caisson

caisson is defined in Webster's Unabridged Dictionary (1913) with 2 senses. The full text of each entry is reproduced verbatim below.

Definitions

1.(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.
2.(Arch.) A sunk panel of ceilings or soffits. 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.

Source: Webster's Unabridged Dictionary, 1913 edition (public domain, via GCIDE / Project Gutenberg).

Definitions: Webster's Unabridged Dictionary, 1913 edition (public domain, via GCIDE / Project Gutenberg).
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