FLUMES are most commonly built of wood, with a rectangular cross-section, but in recent years steel has been employed, particularly in precipitous rocky locations, er where crossing, streams or deep ravines. The ordinary flumes of boards or plank are subject to leakage. To avoid this, and also to give a channel better adapted to the flow of water, wooden staves are now being employed, formed into a semicircular or other shape designed to give a curved bot tom. The staves are held in place by steel rods or hands, so arranged that they can be tightened by turning nuts. The steel flumes arc made of thin plate; riveted together. Flumes may rest on mudsills, or timbers placed ou the ground, Seepage should be guarded against. To this end lining with stone laid in mortar or with con crete or with cement mortar may be employed. A great advantage in linings, if reasonably smooth, is that they increase the carrying ity of the canals by lessening the friction, and aid in maintaining it by lessening the sediment but, being generally designed to cross depres sions, they are more frequently supported on trestles. The trestles. like the flumes, are gen
orally of wood, but they are sometimes of steel, particularly where the flume proper is of that material. or where the flume support must be in spans, as at a stream crossing.
When. instead of valleys or streams, hills are encountered, necessitating a long detour for canals, tunnels are often employed. They do not differ materially from other tunnels. If lin ing is necessary. as in earth, or to reduce the friction when in rock, it may be of brick, con crete, or stone, and resembles that for masonry aqueducts in tunnels. See TUNNELS; AQUEDUCTS.