A caisson lock is a type of canal lock in which a narrowboat is floated into a sealed watertight box and raised or lowered between two different canal water levels. It was designed primarily as a water-saving measure, and also to minimise construction costs compared with other engineering solutions of the time. In use it was capable of replacing up to seven conventional locks. Other design benefits were speed of boat descent/ascent, and only a little loss of water when operating compared with a conventional boat lock.
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Bath and North East Somerset