lock, canal, stretch of water enclosed by gates, one at each end, built into a canal or river for the purpose of raising or lowering a vessel from one water level to another. A lock may also be built into the entrance of a dock for the same purpose. When the ship is to be raised to a higher level, it enters the lock and a gate is closed behind it. Water is let into the lock until its level equals that of the water ahead. The forward gate is then opened, and the ship progresses on the higher level. The procedure is reversed when the vessel is to pass from a higher to a lower level. As many locks as necessary are used in a given waterway. Most modern locks are made of concrete, although some have walls of steel-sheet piles or floors of natural rock or sand. The mitre gate, frequently used in the United States, consists of two swinging sections forming an arc or shallow V, with the apex pointed toward high water so that water pressure keeps both sections tightly sealed when closed. Another type of gate in common use consists of one piece of sheet steel that slides across the entrance to the lock on rollers or is lifted into the air or sunk underwater. The gates of most locks are operated by hydraulic or electric power. Water is poured into or out of locks through culverts built into the masonry structure of the lock walls. Among well-known locks are those of the Panama Canal.