Aqueduct, an artificial conduit or channel for carrying water. In one sense of the word a canal or ditch, or even a water pipe, is an aqueduct; but, as used, the term refers to extensive watertight channels or flumes built, it may be, through tunnels and led across valleys on masonry, that an abundant stream of pure mountain water may be brought into a city. Such a system is possible when a city is situated on comparatively low ground. China still uses aqueducts built centuries before the Christian era. Traces of ancient aqueducts are to be found in Palmyra of the Desert, at Jerusalem, and at Athens. Ancient Rome was supplied with water from the Apennines by nine aqueducts. Three of them are still in use. One of these was forty-five miles long. The Marcian aqueduct is carried across the Campagna, for six miles at a stretch, on arches of masonry. An aqueduct at Nimes, in France, crosses a valley 180 feet deep on three tiers of arches. Each tier is narrower than the one below. The entire structure is built of hewn stone, without cement save in the waterway at the top. One of the Roman emperors brought water sixty miles to conquered Carthage through an aqueduct resting on arches of stone work. It still supplies Tunis with water. A much admired aqueduct at Segovia, Spain, also built by the Romans, has in some parts two tiers of arches, one above the other, each 100 feet in height. Water tunnels of antiquity are equally admirable. A water pipe in Lycia, Asia Minor, consists of cubical blocks of stone, each pierced with a hole, nine inches in diameter. The stones are cemented together to form a pipe a mile in length. Aqueducts in ruins and aqueducts in service are among the sights of many a European city. One of the most celebrated American aqueducts is the Croton, leading from the river of that name thirty-eight miles into New York City. It crosses the Harlem river on a bridge 150 feet high. Irrigation canals, true aqueducts, are built on a large scale in the West. Of late water is conducted more frequently through large pipes that follow the undulations of the surface. A water main or tunnel may descend into a valley and rise again; or it may curve under the bed of a river or over a low hill, and still the water will flow freely, provided the outlet is lower than the source and the upward curves are nowhere more than thirty feet above the intake. As to the main aqueducts, which supplied Rome with a daily volume of 54,000,000 cubic feet of water, it would have been impossible to substitute metal pipes for channels of masonry, because the Romans did not know cast-iron, and no pipe except of cast-iron could have supported such enormous pressure.--Lanciani's Ancient Rome in the Light of Recent Discoveries, p. 60. See IRRIGATION; WATER; SIPHON; PUMP; WATERWORKS.