AQUEDUCT

artificial channel for conveying water flowing by gravitation, or under pre ure, to supply the needs of a center of population. In level districts traversed by rivers, such as Egypt, As syria and Babylonia, and parts of the Levant, open canals with reservoirs were used in ancient times. The Phenic i ans constructed waterworks of various kinds—for instance, subterranean channels in Cyprus, and the towers for the artesian wells of Ras-el-Ain (Syria). Ancient Gr. aqueducts were very efficient, and consisted of subterranean conduits, as well as basins and pipes of masonry, the best-known examples being those of Samos, Athens, and Syracuse. The Romans, who devoted so much genius to the science and art of water supply, constructed long aqueducts traversing valleys on a tier or tiers of arches. _Rome was supplied with water by a dozen aqueducts, of a total length of over 300 m. In the provinces the magnificent existing testimonies to Roman enterprise are chiefly the beautiful Pont-du-Gard, part of the aqueduct to Nimes, consisting of three tiers of arches, dry masonry except for the cemented channel on top; the aqueduct bridges at Segovia (800 yds. long), Merida, and ‘Tarragon a; the aqueduct near Metz and Mainz; and many waterworks in N. Africa, Asia Minor, Dacia, and Greece. The aqueduct of Pyrgos, and the aqueduct of Just in i an supplying Constantinople, are notable instances of early medizval engineering. In connection with water supply, few aqueduct (in the sense of bridges) of any length have been made in recent years, as an inverted siphon of pipes serves the purpose, and is much le costly. The material for the construction of aqueducts nece arily varies with its availability and the nature of the country traversed. In western N. America wood has been used; the so-called ‘flumes’ cro valleys and are carried along steep slopes on trestles. The Californian flume, used for timber transport, is over 50 m. in length; the High-Line Canal, Colorado, is in places 28 ft. wide and 7 ft. deep; and the San Diego flume (35 m.) is remarkable for its bold design, cro ing several hundred trestle bridges. With the progre in iron and steel manufacture and the improvements in_ concrete, modern water supply is conveyed through cast-iron or steel pipes, their cheapne generally making large masonry works dispensable. Brick-work and concrete are, of course, nece ary where tunnelling, so-called cut-and-cover work, and foundations for pipes have to be resorted to. Manchester is supplied with about 50,000,000 gal. of water daily by the Thirl mere aqueduct, 96 m. long, of which 14 m. are tunnels. Liverpool receives about 40,000,000 gal. from N. Wales by the Vyrnwy aqueduct. The water supply of Glasgow—one of the best in Europe; up to 100,000,000 gal. from Loch Katrine—is conducted through tunnels and pipes and over bridges of masonry, or through cast-iron troughs acro smaller glens. Vienna has a famous aqueduct from springs in_the Styrian Alps, 60 m. distant. The Coolgardie aqueduct in Australia is the largest yet made (350 m.), and delivers water at a much greater height above sea-level than its source. New York can receive up to _ 800,000,000 gal. through the Old and New Croton aqueduct, and the Cat skill aqueduct. The latter, completed in 1915, is the largest engineering work of its kind, but the aqueduct of Los Angeles is by far the longest in America. Among the best known canal aqueducts in Great Britain are the Barton aqueduct, carrying the Bridge water over Manchester Ship Canal, and the Ellesmere Canal, aqueduct acro the Dee.