SCALE 300  . crooked for the purpose, causing them to revolve in a quadrant of a cylinder under the sill, known as the drum; and they can be readily lowered by cutting off the flow from the upper pool and putting the drum in communication with the lower pool, which connexions can be adjusted by see-saw sluice-gates, so as to put the upper paddles in any intermediate position between vertical and horizontal (fig. to) . The merits of this weir in being easily raised against a strong current and in allowing of the perfect regulation of the discharge, are unfortunately, under ordinary conditions, more than counterbalanced by the necessity of carrying the drum and its foundations to a greater depth below the sill of the weir than the height of the weir above it . Accordingly, for several years its use was restricted to the Marne; but in 1883–1886 drum weirs were 2 Proc . Inst . C.E., vol. exxix., p . 258 and plate vi., fig . 2 . Shutter weir . adopted for closing the timber passes alongside the needle weirs placed across the Main, with a single upper paddle 393 ft. long and 5 ft . 7 in. high in each case; and a still larger drum weir was erected about the same time for closing the navigable pass of a weir across the Spree at Charlottenburg, with an upper paddle 324 ft. long and 9€ ft. high (fig. to) . A peculiar and cheaper form of drum weir has been constructed across ten bays each 75 ft. wide on the Osage river near its confluence with the Missouri, where a hollow, wooden, cylindrical sector, stiffened inside by iron framing and revolving on an axis laid along the crest of the solid part of the weir, fits into a drum at the back lined with planking, having a radius of 9 ft .

The weir is raised by admitting

water from the upper pool into a wedge-shaped space left below the sector when it is lowered in the drum, which by its pressure lifts the sector out of the drum, forming a barrier, 7 ft. high, closing each bay of the weir . Provision has also been made for rendering the sector buoyant by forcing air into it, so that it can be raised when the head of water is insufficient to lift it by the pressure of the water from the upper pool . In spite of its high cost, the drum weir furnishes a valuable hydraulic contrivance for situations where it is very important to be able to close a weir of moderate height against a strong current and to regulate with ease and precision the discharge past a weir . (L . F .