Besides these chemical means there are rubber tubes floated by buoys, with nozzles which protrude above the wash of the surface water. There is in each tube an automatic valve, which prevents water coming through the pipe at the time the air is being pumped in, and the depth below the surface to which outside air can be supplied is limited only by the length of the pipe. In the plate, A represents a patented interchangeable boiler, in which either hydro-carbonate fuel or caustic soda can be used, in both cases steam being the motive power. The interior boiler for the use of the caustic soda is surrounded by a jacket, into which the steam exhausted from the engine can be used before it becomes so saturated as to create a back pressure on the engine, that is, for a period of twelve hours. When this limit is attained, and the surface is reached, the soda can be blown off into an outer receptacle provided for the purpose, and then reheated and recharged. The hydro-carbon fuel is ordinary mineral oil, carried in tanks of sufficient capacity for a surface run of a week. It may be emphasized as an important fact that this method of exhausting into the jacket of the boiler avoids the possibility of any bubbles appearing on the surface, as was notably the case with the earlier Lay boats. Before diving, the caustic soda, which has been already heated by the combustion of the oil to the proper degree, acts in place of the ordinary fuel, thus constituting a sort of perpetual motion, until the point of saturation is reached, and back pressure in the engine results. The boat, when on the surface, is run with the oil fuel, but as soon as it becomes necessary to dive this fire is extinguished, the after-hatch is opened by unlocking the door of the bulkhead separating the after from the bulkheaded end of the vessel, and by a system of fans the hot air from the fire-room is driven outboard. Then the after telescopic hatch is reefed and secured, the soda is thrown from the receptacle where it has been heated into the jacket of the caustic-soda boiler, the fires are put out, the smoke-stack is taken in and securely fastened, and the machinist, leaving the engine-room, goes through the bulkhead door into the forward compartment, where he has complete control of the machinery and boiler by means of a duplicate set of gauges and levers. In case of an attack, the man detailed for operating the main torpedo is left in the after compartment, where he has access to that weapon and to the buoy, reel, and other mechanical appliances employed in its operation.