404 W A T E K-S U P P L Y oxidized and eliminated in the passage of the water through the ground. The water, however, collects any soluble gases and salts which exist in the strata through which it flows ; and most springs contain some inorganic compounds in solution, depending upon the nature of the strata and the distance traversed. Occasionally springs are so strongly impregnated with certain substances as to receive specific names, such as sulphuretted, chalybeate, and saline springs ; but in such cases they are of more value for medicinal purposes than as sources of water- supply. The abundant springs derived from the chalk, though containing considerable quantities of carbonate of lime, are quite suitable for domestic purposes. Springs from large underground supplies possess the advantage of a fairly constant temperature, as their sources are pro tected from atmospheric changes ; but underground waters are subject to the rise of temperature, experienced in descending below the surface of the earth, of 1 F. 5 on the average, for each 52 feet of depth. Small springs frequently supply little hamlets ; and a shallow tank, formed at the spring, into which the water trickles, serves as a reservoir, which is gradually filled, and from which water is readily drawn. Large springs may afford adequate supplies for towns ; but before relying upon such a source it is essential to gauge their discharge at the close of the autumn, in a dry year, so as to ascertain their sufficiency under unfavourable conditions. When springs have been selected, it only remains for the engineer to design suitable conduits for conveying the water to the place to be supplied. The fine chalk-water springs of Amwell and Chadwell, in Hertfordshire, have been thus utilized by the New River Company for supplying London, by means of a conduit 40 miles long, completed in 1613 ; and the springs issuing from the Malvern Hills furnish Malvern with a plentiful supply of the purest water. Springs were naturally much prized in ancient times, when the simplest means of procuring water had to be resorted to ; and ignorance of their real origin led to their being the subject of mythological legends. They were used for the supply of public fountains by the Greeks and Romans (see FOUNTAIN), and provided water for some of the Roman aqueducts. Streams and Rivers. In olden times, the only other obvious sources of water-supply, besides rain and springs, were the watercourses which carried off the surplus rain fall. Streams and rivers afford the most ample supply, but they become turbid in flood-time ; and when they have a rapid fall, and drain an impermeable basin, they fail in times of drought. These objections have, however, been overcome by settling-tanks, filter-beds, and storage- reservoirs, so that now the principal supplies are drawn from these sources. The increase, indeed, of population, and especially the introduction of the system of discharg ing the sewage of towns into the adjacent streams, have polluted many rivers. Fortunately, in process of time, some of the organic impurities are removed by aquatic animals and plants, and some become oxidized and thus rendered innocuous (see WATER), so that, after a sufficient length of unpolluted flow, the river again becomes suit able for supply. The enactments against river-pollution have attacked this evil at its source ; but some rivers, passing through large manufacturing centres, are hope lessly contaminated with refuse products. The best sources of water are found in streams draining uncultivated mountainous districts, where a plentiful rainfall on steep impervious strata affords a very pure though somewhat intermittent supply. The freedom from habitations, the rapid flow of rain off the surface, the absence of organic impurities and of soluble salts, prevent any chance of contamination beyond occasional discoloration by peat. Distance, however, from hilly regions and a considerable population render it frequently necessary to resort to larger rivers, which, passing towns and villages in their course, become more or less contaminated, and, being fed by springs from permeable strata, contain the salts dis solved by those springs. London, for instance, draws its principal supplies from the Thames and the Lea ; and, though microscopical investigations and the reports of the water-examiners are not always reassuring about the qualities of the water thus supplied and the recuperative powers of nature, waters containing some organic matters and certain kinds of micro-organisms do not appear to be injurious, as is evidenced by the general absence of specific diseases in the inhabitants supplied from these sources. Wells. There are two distinct kinds of wells, namely, shallow wells, sunk into a superficial permeable stratum ; and deep wells, sunk through an impermeable stratum into an underlying permeable stratum. Both kinds of wells tap the underground waters which are the sources of springs, and furnish artificial outlets for waters which would either find a natural outlet in springs at the outcrop, or which, owing to a depression of the strata, may not possess a natural outlet at a low enough level ever to drain the lower part of the underground reservoir. Shallow wells, sunk in the ordinary manner, have long been used for collecting moderate supplies of water, where a permeable stratum, such as the Bagshot sands, or the gravel covering parts of the London basin, overlies a watertight stratum such as the London Clay, especially where a slight depression in the impervious stratum towards the centre, or a considerable expanse of the surface stratum, prevents a ready outflow from the per meable beds. Many parts of London were supplied for a long time in this manner ; for the rain percolating the bed of gravel flowed into the wells sunk in it, from whence the water could be drawn up. Indeed, as pointed out by Professor Prestwich, the growth of London was restricted, till the regular establishment of waterworks, to those districts possessing a gravel subsoil, in which water could be readily procured, as no such facilities existed where the clay rose to the surface. The sites also of many of the older towns and villages were doubtless determined by similar considerations. Shallow wells are still very useful in supplying scattered populations, but they are exposed to the worst forms of contamination when the houses are near together. Any surface impurities are washed in with the rain ; frequently cesspools are given an outlet into the permeable stratum from which the water-supply is derived ; and pumping in the well, to increase the supply, creates a flow from the contaminated areas to the well. Such utter neglect of sanitary pre cautions has led to serious outbreaks of illness ; and the sparkling water from some of the old wells in the City of London has proved very deleterious, from the infiltration into them of the decaying matter from graveyards and elsewhere. Shallow wells, in fact, must be resorted to with great caution, and only when an absence of habita tions, or a thorough inspection of the district drained by the well, affords assurance of freedom from organic pollution. Deep wells, passing generally through impervious beds into a permeable water-bearing stratum to a depth at which an adequate supply of water is obtained, are mostly free from organic impurities, partly owing to the protection of the superincumbent impervious stratum, and partly to the filtration any impurities must undergo before reaching the well. The wells are usually formed by sinking a shaft lined with brick for the upper portion, and then carrying down a boring below to the requisite depth. The level of
the water in the well depends upon the water-level in the