Caisson

Caisson, in relation to shipping, is an apparatus for lifting a vessel out of the water for repairs or inspection. It is usually a hollow structure, sunk by letting water into it. There is an air-chamber inside, which allows it to sink only to a certain depth. In that state it is hauled under the ship's bottom, the traps or openings are closed, the water is pumped out, and the caisson rises with the ship upon it. Pontoon is another term for the same apparatus.—The name caisson is also given to the pontoon or floating gate used to close a dry dock or similar place.