\Cais"son\, n. [F., fr. caisse, case, chest. See 1st
{Case}.]
1. (Mil.)
   (a) A chest to hold ammunition.
   (b) A four-wheeled carriage for conveying ammunition,
       consisting of two parts, a body and a limber. In light
       field batteries there is one caisson to each piece,
       having two ammunition boxes on the body, and one on
       the limber. --Farrow.
   (c) A chest filled with explosive materials, to be laid in
       the way of an enemy and exploded on his approach.
2.
   (a) A water-tight box, of timber or iron within which work
       is carried on in building foundations or structures
       below the water level.
   (b) A hollow floating box, usually of iron, which serves
       to close the entrances of docks and basins.
   (c) A structure, usually with an air chamber, placed
       beneath a vessel to lift or float it.
3. (Arch.) A sunk panel of ceilings or soffits.
{Pneumatic caisson} (Engin.), a caisson, closed at the top
   but open at the bottom, and resting upon the ground under
   water. The pressure of air forced into the caisson keeps
   the water out. Men and materials are admitted to the
   interior through an air lock. See {Lock}.