caisson

n. [ F., fr. caisse, case, chest. See 1st Case. ] 1. (Mil.) (a) A chest to hold ammunition. (b) A four-wheeled carriage for conveying ammunition, consisting of two parts, a body and a limber. In light field batteries there is one caisson to each piece, having two ammunition boxes on the body, and one on the limber. Farrow. (c) A chest filled with explosive materials, to be laid in the way of an enemy and exploded on his approach. [ 1913 Webster ]

2. (a) A water-tight box, of timber or iron within which work is carried on in building foundations or structures below the water level. (b) A hollow floating box, usually of iron, which serves to close the entrances of docks and basins. (c) A structure, usually with an air chamber, placed beneath a vessel to lift or float it. [ 1913 Webster ]

3. (Arch.) A sunk panel of ceilings or soffits. [ 1913 Webster ]

Pneumatic caisson (Engin.), a caisson, closed at the top but open at the bottom, and resting upon the ground under water. The pressure of air forced into the caisson keeps the water out. Men and materials are admitted to the interior through an air lock. See Lock. [ 1913 Webster ]