The principal features of the Nordenfelt boat are its method of submersion and its propulsion by steam. The boat is one hundred and twenty-five feet long, twelve feet beam, and displaces two hundred and fifty tons when entirely submerged, one hundred and sixty tons when running on the surface. Her propelling machinery consists of two double cylinder compound engines, with a horse-power of one thousand, and propelling the boat at fifteen knots on the surface. The submersion of the boat is effected by means of two horizontal propellers working in wells at each end. Two conning-towers project about two feet above the deck, of one-inch steel, surmounted by glass domes, protected with steel bars, for purposes of observation. The boat usually runs on the surface with these towers showing, unless the buoyancy, which is never less than half a ton, is overcome by the horizontal propellers, when the boat becomes partially or totally submerged according to their speed. To ascend to the surface it is only necessary to stop the horizontal propellers, which also stop automatically on reaching a set depth. In the forward tower are the firing keys, machinery and valves necessary for driving or steering the vessel, for controlling the horizontal propellers, and for discharging the Whitehead torpedoes. Four of these are carried, and they are discharged with powder from two tubes in the bows. In the conning-tower are also placed the instruments indicating the depth, level, and course. When the boat is awash, the funnels have to be unshipped and the boat closed up before submersion. The length of time, twenty-five minutes, required for this operation is an objection to this boat, though when submerged it does not get unpleasantly hot. The temperature after a three hours' submerged run was only ninety degrees Fahrenheit. The crew consists of a captain and eight men. The Tuck also comes from America. It is of iron, cigar-shaped, thirty feet long and six feet in diameter. It is submerged by means of a horizontal rudder in the stern and a horizontal propeller acting vertically amidships beneath the boat. It is driven by electricity, supplied from storage batteries packed closely in the bows. Compressed air is carried in reservoirs, but a supply is usually obtained when the boat is not far from the surface, by means of an iron pipe twenty feet long, which usually lies on deck, but which can be raised to an upright position by gearing from within. The top then rises above the surface of the water, and by opening a valve in the foot and attaching a pump, fresh air is drawn into the interior. The crew need not exceed three men. [Illustration: Section of the Goubet Submarine Boat.]