SCALE  'M . up the curtains, and then lifting the hinged frames to a

horizontal position under the overhead bridge by means of chains worked by a winch on the bridge . This system, which has been employed for the lowest weir on the Moldau, and for a weir at the upper end of the Danube canal near Vienna to shut out floods and floating ice, as well as on the Seine, possesses the merits of raising all the movable parts of the weir out of water in flood-time, and rendering the working of the weir very safe and easy . On the other hand, it involves the expense of a wide foot-bridge for raising the frames, and wide and high river piers, especially for the navigable passes where the bridge has to be raised high enough to afford the regulation headway for vessels at the highest navigable flood-level (fig . 7), so that its adoption should be restricted to positions where the conditions are quite exceptional .