The mode of working is represented diagrammatically in figs. 37 and 40. After putting on the helmet, the bag is first filled with fresh air, the air valve is then closed, and the valve of the oxygen cylinder unscrewed so as to permit of the flow of the oxygen which, mixes with the air in the bag, and begins to circulate; the expired air passes through the caustic potash pellets P, which free it of carbonic acid gas, so that, with a fresh supply of oxygen from the cylinder through the pipe C, it is regenerated and made fit for breathing again. The pressure in the cylinder is measured by a manometer, which indicates also when the supply of oxygen gives out. [Illustration: FIG. 38.—Showing the Potash Cartridge No. 2 with Change Mechanism X; No. 2 Oxygen Cylinder with Spanner V; and on the Left a Hexagonal Socket U, for unscrewing the Locking Nuts of Reserve Cylinders (_R. Jacobson_)] [Illustration: FIG. 39.—‘Proto’ patent self-contained breathing apparatus (_Siebe, Gorman & Co._)] Another apparatus—the ‘Proto’ patent self-contained breathing apparatus (Fleuss-Davis patents)—is also illustrated in fig. 39. Illustration 40 gives a diagrammatic view of the principle upon which it is designed. The instructions for using the ‘Proto’ apparatus are as follows: _The oxygen cylinders_ (B, B), having been charged with oxygen through the nipple at (H) to a pressure of 120 atmospheres (about 1800 lbs. per square inch), are to be re-attached to the belt as shown, and the reducing valve, with its tubes, &c., is to be connected to the nipple at (H). This supply is sufficient for fully two hours. _Charging the breathing bag._—Put 4 lbs. of stick caustic _soda_ into the bag (D), i.e. 2 lbs. into each compartment, and immediately fasten the mouth of the bag by means of the clamps and wing nuts (O). If the apparatus is not to be used at once, but is to be hung up for use at some future time, the indiarubber plug which is supplied with the apparatus should be tightly fitted into the mouthpiece in order to prevent access of air to the caustic soda, and to preserve it until required for use. See that the inlet and outlet valves (T and S) and the connection (N) are screwed up tightly. The small relief valve (K) is only to be opened (by pressing it with the finger) when the bag becomes unduly inflated through excess of oxygen. This may occur from time to time, as the reducing valve is set to deliver more than the wearer actually requires. _Equipment._—The whole apparatus is supported upon a broad belt which is strapped round the body. The bag is also hung by a pair of shoulder braces.