On the weir 4000 feet long across the river Chenab at Khanki in the Punjab, the falling shutters, 6 feet high and 3 feet wide, are hinged at the base and held up by a tie-rod on the upstream side. The trigger which releases the rod is actuated by means of a wire rope carrying a steel ball, and worked by a winch from the abutment of the weir or from one of the piers, which are 500 feet apart. A winch is fixed on the top of each pier, and communication with the piers is effected by means of a cradle slung from a steel wire rope, which rests on standards and runs across the weir. The wire rope which carries the steel ball passes over a series of forks, one on each shutter. When one trigger has been released, that shutter falls and the ball hangs loose. A further haul on the rope causes it to actuate the trigger of the next shutter, and so on. If it is desired to drop only some of the shutters, the rope is passed over the forks of those shutters only. The shutters can be raised by means of a crane which runs along the weir on rails downstream of the shutters or, if the water is too high to allow of this, by a crane in the stern of a boat which is moored upstream of the weir and allowed to drop down. [Illustration: FIG. 46.] [Illustration: FIG. 47.] 7. =Adjustable Weirs.=--Drum weirs, invented by Desfontaines, have been used in France and Germany. Two paddles (fig. 47) are fixed on a horizontal axis and can turn through about 90°, the lower paddle, which should be slightly the larger, working in a “drum,” which is roofed over and can, by means of sluices, be placed in communication with either the upper or lower reach of the stream. According as the upper paddle is to be raised or lowered, water is admitted from the upper reach above or below the lower paddle, the water on its other side being at the same time placed in communication with the lower reach. On the weirs first made on the Marne, the height of the upper paddle was 3 feet 7½ inches, and there were, in a weir, a number of pairs of paddles, each being 4 feet 11 inches wide. By having sluices at both abutments communicating with both reaches, and by opening or closing each of them more or less, the various paddles can be made to take up different positions, and thus perfect control over the discharge is obtained by simply turning a handle to control a sluice gate. A weir has since been made with a single pair of paddles extending right across the opening (33 feet), and the height of the upper paddle is over 9 feet.[16] The chief objection to drum weirs is the necessity for the hollow or drum, which renders the work very expensive, except when only a small depth of water is held up. [Illustration: FIG. 48.] The old sluice gates of the Nile barrages were made segmental (fig. 48), turned on pivots in the piers, and were raised by chains.