The system of using locks allows the water in different parts of the canal to be at different levels. This is done by closing both ends of each section of the canal with gates; a second pair of gates is placed a short distance beyond, and the space between these is called a "lock." If a vessel is to be taken into a section of the canal higher than that from which she has come, she goes into the lock; water is then let into this lock from the higher level by opening a water-gate until enough has entered to float the vessel up to the level of the higher section of the canal; the gates before the vessel are then opened and she passes out into the new section. If she is to be taken to a lower section, the reverse of this operation accomplishes this: the water is let out until she is on the lower level.

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