Locks are watertight chambers which enable boats to ascend or descend to different levels in a canal or river.
The lock is usually rectangular in shape with gates at either end. If a boat has to go to a higher level it enters through the bottom or downstream gates of the lock, which are then closed. The water level in the lock is raised to that of the higher part of the canal by filling from the upper level and the upstream gates are opened to let the boat out. The opposite procedure takes place when a boat needs to descend.
Locks used to be made of timber, brick or stone, but now concentrate and steel piling are more usual. Originally the chamber was filled or emptied by sluices in the gates. Now a day, as locks become bigger, these are often replaced by conduits or pipes running the whole length of the structure, with offshoot pipes running into the lock to give an even discharge of water. Old locks may be manually operated but new ones are worked by hydraulic power.
Locks vary tremendously in size from about 123 by 17 feet (38 by 5 meters) on small canals to the giant locks on the Mississippi River in America, which are 1,200 by 110feet (366 by 33.5 meters).
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