6. _Electricity._—All modern submarines rely on an electric motor for under-water propulsion, the current being derived from accumulators. The ideal primary battery and the ideal accumulator are still to seek, but the latter improves yearly, and there is little doubt that some few years hence the current available will enable the submarine to make long voyages under water with greatly increased speed. It is said that the _Holland_ has on no fewer than four occasions burned up the armature of her motor, and some device seems to be wanted to keep the armature cool. 7. _Carbonic Acid._—Many attempts have been made to construct an engine which can be worked by liquid carbonic acid, but the general result, as some one has said, has hitherto been that the inventors have been more or less broken up in body and mind. 8. _Liquid Air_ has been suggested as a propelling agent for submarines, but up till now it has not been applied for such a purpose. A motor-car propelled by liquid atmospheric air was shown at the Agricultural Hall in April last, and some energetic Americans are endeavouring to “boom” this elusive substance in this country. More sober investigators have, however, little faith in an immediate commercial future for liquid air. [Illustration: INSIDE THE “GOUBET.” ] PROPULSION OF THE SURFACE. While the “Holland” boats for the British and United States navies are driven on the surface by a gasoline engine, this type of motor has not yet been used on the French boats, a steam engine fed with liquid fuel being employed in the _Narval_ and vessels of this class, while in the _Gustave Zédé_, _Morse_, &c., electricity is the sole motive power both above and below the waves. _Le Yacht_ states that France has always avoided the use of gasoline owing to the danger which arises from its presence on board submarine craft. [Illustration: THE GASOLINE ENGINES OF THE FIRST BRITISH SUBMARINES. ] (_a_) _The Steam Engine._—In the Nordenfelt boat steam was raised, when running on the surface, by the burning of coal, but of late the advances that have been made by the employment of liquid fuel have led to the employment of this combustible for submarine boat propulsion in preference to coal. The great drawback at present to the use of the steam engine is the length of time necessary for the unshipping of the chimney, the cooling of the engines, &c. (_b_) _The Oil Engine._—At the present time there is no oil motor in existence of sufficient power to give even moderate speed to a large boat, but the ingenuity of the engineer will probably overcome this drawback. Those who make the dogmatic assertion that the submarine boat cannot be very fast because she cannot be endowed with much power, remind one of the wiseacres who were so convinced that steamboats would never replace sailing-vessels, nor steam locomotives the horse-drawn coach.