408 W ATE E -SUPPLY Accessory Works, Sometimes a small dam is placed across the upper end of a reservoir, so as to form a small settling reservoir, in which the inflowing stream can deposit any sediment before passing into the main reservoir. A waste weir is provided at a suitable place in the dam with its sill slightly lower than the highest proposed water-level in the reservoir, so that the surplus water, when this level is reached, may flow over into the lye- wash. The length of the weir should be made sufficient for the discharge over it to pass off the inflow during a flood, so as to ensure the dam against being overtopped by a rise of water in the reservoir, which would be fatal to an earthen dam. To provide for a large discharge without a great length of weir, the sill of the weir may be placed somewhat lower, and planks placed temporarily across it to retain the water at its highest level on the approach of the summer. The water is drawn off from the reservoir, as required for supply, through an outlet culvert passing from a low level in the reservoir into a conduit in the valley below. This culvert was formerly frequently placed through the lowest part of the dam, being readily formed during the construction of the dam, and giving command of all the water in the reservoir. Accidents, however, have often been traced to the unequal settlement of the earthen embankment near the culvert, or to infiltration of water into the embankment, either by escaping from the culvert fractured by settlement, or by finding a passage along the outside line of the culvert or pipes. Thus the bursting of the Dale Dike embankment, 95 feet high, near Sheffield, in 1864, on the occasion of the first filling of the reservoir behind it, having a capacity of 114 million cubic feet, which entailed the loss of 238 lives, was attributed to the unequal settlement and consequent cracking of the puddle wall over the trench in the rock in which the outlet pipes were laid, aggravated in this instance by the defectiveness of the material in the main bank, the rough manner in which the bank was raised, and the rapid filling of the reservoir. The percolation of the water under pressure along the line of outlet pipes was the cause of the gradual failure of the embankment, 41 feet high, across the Lynde Brook, Worcester, Mass., which burst in 1876 and set free a reservoir with a capacity of 110 million cubic feet. No possible considerations can justify the burying of outlet pipes at the base of a high embankment, with the valves regulating the discharge at the outer extremity, whereby the water-pressure always acts along the whole length of the pipes, and their inspection is impracticable. In some cases a valve-tower is erected in the centre of the embankment, by which means the water can be shut off from a portion of the pipes. If, however, the outlet pipes are carried under the embankment, they should be laid in the solid ground, and should be commanded along their entire length by a valve-tower placed at the inner toe of the embankment (fig. 4). Nevertheless, it is far safer to carry the outlet pipes in a tunnel constructed through the side of the valley, beyond one end of the embankment. In the case of masonry dams, the outlet is generally con structed in the solid rock distinct from the dam ; but at Villar the outlet culvert has been carried through the centre of the dam (fig. 7). CONVEYANCE OF SUPPLY. A reservoir in a mountain valley is at a sufficient elevation for the water to flow by gravitation to the locality to be supplied ; and it is only necessary to form a conduit by canals, tunnels, aqueducts, and pipes, of adequate size in relation to the gradient, to convey the daily supply required (see HYDROMECHANICS). In olden times hills were contoured, and valleys were crossed by colossal aqueducts, at great expense, to obtain a regular inclination, which was reduced by the circuitous route that had to be taken (see AQUEDUCT). Now, however, hills are pierced by tunnels; and, by the employment of cast- iron, deep wide valleys can be crossed by inverted siphons following the depressions of the land, so that a much straighter course is attainable, affording better gradients, and therefore enabling smaller conduits to be adopted, which is of great importance when long distances have to be traversed. Thus the waters of Thirlmere, after being discharged through a tunnel formed under the Kirkstone Pass at the south end of the lake, instead of at the natural outlet to the north, will be conveyed to Manchester by a conduit 96 miles long, with a total fall of 178 feet. Portions of the route are in tunnels, 7 feet square, the longest tunnel being a little over 3 miles long ; and there are several inverted siphons, to be formed of five cast-iron pipes, each 3J feet in diameter, the longest siphon being 3J miles, and that under the river Lune having to bear a water-pressure of 416 feet. The water from the Vyrnwy reservoir will be conveyed to Liverpool in a conduit 67 miles long, of which 4 miles will be in tunnel, and will furnish a supply of 40 million gallons per day. A large supply of water from the river Verdon for the district round Aix, serving for irrigation and manufactures, as well as for domestic purposes, is conveyed across the valley of St Paul in an inverted siphon formed of two wrought-iron tubes, each 5| feet in diameter. The water obtained from rivers in low districts, and from wells, has to be raised by pumping to the height necessary to obtain a proper pressure for supply ; and the pumps have to be in duplicate, to prevent a failure in the supply in the event of a break-down. Thus the water- supply of London has to be raised by pumping to fill the service reservoirs. PURIFICATION. The water obtained for supply is frequently not sufficiently pure to be at once distributed for domestic purposes. The impurities to which water is liable are of three kinds, namely, particles of matter in suspension, inorganic substances in solution, and organic matters in solution or of extreme minuteness. Suspended matters are readily removed by subsidence if the particles are heavy, and by filtration if the particles are flocculent or light. Some inorganic compounds are readily removed, whilst others cannot be dealt with in a practical manner. The removal of organic impurities is of the most im portance, and the most difficult, which has led to the great care exercised in selecting unpolluted supply. Settling-Reservoirs. Allusion has already been made to subsiding reservoirs formed at the head of large storage reservoirs ; the object of these, however, is rather to pre vent the accumulation of silt and sand in the principal reservoir than for the purpose of purification, but the principle is the same. Supplies from large reservoirs are generally free from matters in suspension, except some times during heavy floods, owing to the subsidence in the reservoir itself ; but river supplies have often to be led into settling-reservoirs, where the water, whilst at rest, can deposit its heavier particles before passing on to the filter-beds for the removal of the lighter portions. Filter-Beds. Over the bottom of brick tanks layers of clean material are spread, decreasing in coarseness from small rubble to sharp sand, with a total average thickness of about 4 feet. The actual filtration is effected by the upper layer of sand ; and the lower layers allow the passage of the water unaccompanied by the sand. The efficiency of the filtration depends upon the slowness of
the passage of the water, which should not exceed a flow