CAISSON

(Fr.) In Architecture, a sunken panel in a flat or vaulted ceiling, or in the sofflt of a cornice. In ceilings they are of various geometrical forms, and often enriched with rosettes or other ornaments. Cai on, In bridge building, is a large chest or ve el in which the piers of a bridge are built. This sinking as the work advances,its bottom at last comes in contact with the bed of the river, when the sides are disengaged, its construction being such as to admit of their being thus detached without injury to the floor or bottom. [s. 194]