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If the lock were empty, the boat would have had to wait 5 to 10 minutes while the lock was filled.
For a boat travelling upstream, the process is reversed; the boat enters the empty lock, and then the chamber is filled by opening a valve that allows water to enter the chamber from the upper level.
The upper chamber rises 60 feet (18 m) and is connected to the lower chamber by a tunnel, which when descending does not become visible until the chamber is nearly empty. The chamber may be the same size (plus a little manoeuvring room) as the largest vessel for which the waterway was designed; sometimes larger, to allow more than one such vessel at a time to use the lock.
The chamber is said to be "full" when the water level is the same as in the upper pound; and "empty" when the level is the same as in the lower pound.
This type can be found all over the world, but the terminology here is that used on the British canals. Both locks are amalgamations of two separate locks, which were combined when the canals were restored to accommodate changes in road crossings.
Boaters approaching a lock are usually pleased to meet another boat coming towards them, because this boat will have just exited the lock on their level and therefore set the lock in their favour — saving about 5 to 10 minutes.
A pound lock has a chamber with gates at both ends that control the level of water in the pound.
In contrast, an earlier design with a single gate was known as a flash lock.
The gates were 'hanging gates'; when they were closed the water accumulated like a tide until the required level was reached, and then when the time came it was allowed to flow out.
A famous civil engineer of pound locks in Europe was the Italian Bertola da Novate (c.
A lock chamber separated from the rest of the canal by an upper pair and a lower pair of mitre gates.