A self-contained underwater breathing apparatus (scuba) in which gas flows from a cylinder carried on the diver’s back, through a regulator and tube to a second regulator at the mouthpiece through which the diver breathes. Gas that the diver exhales passes into the water, so that the device is an open-circuit system. The version most widely used was invented in 1942–3 by the French engineer Émile Gagnan and (then) naval lieutenant Jacques Cousteau.