Page:The American Cyclopædia (1879) Volume XVI.djvu/530

 510 WATER WORKS water to towns : 1, the reservoir system ; 2, the stand-pipe system ; 8, the Holly system. The two first are sometimes classed under one, called the gravity system ; the Holly is an artificial power system. The first system was the only one employed by the ancients; but in their aqueducts they neglected the principle of hy- draulic pressure, constructing them with a con- tinuous descending grade. Reservoirs are con- structed, according to their situation and ser- vice, with earth embankments and puddled bottoms and sides, or of solid masonry of stone or brick and hydraulic cement, and tho latter are sometimes surrounded with earth embankments. In constructing the walls, the engineer takes into consideration the degree of hydraulic pressure to which they are to be subjected and the nature of the material of which they are constructed, and also tho lia- bility of such material to become weakened or strengthened by time, always bearing in mind the hydrostatic law that the pressure of fluids against equal lateral surfaces increases from the top downward as the odd numbers 1, 8, 5, 7, &c., and also tho practical rule that the walls of such structures should be able to with- stand a much greater pressure than is likely to be brought to bear against them. In order to have the service pipes in a city freely deliver their water at all points, the surface of the water in the distributing reservoir should be considerably above the level of any of the service pipes, as tho retardation of the flow and the decrease in pressure produced by the numerous discharging orifices at various levels will prevent the delivery of water in such pipes at the same level. Tho water is usually carried from the distributing reservoir in large mains, which are sometimes mado of cement, but usually of cast iron ; these mains run through the principal streets, and from them distribu- ting pipes are carried through the different streets and are tapped by the service pipes of the several buildings. Tho distributing pipes are usually of iron, and in some cases also the service pipes ; but the latter are generally of lead or of lead lined with tin. The reservoir system has a good and perhaps its best exam- ple in tho supply of Croton water to the city of New York. Supplementary to the main system, there is a small reservoir in a tower at the High bridge, into which the Croton water is forced by two large pumping engines, for supplying the higher portions of the city ad- jacent. Boston is also admirably supplied by this system from Cochituate lake. The stand- pipe system is not mueh used, especially in cold climates, as the water is liable to freeze in the stand pipe ; but it may be employed with ad- vantage in localities where the thermometer rarely falls below the freezing point. The system comprises a dam for the collection of water, or a large well or several wells from which tho water is pumped either by steam or water power into a tall tower, usually con- structed of iron, which is called a stand pipe. This stand pipe is in fact a small reservoir, which requires to be constantly supplied in order to maintain the necessary pressure in the mains and distributing pipes; and that sys- tem which employs only a very small reservoir needing constant supply is only another form of the stand-pipe system, with the advantage of having a body of water even in its small reservoir which is not liable to be frozen. In all systems of water works it is of the greatest importance, not only on account of economy and convenience, but also of the more impor- tant subject of health, that the supply pipes to their discharging orifices should be constantly filled with water, and that they should not, as unfortunately they often do, contain air in con- sequence of insufficient hydrostatic pressure, so that the inner surface of tho pipes becomes corroded with soluble oxides and salts derived from the material of the pipes, the oxygen of the air and of the water, and other matters. The city of Brooklyn is supplied with water from an extensive rainfall drainage basin com- prising about 60 sq. in. on Long Island. The upper side of this basin begins in the range of hills on the north side of the island, the water being carried through extensive and deep beds of gravel and sand toward the southerly shore of the island, where it is discharged into nat- ural and artificial collecting reservoirs. These are: Jamaica pond, f>i m. distant from the pump well, with an area of 40 acres, a storage capacity of over 6,000,000 gallons, and a daily supply capacity of over 8,000,000 ; the Brook- field reservoir, 8 m. from the pump well, with an area of 8} acres, a storage capacity of 15,- 500,000 gallons, and a minimum flow of nearly 2,000,000 gallons daily; Clear Stream pond, 8| m. from the pump well, with a storage capa- city of 800,000 gallons and a daily flow of 750,- 000 gallons; Valley Stream pond, nearly 10 m. from the pump well, with an area of 17$ acres, a storage capacity of nearly 19,000,000 gallons, and a minimum flow of 2,433,000; Rockville pond, about 12 m. from the pump well, with a storage capacity of over 3,000,- 000 gallons, and a minimum daily flow of over 2,500,000; Hempstead pond, about 12| m. from the pump well, having an area of 23 acres and a storage capacity of over 5,000,000 gallons; also Willet's and Smart's ponds, and Freeport creek, with a daily flow of 5,000,000 gallons ; and One-milo pond, at tho head wa- ters of Jamaica stream. The quantity which may be supplied annually is therefore estimated in the neighborhood of 19,000,000,000 gallons. The consumption of the water in the city in 1872 was about 8,000,000,000 gallons, so that about 11,000,000,000 gallons flowed over the weirs during the year. Beginning at Hemp- stead, a closed conduit of masonry, 8 ft. 2 in. wide, conveys the water with a descent of 6 in. per mile to Rockville, where the conduit is increased to 8 ft. 8 in. in width, at Valley stream to 9 ft. 2 in., at Clear stream to 9 ft. 4 in., and at Brookfield branch to 9 ft. 8 in., while