Page:The New International Encyclopædia 1st ed. v. 10.djvu/897

IRRIGATION. ground. Where grades and other conditions permit canals should be made narrow and deep rather than wide and shallow, in order to lessen the surface exposed to evaporation.

Seepage should be guarded against. To this end lining with stone laid in mortar or with concrete or with cement mortar may be employed. A great advantage in linings, if reasonably smooth, is that they increase the carrying capacity of the canals by lessening the friction, and aid in maintaining it by lessening the sedimentary deposits and plant growths on their sides and bottoms. Sometimes leakage may be diminished by throwing powdered clay into the water at the head of the canal. The sedimentary matter naturally carried by the water will often reduce the leakage in a few months or years.

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, or 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 bottom. The staves are held in place by steel rods or bands, so arranged that they can be tightened by turning nuts. The steel flumes are made of thin plates riveted together. Flumes may rest on mudsills, or timbers placed on the ground, but, being generally designed to cross depressions, they are more frequently supported on trestles. The trestles, like the flumes, are generally 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 lining is necessary, as in earth, or to reduce the friction when in rock, it may be of brick, concrete, or stone, and resembles that for masonry aqueducts in tunnels. See ;.

of some kind are required for nearly all canals, flumes, or pipe-lines. In America they are generally of timber, but in much of the foreign work they are permanent structures of stone. The essential features are a bulkhead, gates, and wasteway. Where there is a dam at the head of the canal, the headworks may be at one end of it, or form a part of it.