In some factories in Bavaria and Switzerland there are self-acting shutters which revolve on a horizontal axis at the lower edge, and are counterbalanced by cylindrical weights which roll on ways in the side wall. This arrangement is suitable when there is only one span, which can, however, be as great as 30 feet. An adjustable weir used at Schweinfurt on the Maine, consists of a hollow iron cylinder, 59 feet long and 10 feet in diameter, running across the stream. The cylinder is pear-shaped in cross-sections, and can be made, by means of mechanism, to revolve, the water passing over it. Another kind used at Mulhausen on the Rhine consists of a hollow iron cylinder 85 feet long and 9·8 feet in diameter. The whole cylinder can be raised by winches (_Min. Proc. Inst. C.E._, vols. cliii. and clvi.). 8. =Remarks on Sluices.=--In all kinds of sluice openings or regulators, the principles of design as regards protection of the bed and sides, splaying and curving of walls and piers, thickness of floor, and prevention of the formation of streams under the structure are the same as laid down for weirs. In order that a pier may be safe from being overturned by the pressure of the water when the gates or timbers are down, the resultant of its weight, including that of anything resting on it, and of the water pressure on it, must pass through the middle third of its length. This generally occurs when there is an arched roadway. Otherwise it must be arranged for by prolonging the base of the piers downstream, and giving the downstream side a batter or steps. The floor should usually be placed at a level somewhat lower than the mean bed-level of the stream. The bed may possibly be lowered in course of time. Lowering the floor also gives a greater thickness of water cushion to take the shock of water falling over the gates or planks. It is convenient to build, on the floor, a low wall or sill, reaching up to the level of the bed or thereabouts, and running across from pier to pier under the line of gates or needles. The height of the gates or needles can thus be reduced, and there is little chance of silt or stones collecting and interfering with them. In the case of needles the wall must be strong enough to resist their horizontal pressure. If ever the bed is lowered, the wall can easily be cut down or removed. Sluices with gates are, of course, used in connection with works other than weirs or regulators, as, for instance, in reservoirs or locks, or generally for communication between any two bodies of water. The gate may or may not be wholly submerged. If it is not wholly submerged, planks can be used. Needles can be used if the flow is always in one direction and never in the reverse direction. In all cases protection downstream of the opening is required.