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Related to pneumatic caisson: Cassion
pneumatic caisson[nu̇′mad·ik ′kā‚sän]
A caisson having a chamber filled with compressed air at a pressure equal to the pressure of the water outside.
1. A watertight structure or chamber, within which work is carried on in building foundations or structures below water level.
2. A sunken panel, esp. in a vaulted ceiling or the inside of a cupola; a coffer.