WEIR (AS. 'leer, Ger. Wehr, weir, damn, dike; connected with AS. werian, Goth. warjan. OHG.
werjan, Ger. webren, to guard, protect, Skt. rear, to protect, prevent, cover). A barrier or dam thrown across a stream to facilitate the measure ment of its flow, to raise its level, or to direct the water for any useful or ornamental purpose. The word is also used to denote several varieties of fish traps. (See in the Limited States the term is but little employed except for measuring weirs and waste weirs or spill ways; but in Great Britain. Egypt. India, and Australia it is quite commonly to such dams as have water flowing over their tops, either continuously or in floods. For ordinary dams for wa7ste weirs or spillways, see DAMS AND BEsEn voins, MEastnum; Wmas are generally restricted to gauging the flow of comparatively small natural streams, sewers, and the like, and to serve as checks on other methods of measuring flows, such as meters, or the displacement of pump. As rule they are temporary structures and there fore are more commonly built of wood. Some times metal is employed. but. such is rarely the ease. except for the crest of time weir. Ordinary flow dams may be used as w•eir4, either Imy chang ing their crests to make them conform to the accepted types, or by making aHowainTs in com putations. The standard measuring weir has a perfectly horizontal knife-edge, with vertical ends, and its back, or up-stream face, a vertical plane surface. This is known as a sharp-crested rectangular weir. If the ends of the weir proper
In not extend the full width of the stream, allow ance must be made for what are called end-con tractions. When the two ends of the weir are inclined to the crest the weir is known as trape zoidal. If two such inclined ends join in it com mon point. so there is no lio•izontal crest. time weir is called triangular. In determining the flow of water over weirs, the observer merely de termines the depth of the water on the weir, taking care that the measurement is made far enough above the weir to avoid the curved sur face which is formed as the water pitches over. The readings are taken at sufficient intervals to include all marked variations in the depth of the water, or automatic recording gauges may lie employed. The actual volumes are finally deter mined from the tables giving the discharge for various lengths of overflow. depths on the weir, and time character of the weir. See i RRIGATION. BIBLIOGRAPHY. For further details, with ac counts of various experiments, formulas, tables, etc., and extended bibliography, consult pp. 218 225 of Turneaure. W1•atcr Supply Engineering New York, 1901), and papers in Transactions American Society of Civil Engineers (New York, March and May, 1900). Both are technical. The latter gives the results of important experiments at the hydraulic laboratory of Cornell University.