It'ater-supp/y.—Altliongh there is no flow of water in the channels of canals, yet, owing partly to evaporation and percolation and partly to leak age through the locks and the loss of a certain volume of water at each passage of a boat, there will be a constant waste that must be replenished from §onte source of supply. Canals that are not built along the course of a river or that do not open into a river at their highest level inust receive a supply from small streams along their course. In case a constant and sufficient supply cannot be obtained from such sources, it will be necessary to construct large storage-reservoirs for the purpose. To this end, a deep and narrow valley or gorge is selected, in which the water is impounded by building across its course a dam of masonry or earth. For conveying the water from the reservoir to the canal cast-iron pipes are used; these pass through the body of the dam, and are opened or closed by means of common gate-valves. To permit the repairing of the pipes, it is necessary to provide them at the in-take with an additional hinge valve (fig. 6), which in case of necessity may be closed by suitably operating the lever, to which a chain is attached for the purpose.
Surplus Ifirter.—For the discharge of surplus water which may enter the canal during floods, and mav overflow the tow-path and cause breaches in the banks, waste-weirs must be provided. The number and location of these will be governed bv circumstances. They should be located at the top water-level of the canal, and at suitable intervals along its course where dangerous accessions of flood-water may be apprehended. The syphon sluice (p/. 52, fig-. 7) is a self-acting device which discharges the surplus water when it reaches a certain height. Another form of self-regulating waste weir (fig. 8) is operated by a float which rests on the surface of the over flowing water, and is connected by levers with the draw-door of the weir.
Loss of certain quantity of water, corresponding to the vol ume required for filling the lock-chamber, is used in effecting the passage of each vessel throngh the lock. In dry seasons the amount of water at command is occasionally insufficient to provide for the needs of canal traf fic. Various plans have, therefore, been devised for reducing the large quantity of water that may be required for locking. Near the locks are arranged basins which receive the water flowing out of the former when they' are emptied, and return it when they are to be filled. The same ob ject is accomplished by building two locks side by side, one serving as the basin for the other. On the Great Western Canal, connecting the Thames
and the Severn, to overcome a lift of 44.6 feet are employed movable wooden chambers Nyhich can be made to counterbalance one another, and may thus be raised to the upper level or sunk to the lower, carrying the vessels with them. (See p. 320.) Locks (fig. 3) are side-channels which are separated from the body of the stream by solid walls and bottom, and whose ends are formed of mov able parts (gates) ‘vhich may be opened or closed, as may be required; they afford a means whereby a vessel may be transferred from a higher to a lower level, or vice versa, the vessel being floated into the chamber of the lock, where it remains while the water therein is either raised or lowered. Locks are usually built of sufficient size for a single boat only, but are occasionally built large enongh to accommodate several at a time (double locks). In this case, that the boat entering the lock first May also pass out first, the ends of the structure are not placed opposite to each other, but at a suitable angle.
The floor of the tail-end of the lock (tail-bay) is on a level with the bot tom of the lock-chamber, the ti.vo together forming the lower floor, while that at the head lies as much higher as the fall of the lock, and is called the upper floor, or head-bay. The difference in elevation between the bot tom of the lock- and the top of the head-bay is called the "lift." The bottom between the two portions is steeply inclined. At each end of the lock is the gate-chamber—that is, the space within which the two wings of the gate move in opening and closing. On each side of this chamber suitable recesses are provided for the reception of the leaves of the gate, so that when opened they may lie flush with the side (wing) walls of the lock. That portion of the recess just named in which the gate-post (quoin) turns is called the " hollow quoin." When closed, the gates rest against a bottom sill, and are pressed by the water of the npper level so firmly against the sill, the hollow quoins, and each other that the lock is thus rendered prac tically' water-tight.
Shticcs.—To admit the water above into the lock, and thus raise the vessel or vessels in them to the upper level, or to let out the water from the filled-up lock, and thus sink the vessel to the lower level of the canal, either sluices formed in the gates are used (pi. 52, fig. 3) or the water is admitted through suitable channels provided in the side wall of the lock, the opening or closing of said channels being controlled by a valve.