Operaiing-mcchanism of Draw-doors.—These draw-doors may be so arranged as to slide vertically in grooved guides formed at the sides of vertical frames or piles. By closing, these doors the water behind the dam is retained; by partly or entirely raising them the discharge may be controlled at pleasure. in important structures of this kind the draw doors and the frames in which they slide are constructed of iron, and to reduce the friction—which, in the case of large doors worked against a strong head of water, is considerable—the doors are sometimes 'uncle to slide over rollers disposed in the sides of the frames. The frames may be firmly secured in position by fastening- their upper ends to a stout horizon tal beam of timber, or by employing some other convenient mode of bracing. Entirely across the dam extends a foot-bridge, which carrics the appliances by which the doors are raised or lowered. These may vary greatly. In one form the door is suspended from a horizontal axle or drum, and is raised and lowered by a lever or crank attached to the same ( jig. 18); in other cases the object is accomplished by suitably-disposed gearing, etc. Whatever be the method adopted, however, the hoisting-mechanism must be powerful enough readily to permit the operation of the doors against the pressure of the highest flood-waters with the force of attendants on diity. To reduce the power required, as well as to avoid the Ilse of complicated mechanism, it is customary, in the case of heavy doors, to form them of sections, which lie one over the other, and which are raised or lowered successively (fig. 19). In place of the draw-door, other devices
have been sugrgested and applied to accomplish the same purpose; these, however, it will be unnecessary to describe.
Sick-channels and Locks.—The combination of the overfall dam with sluices (figs. 14, 15) serves, as above explained, to lessen the damming back of the water during floods by providing means for its more rapid dis charge than by overflow alone, and likewise to render navigation as well as rafting practicable in streams affected by a very variable water-level. The dam-openings for the passage of vessels are usually made in the form of locks (N. 32, fig-. 3), which mainly consist of an enclosure forming a side channel connecting the upper and lower reaches of the stream; this chan nel, which is provided at both ends with gates, is of sufficient length to receive the largest vessel required to be passed. By properly opening- and closing these gates the water-level in the lock may be raised to that of the upper reach or lowered to that of the lower reach. The passage of vessels from the upper level of the dam to the lower level below it, and z.frezrrsa, is thus rendered easy. (See Canals.) As the locks require to be protected against the entrance of flood-water, it is customary, where it may bp found necessary, to provide the dam with a munber of draw-doors, through which the water may be freely discharged should it rise high enough to threaten the invasion of the lock.