Locks in canals and rivers are used for raising or lowering vessels from low to high level water, or vice versa. At the place where, owing to the presence of a weir or otherwise, the level of the water changes, a sufficient length of the river or canal bed is provided with a gate at each end, opening in such a direction that the pressure of the water tends to keep them closed. The gates are provided with sluices, or openings which may be closed by slides. When the lock is full the upper gates can be opened to admit vessels; the gates being closed, water is allowed to escape through the sluices until that in the lock reaches the lower outside level; the lower gates can then be opened to allow vessels to go out, or those coming up stream to enter. The lock is then refilled by closing the lower gates and opening the sluices in the upper ones.