One of the most remarkable existing aque ducts is the Allegliany wire suspension aque duct, built at Pittsburg in 1816. It consists of seven spans, of 160 feet each. The trunk or channel of the aqueduct is made of wood, 1140 feet long, 14 feet wide at bottom, 161 feet at top, and 81 feet deep ; it is formed of double layers of thick plank, united in a very durable manner. The bottom of the trunk rests upon transverse beams ar ranged in pairs, four feet apart ; and between these are the posts which support the sides of the trunk. There are outside posts, which support the side walk and towing path. Each Pair of beams is supported on each side of the trunk by a double suspension rod of 1/ round iron, bent in the shape of a stirrup, and mounted on a small cast-iron saddle, which rests on the cable. This cable (of which there are two, one on each side of the trunk) is 7 inches in diameter, perfectly solid and com pact, and constructed in one piece from shore to shore of the Ohio, .1175 feet long ; it is composed of 1900 wires, / of an inch thick, laid parallel, and subjected to equal tension; each wire was separately coated with a protec tive varnish, and the whole bundle is encircled by a close, compact, continuous wrapping of annealed wire. The whole cable appears like
a solid cylinder; it is stronger and more elastic than an equal weight of iron chain. The cable passes over the tops of short pyra mids, built of masonry, and placed in a line between the trunk and the towing path ; the pyramids are so small and so simple that they offer no obstruction to the tow-rope passing over them. The extremities of the cables do not extend below ground, but are connected with anchor-chains which are buried beneath masses of masonry sunk in the ground. The theory of construction throughout has been to form a wooden trunk strong enough and firm enough to support its own weight, and to combine this with wire cables of sufficient strength to bear the weight of water in the trunk ; and a very efficient and economical structure has been thus produced. Each cable weighs 110 tons ; the weight of water in the aqueduct is 2100 tons ; the deflexion of the cables between the piers is 141 feet.