford the most com mon sources of supply', though their water is frequently very turbid and requires purification before being used. This is accomplished either by conducting it into large basins, where, after a short period of repose, the major portion of the suspended sediment subsides, or by some form of fil tration, or by a combination of both these methods; or the purification may be effected by chemical means. Clearing in subsiding-reservoirs is tile oldest method. For this purpose, as the water should stand a few days before drawing off, several sufficiently capacious basins, so contrived as to permit of periodical cleansing, are necessary. To facilitate this, such sub siding-basins are occasionally built in compartments, from any one of which the water may be withdrawn.
Filtration is the more expeditions and effective method of purification, as it suffices to remove not only the suspended mud and dirt, but also, to a notable extent, the organic impnrities. For this purpose either natural or artificial filtration may be employed.
Natural Filtration is admissible in cases where deposits of sand or gravel free from clay and sufficiently deep occur adjacent to a river. It will then suffice to dig long trenches, extending below the level of the g,round-water and from 5o to 15o feet distant from the river-bank, and to wall these trenches with masonry at the sides and top, leaving the joints open (/5/. 52,fig. ir), or to lay in them perforated iron pipes. These pipes
are then placed in suitable communication with underground conduits, and the trenches are then filled up. With this arrangement, no impurities can enter from the outside. The water filters through the gravel, enters, quite clear, throug-,11 the open joints of the masonry or through the perforation of the iron pipes, and flows through the underground conduits founded for the purpose into properly-appointed storage-wells, from which it is further distributed by plimping-machinery (Dresden, Leipsic, Halle, Riga).
The for town and city supply usually comprise a stratum of sand several feet thick resting upon a heavier stratum of gravel and small stone, which latter are provided to sustain the sand and to afford channels through which the filtered water can run off. The principal portion of the impnrities collect upon the upper snrface of the layer of sand, and pro vision must therefore be made for cleansing this from time to time or for renewing it as may be found necessary. It is essential that the water should come to the filter-bed under some pressure, and it is desirable, where this is practicable, to permit the water to clear measurably in subsiding basins before it is admitted to the filter-bed. In Figure ro is shown a section of one of the filter-beds at Chelsea, near London, of which the largest is about 35o feet long by 175 feet wide. The floor is a stratum of impervious clay Y, feet thick. The side-walls are 12 feet high. Along the floor of the filter-bed and parallel with one another are eleven pipes having loose joints, to permit the percolation of water into them. They are surrounded by a stratum of small stones; upon this is placed a layer of shells; then follows a layer of coarse sand r foot thick, and the upper most layer, filially, is formed of 2 feet of fine sand. The turbid water per colates through these various strata successively, and finds its way into the open-jointed pipes above named, where the ends terminate in a collecting pipe. Through the latter the filtered water is conducted to the reservoir, from which it is sent through the mains and service-pipes.