CAISSON, in engineering work, is a chamber of iron or wood which is used in the construction of subaqueous foundations, - such as those required for the piers of bridges, &c. Its object is the same as that of a coffer-dam, viz., to allow the work to be carried on below the waterlevel, - but it is used in places where either the water or the permeable soil is too deep to allow a darn to be erected. In cases where the bridge piers are hollow cylinders of iron, they not unfrequently form their own caissons, - their own weight, or that of ballast placed upon them, forcing their lower edges into the ground. The material left within them is dredged up or excavated as they descend. Where, however, the soil is not so soft, or is mixed with stones, this self-lowering becomes impossible. The lower part of the caisson is then commonly formed into an air chamber, open at the bottom, and resting upon the bed of the river Air is pumped into this at a pressure corresponding to its depth below the surface of the water, and the excavation is carried on by men working in the compressed air as in a large diving-bell. In some cases the masonry of the pier is built within the caisson on the top of the chamber as it descends, the chamber itself being eventually filled up with masonry or concrete, and left to form the permanent base of the structure ; in others the caisson is lowered (as the excavation goes on) by weights ; and when the required depth has been reached, the masonry is commenced within the air-chamber, and the whole caisson raised again as the building proceeds. Probably the largest caissons ever used are those of the East River Suspension Bridge (a structure still unfinished) at New York, of which one was 172 feet long by 102 feet wide. See BRIDGES. For military caissons see FORTIFICATION.