A lock

These three diagrams show the movement of a boat along a canal over a lock.
The process involves a structure which is built on the canal to let boats move from one water level to another.The construction consist of two identical gates and a large section of the canal to take one or more boats.The gates have valves,called ‘paddles’ which are used for lowering the water level in the lock.
The diagrams illustrate that when the upper gates are open the boat moves into the space between the gates.Then, the upper gates are closed and the ‘paddles’ are opened and as a result, the water level decreases.When level inside the section and behind the lower gates are equal,the last gate is opened.And the boat can leave the lock.When the boat moves in the opposite direction from lower to upper water level,the lower gates are opened first. The boat comes into the section between the gates and they are closed behind boat.The ‘paddles’ of the upper gates are opened,so after some time levels behind upper gates and inside the section become identical.Then the upper gates are opened and the boat moves out of the lock.

In conclusion,we can see that this structure is useful for moving boats along canals with different levels of water on both sides:upward and downward.