Of canal structures, the most important probably is the lock, a device to raise or lower boats from one level to another. A lock, placed at the meeting of two canal levels, is simply a chamber or basin which has a gate or set of gates at each end and side walls extending from a little above the water-surface of the higher level to the bottom of the lower level. When a boat enters a lock the closed gates at the other end maintain the difference in elevation between the two levels. After the gates are closed behind the boat, water is ad mitted to or drawn from the lock chamber, so as to raise the boat to the upper level or lower it to the lower level. Then the gates ahead of the boat ate opened and it passes to the new level, the point of difference in elevation be tween the two levels having been transferred to the gates through which the boat entered the lock.
Various kinds of lock-gates have been em ployed, the most common being the mitre-gate. This consists of two leaves swinging on ver tical axes from the side walls and meeting at an angle in the centre with the apex toward the upper level. Wood was the material formerly used for mitre-gate construction, but now steel is generally used. Improvement in gate design has made possible locks of more than 40 feet lift, whereas 10 or 12 feet was the usual maxi mum. The tumble-gate, sometimes used at the upper end of the chamber, is mounted on a horizontal axis at the bottom and is raised or lowered mechanically. Lift-gates, which can be raised high enough to clear the boats, and rolling-gates, which roll back into a recess formed outside the lock at right angles to the side wall, have also been used.
On the smaller locks, when mitre-gates were used, the water in the lock chamber was con trolled by means of wickets or valves in the gates, but where the other kinds of gates are used and in all large locks, culverts are em ployed. These are built in the side walls or under the floor of the lock chamber, and in the larger locks usually extend just beyond the gates at both ends of the lock, connecting with the pools above and below the lock. They have several openings to the lock chamber, in order that the lock may be filled or emptied in a reasonable time with the least disturbance to the craft in the lock. The flow in these culverts is usually controlled by means of lift-gates or valves, located near each end. The Stoney gate, which operates on a nest of rollers, is being used largely on the New York State Barge Canal. There are a few instances of locks in which the culvert has been formed near each end into a siphon that has its neck above the highest water in the upper pool. A closed tank built in the masonry, which is filled from the upper pool and has an outlet to the lower pool and air pipe connection with the neck, primes the siphon, the water flowing from the tank tending to create a vacuum, which is filled by air from the neck.
The quantity of water required for one complete lockage is equal to the volume of. the lock chamber between the elevations of the two pools. However, when the lockages alternate up and down, this quantity of water will serve to lock a boat each way. Where conditions permit, a saving of water is sometimes effected by the construction of a side pool. When the lock is to be emptied, some of the water is drawn into the side pool and retained above the level of the lower pool, thereby being available for filling the lower portion of the lock cham ber at the next lockage. Evidently the quantity of water thus saved must be somewhat less than half the quantity required for one lockage.
There are a few examples of another type of lock—a structure consisting of a tank into which the boats pass and which is raised or lowered by power. Gates are provided at each end of the tank and also in the canal adjacent to the tank. Sometimes two tanks are operated side by side on a kind of balance. Since the weight of a tank and its contents is nearly constant, very little power is required and that mostly to overcome friction. With this type no water is consumed for lockage and but a small amount for power. On account of cer tain structural difficulties, however, very few locks of this variety have been built.
Inclined planes have been used occasionally in small canals in place of locks, the boats being hauled out of the water and up or down the incline in a saddle. Obviously these are not suited to large boats on account of the difficulty of safely supporting the boat and also because of the great amount of power required.
Of the other structures used in canals the movable dams deserve special mention, because of the part they play in the. utilization of nat ural streams for canal purposes. These struc tures are designed to fulfil all the functions of a fixed dam, when it is desired to maintain the pools for navigation, and at other times, when a fixed dam would cause excessive floods, to be capable of being removed from the channel area either by being lowered to the river bed or by being raised above the flood level. Many kinds have been devised and used, the bear trap, of which there are several varieties, the Chanoine wicket, the Boule gate, the Taintor gate, the rolling drum and the Poiree needle dam being best known. Boule gates are often used in connection with a sort of bridge superstructure, forming what is com monly called a bridge dam. Many types of automatic crests and also of controlled flash boards have been devised for special cases, to serve partially the same purpose as the movable dam, but their use is less general.