Now in a modern submarine, of five hundred tons submerged displacement, for instance, this reserve should be about one hundred and twenty-five tons, according to the best practice. This means that before the vessel could sink beneath the surface the ballast tanks must be filled with one hundred and twenty-five tons of water. On the surface these tanks are filled with air. The water is permitted to enter by the opening of valves for that purpose. These ballast tanks are located within the main hull and in the superstructure. =Propelling Machinery.=--When on the surface the submarine may be propelled by steam, internal-combustion engines, or any other kind of motive power adapted to the propulsion of surface ships. For propulsion when submerged many types of engine have been tried: compressed air engines; steam engines drawing the steam from boilers in which water has been stored at high temperatures; carbonic acid gas engines, and the internal-combustion engines receiving their air supply from compressed-air tanks. Most modern submarines use internal-combustion engines for surface navigation and storage batteries delivering current to electric motors for submerged propulsion. The internal-combustion engine is best suited for surface work because it can be started or stopped instantly, which is a desirable feature in submarine work. It is not fitted for submerged operation because of its great noisiness, and also because its spent gases must be discharged from the boat, in which case these gases ascend to the surface in the form of bubbles and thus betray the presence and position of the submarine. The storage battery, on the contrary, permits the use of practically noiseless machinery and does not require any outboard discharge of gases, as the battery gives off no material quantity of gases when delivering its stored-up power. I was the first to use successfully an internal-combustion engine in a submarine boat, the _Argonaut_. This first engine was a heavy-duty engine of rugged construction, and gave but little trouble. This type of engine, with but slight modifications, was installed in six other boats built subsequent to the _Argonaut_. They also worked satisfactorily for several years, and so long as I had knowledge of them they always gave satisfactory and reliable service. The first gasolene (petrol) internal-combustion engines installed in the Holland boats were also of rugged construction, and I have been informed by various officers in our submarine service that they were reliable and gave but little trouble. It is known that, after twelve years' service, some of them are still doing good work. The boats in which these engines were installed were slow-speed boats, making only from eight to nine knots on the surface.