CAISSON. A floating gate with both ends alike, and resembling a short but very deep ves sel, which is used to close the entrance to a dry dock. Its framing and plating resemble those of a ship, and it frequently contains a boiler and pumps for clearing it of water after being sub merged. The term is also used, in a nautical sense. to designate a hollow floating structure for lifting ships. Caissons for this purpose are now generally made of steel. and are divided into compartments which can be separately con ne•ted with a pump. TO operate a caisson it is submerged to the required point. and then, having little or no buoyancy. it is drawn under the vessel to be raised or, with others. lashed to her with chains or ropes.. The water is then pumped out, and the ship rises, supported by the enisAons. Sectional floating docks, not now much consist caissons which are linked together to form a bed long enough to support a ship. in New York and in one or two other
ports there are contrivances for lifting vessels ealled screw-docks. These consist of caissons which are sunk sifilici(mtly to permit the vessel to pass over them. They are then emptied, and after they have lifted the vessel is far as their buoyancy permits, the caisson and vessel arc raised by in a side framework.
The pneumatic caisson used in hydraulic engi neering consists of a steel box without a lower side. but with sides and top air-tight. It rests on the bottom. into which the sharp edges of its *ides are forced by pressure or weight. To fur ther assist in keeping the water out. an adequate air-pressnre is maintained in it. The men who work in the caisson enter through air-locks which have air-tight doors on each side, so that very little pressure is lost. See Forxmyriox, and DOCK.