W A T E R-S U P P L Y 405 stratum : and generally the water has to be raised by pumping to the surface. Occasionally, owing to a de pression of the strata, the top of the well is below the level of the lowest part of the outcrop of the permeable stratum into which the well is sunk, and the water rises in the well directly this stratum is reached, and flows over if the hydrostatic pressure is sufficient to overcome the friction. These latter wells, known as Artesian wells, have been already described (see ARTESIAN WELLS) ; and the methods of boring other deep wells are precisely similar. The most favourable strata for deep wells in England are the Chalk, Oolites, New Red Sandstone, and. Lower Greensand. The yield of these wells depends upon the extent of the portion of the underground reservoir which they can drain ; and the reservoir depends for its supply, as in the case of springs, on the extent of the stratum exposed at the surface, the drainage it may receive from adjoining impermeable strata, and the amount of rainfall over these areas. As these points can only be roughly estimated, it is impossible to judge of the yield beforehand ; and much depends on the fissures the well may happen to pierce, as the main flow in many rocks takes place along their fissures. It is dis advantageous to sink a well where the superincumbent impervious stratum is very thick, not merely because of the depth that has to be sunk before reaching water, but also on account of the slow rate of the underground flow at a long distance from the outcrop, and owing to the com pression of the porous stratum by the mass above it. For instance, the deep well on Southampton Common, sunk through 465 feet of impervious beds, has only yielded a small supply of water, though carried 852 feet into the chalk. A well may also prove a failure owing to a fault or an impervious barrier interrupting the underground flow, if it is sunk on the side of the fault or barrier away from which the dip of the stratum inclines. Thus a well sunk at A (fig. 2) will receive the water flowing along the permeable stratum ; whilst an adjacent deeper Hlf well at B will yield no water, on account of the interruption of the flow. As there is a limit to the underground waters, only a limited number of wells can be advantageously sunk within a certain area ; and a multiplication of wells, such as has occurred in the London basin, permanently lowers the water-level of the underground reservoir, and involves an increased lift in pumping to maintain the supply. Wells drawing their supplies from the same sources as springs reduce the yield of the springs issuing from the strata which they pierce ; and when these springs feed rivers the volume of these rivers is thereby somewhat diminished. Wells, however, sunk into strata draining to the sea-coast merely intercept water which otherwise would be absolutely lost. A useful well for small sup plies is a tube well, which consists of a series of strong wrought-iron pipes, between 1 and 2 inches in diameter, the bottom length being terminated in a point, and per forated with little holes for a short distance up. The point is driven into the ground by a falling weight, as in pile-driving ; and, as the tube descends, fresh lengths of pipe are screwed on the top. When the perforated pipe reaches a water-bearing stratum, the water enters through the holes and is raised by a pump (compare descriptions and diagrams of tube-wells under PETROLEUM, vol. xviii. pp. 716-718). EXTENSION OP SUPPLY. A supply obtained from wells may be increased by reaching the water flowing through undrained fissures or lying in untouched cavities, either by sinking fresh wells, or by driving headings from the bottom of existing wells in various directions, both of which courses were adopted for extending the Brighton water-supply. Continued pumping sometimes improves the supply when the stratum is well saturated and the drain is not sufficient to lower the water-level permanently. This result is due to the steepening of the gradient of flow towards the well by the depression of the water-level in the well, which increases the velocity of flow, whereby the channels of access are cleared out and enlarged, so that the water flows more readily and quickly into the well than at the commence ment. The supply from springs and streams can only be increased by storing up the excess of supply in the wet season, to make up for the deficiency in the dry season. This can be accomplished by means of storage reservoirs, which sometimes are found suitably provided by nature in the form of lakes, or may be constructed in mountain valleys by means of dams. RESERVOIRS. Lakes as Reservoirs. A lake is a natural reservoir of water, caused by the influx of a stream into a depression of an impermeable stratum, which is barred to a certain height by a ridge across its outlet, over which the water has to rise before it can flow away (see LAKE). The water of lakes is generally of exceptional purity, owing to its being usually supplied by the drainage from the impervious uncultivated ground of uninhabited mountainous districts, and its general freedom from pollu tion, and on account of the lake serving as a deep subsiding reservoir for any matters in suspension contained in the inflowing streams, of which the Lake of Geneva in relation to the turbid upper Rhone is a notable instance. Glasgow is supplied with excellent water from Loch Katrine (see AQUEDUCT, vol. ii. p. 224) ; it was at one time proposed to supply Edinburgh from St Mary s Loch, and London from Bala Lake ; and works for the conveyance of water from Thirlmere, in Cumberland, to Manchester are in progress. To increase the storage capacity of a lake intended to serve as a reservoir, and avoid injury to vested interests, the ordinary water-level of the lake has to be raised by heightening the barrier at its outlet. By this means the lake is not unduly lowered by the drain upon it during the dry season, and compensation water is pro vided to supply the water-rights along the stream below. The extent to which the water-level of the lake has to be raised depends upon the area of the lake, the influx into it, and the supply drawn off ; thus Loch Katrine, with an area of 3000 acres, needed only a rise of 4 feet in order that, with a maximum lowering of 7 feet, it might provide a storage of 5687 million gallons for a supply of fifty million gallons per day ; whereas the water-level of Thirlmere, with an existing area of only 350 acres, requires raising 50 feet to furnish a storage of 8100 million gallons for a similar daily supply. The amount of water that can be collected depends on the catchment area, and the rainfall of the driest years, less the loss from evaporation ; and the supply which can be relied
upon from any definite gathering ground is given by the