With the increasing use of self-contained underwater breathing apparatus in archeological collection work, geological and petroleum prospecting, and biological collecting, diving equipment has taken on real importance as a scientific tool. Numerous techniques have evolved to make diving units easier to breathe from so that divers can make longer work or exploratory dives and so that neophytes will be less likely to suffer lung fatigue. This report (1) presents a brief resume of some of the principles of existing apparatus along with concepts leading to more advanced forms. The principal emphasis is on "open circuit" equipment in which air is inspired from a high-pressure tank and expired into the surrounding water. In such cases the compressed air is also an energy source, which allows incorporation of certain forms of regenerative feedback leading to easier breathing.Such equipment uses a regulator which feeds the diver air at a pressure just equal to that of the surrounding water. In essence, a regulator is a box with a flexible side which activates an internal valve. If the air pressure within this box is reduced to below that of the surrounding water, then the valve will be opened by the diaphragm, thus allowing air from the high-pressure cylinder to enter the box, until the equality of internal and external pressure is returned. Through a hose, the diver breathes from this box. When he exhales, his breath travels through the other hose attached to the mouthpiece or mask and out through a Instructions for preparing reports. Begin the report with an abstract of from 45 to 55 words. The abstract should not repeat phrases employed in the title. It should work with the title to give the reader a summary of the results presented in the report proper. (Since this requirement has only recently gone into effect, not all reports that are now being published as yet observe it.)