(820 ILCS 245/1) (from Ch. 48, par. 261)
    Sec. 1. When used in this Act, unless the context indicates otherwise, the term
    (a) "Caisson" means a wood, steel, concrete or reenforced concrete air-tight and water-tight chamber in which it is possible for men to work to excavate material.
    (b) "Lock" means a chamber designed to facilitate the passage of men and materials from an air pressure greater than normal, as in a compartment, caisson or tunnel, to the ground or water level or normal air pressure.
    (c) "Pressure" means gauge air pressure in pounds per square inch.
    (d) "Tunnel" means a subterranean passage or chamber.
(Source: Laws 1939, p. 614.)