CAISSON
, in Architecture, a kind of chest or flat- bottomed boat, in which the pier of a bridge is built, then sunk to the bottom of the water, and the sides loosened and taken ofs from the bottom, by a contrivance for that purpose; the bottom of the caisson being left under the pier as a foundation to it. The caisson is kept afloat till the pier is built to above the height of low-water mark; and for that purpose, its sides are either made of more than that height at first, or else gradually raised to it as it sinks by the weight of the work, so as always to keep its top above water. Mr. Labelye tells us, that the caissons in which he built some of the piers of Westminster bridge, contained above 150 load of fir timber, of 40 cubic feet each, and that it was of more tonnage or capacity than a 40 gun ship of war.
Caisson, in Military Affairs, is sometimes used for a chest; and in particular for a bomb or shell chest, and is used as a supersicial mine, or fourneau. This is done by filling a chest either with gun powder and loaded shells, or else with shells alone, and burying it in a spot where an enemy, besieging a place, is expected to come, and then firing it by a train to blow the men up.
