Caisson
a military term, denoting a wooden chest to hold ammunition; CAITHNESS CALAIS formerly applied to the ammunition- as basins, water bottles, and wagon itself. Also a wooden box containing shells and loose powder, which was buried in the ground and ignited by means of a fuse when the enemy was passing over it. In engineering a caisson is a wooden, metal, or concrete case or frame sunk in the beds of rivers, etc., to keep out the water during the laying of the foundations of a bridge, etc. It is constructed of strong timbers, or other material, firmly and closely joined together. The term in architecture is applied to sunk panels of various geometrical forms symmetrically disposed in flat or vaulted ceilings, or in soffits generally.