Its dimensions and details were as follows: Length 64 feet, beam 9 feet (over sponsons 12 feet), draught 11 feet, displacement 60 tons; speed on measured mile 9 knots; distance travelled without re-coaling 150 miles; depth to which safe descent was possible, about 50 feet. Engines, surface condensing compound type, with two cylinders and cranks at 90°; at pressure of 100 lbs. to square inch, indicating 100 horse-power. Boiler of ordinary marine return tube type, having one furnace, and about 200 square feet of heating surface; two hot-water cisterns, rhomboidal in body with spherical ends. The boilers and cisterns contained about eight tons of water. Both boilers and cisterns were made for a working pressure of 150 lbs. to square inch. One fish torpedo, 14 feet long, was carried outside on the bow and discharged mechanically. The sinking apparatus consisted of two vertical propellers driven by a 6–h.p. double-cylinder engine, and placed in sponsons on each side of the boat. The revolution of these caused the boat to descend horizontally when its buoyancy had been sufficiently diminished. There was one cold-water tank in the centre of boat, holding about four tons of water, for regulating buoyancy. This tank was used as coal bunker when doing long surface runs. In the stern was a four-bladed propeller 5 feet in diameter, and the rudder for port and starboard steering was placed aft of this propeller. In the bow on either side were balanced rudders on one and the same axle, always maintained in the horizontal position. The crew consisted of three men, and when the boat was closed up there was sufficient air to supply three men for six hours without causing discomfort, and this was not supplemented by any storage of compressed air or restorative chemicals. The depth below the surface at which the boat travelled could be varied in two ways; either by varying the speed of the vertical propellers, or by reducing the speed of the engines driving them by an automatic valve controlling the steam supply. On the surface the boat was driven by working the boiler in the usual manner, and the temperature of the water in the cisterns was kept up to a degree corresponding to a steam pressure of 150 lbs. When it was desired to descend, the ashpit and fire door were closed, as also the funnel inside the boat, and the vertical propellers were started. For sub-surface travelling there was available, as propelling power, the steam given off by the heated water (about eight tons), and this was found sufficient for a distance of 14 knots; on one occasion, when the boat was opened up, there was still over 20 lbs. pressure in the boiler. [Sidenote: Nordenfelt II.]