caisson

The word caisson comes from the French caisse, meaning 'case' or 'box,' combined with the augmentative suffix -on, yielding 'big box.' The French caisse derives in turn from Latin capsa, a case or container for books and documents, which is also the ancestor of 'capsule,' 'case,' and 'chest.' In its earliest military uses, a caisson was a wheeled ammunition chest drawn behind artillery — the box that carried gunpowder and shot. This sense survives in the American military funeral tradition of the caisson, the horse-drawn carriage bearing a flag-draped coffin. But the engineering sense, which emerged in the seventeenth century and reached its fullest expression in the nineteenth, describes something altogether more consequential: a watertight chamber sunk to the bed of a river or harbor, in which workers could excavate and pour foundations for bridge piers, lighthouse bases, and harbor structures.

The open caisson — essentially a large hollow cylinder or box with no bottom, which is sunk by excavating the soil inside it until it settles to the desired depth — was used in Roman and medieval construction, though rarely for underwater work. The pressurized pneumatic caisson, which used compressed air to hold back the water and allow workers to labor in dry conditions, was the revolutionary nineteenth-century innovation that made deep underwater foundations possible. Compressed air, pumped into the sealed working chamber at the caisson's base, exerted enough pressure to prevent water from entering through the open bottom. Workers entered and exited through airlocks, spending their shifts in an environment of elevated atmospheric pressure that had invisible and catastrophic consequences that would not be understood for decades.

The building of the Brooklyn Bridge in New York, begun in 1869, brought the pneumatic caisson to public attention and revealed its human cost with appalling clarity. Two massive wooden caissons, each the size of a five-story building, were sunk to the East River bed — the Brooklyn caisson to 44 feet, the New York caisson to 78 feet. Workers labored inside them under compressed air pressures of up to 35 pounds per square inch. Many emerged from shifts with searing joint pain, paralysis, and in some cases death. The illness was called 'caisson disease' and later 'the bends,' caused by nitrogen bubbling out of the blood when workers returned too quickly to normal pressure. Washington Roebling, the bridge's chief engineer, suffered such severe decompression sickness while overseeing caisson work that he was never able to visit the construction site again, directing the project's completion from his apartment window through binoculars.

Caisson construction remains in use for bridge foundations, though modern techniques include bored pile foundations and drilled shafts that avoid the need to put workers in pressurized chambers. The term has also given its name to caisson disease — decompression sickness — which affects deep-sea divers, tunnel workers, and anyone else who experiences rapid changes in ambient pressure. The word travels from Latin book-box through French ammunition chest through Manhattan riverbed to the physiology of dissolved gases in human blood: a single term that connects Roman librarians, Napoleonic artillery, Victorian bridge-builders, and the rules governing how fast a scuba diver may ascend. The big box has left a large footprint.