The Pound lock (usually just called "lock") is an early engineering solution to the problem of connecting waterways at different levels, which relies on the Archimedes principle.
The pound lock has two sets of gates; one set connects to the high waterway, and one to the low waterway. As you can see from this picture, the gates are angled towards the high waterway, so that water pressure closes the gate when there is a difference in water level.
Click on the picture for an animation showing how a lock works.
Although the lock is an ingenious solution, there is a problem. Each time the lock is filled and emptied, that lock full of water is lost. Later engineering solutions are much more water efficient.
The Pound Lock
The Anderton Boat Lift
By contrast, the Anderton boat lift consists of two caissons, which move up and down between two waterways. The two caissons are mounted on pistons which are connected to each other hydraulically and so counter balance each other.
The water level in the top caisson is made slightly higher than that in the lower caisson so that it is slightly heavier. To swap the caissons over (after closing the gates) a valve in the hydraulic system connecting the two pistons is opened, and the greater weight of the upper caisson forces the hydraulic fluid through the valve as it descends. The fluid forces the other piston to rise pushing the other caisson up.
The Falkirk Wheel
The Falkirk wheel in Scotland is another ingenious solution which uses no water. The two caissons are exactly the same weight and counter balance each other so the wheel always remains in balance.
The Falkirk wheel is driven round by a small 18KW electric motor and takes about 2 minutes to rotate. That’s about the same energy as it needs to boil a kettle. The system loses virtually no water at all.
All of these systems rely on Newton’s third law of motion to reduce power and water consumption.