SianoVifies.—Where circumstances are unfavorable to the construction of elevated reservoirs, recourse may be had to stand-pipes (IN. 52, figs. IS, 19). These consist of tall towers in whose tops are water-chambers (one in each tower) with which inflow and outflow communicate. The water is pumped into these elevated storage-chambers, and flows out of them into the service-pipes. These chambers serve as substitutes for the elevated reservoirs iu furnishing the water-serviee with the constant pressure required to cause the water to rise to the upper stories of high buildings, in case of fire, etc. In the United States, the svctern of direct pumping into the mains by what is known as the Holly Direct Supply System is largely employed in the smaller cities and towns.
London Sioragc-basins.—To give an idea of the magnitude of the stor age provision demanded for the water-supply of a great city, mention may be made of the reservoirs of London. The population supplied by the water companies of that city in 1887 was 5,274,542, and the average daily supply (in the month of May of that year) was 16o,388,316 gallons. Of this, more than half, or 82,366,466 gallons, came from the Thames, and the balance from the river Lee, and from certain chalk-springs in the val leys of the Lee and Thames, and from twenty-one deep wells sunk into the chalk-formation to the north and south of London. There are fifty-four subsidiary reservoirs for unfiltered water, with an area of four hundred and sixtv-five acres and an available capacity of 1,29o,roo,000 gallons, and fiftv-three covered reservoirs for the storage of the water after filtration, with a capacity of 160, 002,000 g-allous. The number of filter-beds is ninety nine, with an area of ninety-eight acres.
ancient times the stored and puri fied water was distributed by canals of masonry which were frequently car ried through long tunnels and upon high aqueducts. In modern times works of this nature are not so common, pipes, which are more convenient, less costly, and more readily adapted to irregularities of the gronnd-surface, being almost exclusively used. (Comparefi/. 52, fig. 2o.) Croton York has at this time (1889) nearly completed a splendid aqueduct froin the Croton River Valley to the city, a distance of thirty and three-quarter miles, of which twenty-five miles is cut through the solid rock. Figure 4 (fi/. 54) is a view of the section of this aqueduct whvre it is taken beneath the Harlem River, in the form of an inverted siphon, the excavation being in the solid rock. Provision is being made by this improvement for supplying no less than 200, 000,000 g-allons of water dally—a quantity which will be ample- to meet the growing- require ment of the American metropolis for many years to come. The great reser
voir at Quaker Dam, which will form an important adjunct to this new aqueduct, will have a storage capacity of 3,600,000,000 gallons of water.
Pipe Systems of II systems of distributing water for the supply of towns or cities are in vogne—namely,the "ramification " system and the " circulating " system. In the ramification system, C0111 monly employed, the supply passes from the principal main into tributary mains, from these again into the large branches, and thence, through still smaller branches, into the house service-pipes to the points of consump tion. In the circulating system all the branches communicate at both ends with the principal main; so that the entire system of pipes may be regarded as an underground reservoir that may be tapped at any desired point. The advantage of the ramification system is that the mud in the water is car ried along to its extreme end, and may there be discharged through suit able openings arranged for the purpose; the disadvantages are that the con sumers located at a distance from the principal mains may have their sup ply sensibly diminished by the consumption of those nearer to the main supply, and tbat if a branch main be broken near the chief main all that section of the town receiving its supply through the affected branch will be deprived of water. In the circulating system, these objections are avoided, as the water will always find access to all parts of the system of connected pipes from two sides.
Pifie Aqueduct over Me inter esting example of a pipe aqueduct is that designed by Frederick Graff, chief engineer of the Water Department of Philadelphia, and erected in iS7o, for carrying the water from the Roxborough to the Mount Airy reser voir across the valley of the Wissahickon for the supply of Germantown, a suburb of Philadelphia, as shown in Figure 2. It is formed of two parallel lines of 2o-inch cast-iron flanged pipes, which constitute the top members of a series of inverted bow-string trusses composed of short Plice nix posts 50 inches in diameter, resting on chains formed of long links, or eye-bars, haying a sectional area of to square inches each. The ends of the chains are attached to lugs on top of the pipe at the ends of each span. The structure comprises four spans, each 16V4 feet long, the high est elevation of the centre of the pipes above the water of the creek being Too feet. The three piers are each formed of a group of four 9-inch Plice nix columns, respectively 72, 971A, and 48 feet in height, resting ou low masonry foundations 7 by 14 feet in horizontal section.