5. =Weirs with Sluices.=--The long weirs built across Indian rivers below the heads of irrigation canals generally extend across the greater part of the river bed. In the remaining part--generally the part nearest the canal head--there is, instead of the weir, a set of openings or “under-sluices” (fig. 40) with piers having iron grooves in which gates can slide vertically. The piers may be twenty feet apart and five feet thick. The gates are worked by one or more “travellers,” which run on rails on the arched roadway. The traveller is provided with screw gearing to start a gate which sticks. When once started it is easily lifted by the ordinary gears. The gates descend by their own weight. The gate in each opening is usually in two halves, upper and lower, each in its own grooves, and both can be lifted clear of the floods. In intermediate stages of the river these gates have to be worked a good deal. (See also CHAP. V., _Art. 5_.) Usually the weir has, all along its crest, a set of hinged shutters, which lie flat at all seasons, except that of low water in the river. [Illustration: FIG. 41.] The canal head consists of smaller arched openings, provided with gates working in vertical grooves and lifted by a light traveller. If the floor of the canal head is higher than the beds of the river and the canal, it may be said to be a weir, but otherwise the canal head is merely a set of sluices without a weir. The barrage of the Nile at Assiut (fig. 41), and the old barrages of the Rosetta and Damietta branches, consist of sets of sluices without weirs. At Assiut there are piers five metres apart and gates working in grooves like those, above described, at Indian headworks. [Illustration: FIG. 42.] The “dam” across the Ravi, at the head of the Sidhnai Canal in the Punjab, also consists of sluice openings without a weir. The piers are connected by horizontal beams (fig. 42), against which, and against a sill at their lower ends, rest a number of nearly vertical timber “needles,” fitting close together, which can be removed when necessary by men standing on a foot-bridge. In floods the needles are all removed and laid on the high-level bridge (not shown in the drawing), the foot-bridge being then submerged. With needles the span between two piers can be greater than would be possible with a gate. Needles can be used up to a length of 12 or 14 feet, excluding the handle which projects above the horizontal beam. They can be of pine, about 5 inches deep in the direction of the stream, and 4 inches thick.