Submersion to the various depths required is secured by the motion of the vertically acting screws, S S, driven by small three-cylinder engines. The boat is so ballasted as always to have spare buoyancy, and while a few revolutions of the screws will send her under water, the arrest of their motion is all that is required to bring her to the surface again. In this arrangement, as even the non-technical reader will readily understand, there is a great element of safety, the rising motion being entirely independent of any machinery which might refuse to act at the required moment. Another advantage is also gained in the ease with which the horizontal position is maintained by regulating the speed of the screws. To assist in keeping this position there is a horizontal rudder or fin, R, at the bows, which, by a very ingenious arrangement of a plumb weight with other mechanism in connection with the steering tower, works both automatically and by hand. The torpedoes are carried on the outside of the boat, as shown at F. They are Swartzkoph or Whitehead, as the case may be, and are released by electrical action under the control of the captain, standing on the platform at P. C is a cupola of stout glass by which a view is obtained occasionally when the boat is running submerged. _Construction Details._—The following are the dimensions of the Turkish boat: length 100 feet, beam 12 feet, displacement 150 tons, speed 12 knots, and coal endurance sufficient for travelling 900 miles. The engines (E) are of the ordinary inverted compound surface-condensing type, with two cylinders, and with 100 pound pressure indicate 250 horse-power. The circulating and air pumps being actuated by a separate cylinder, the main engine is left free to work or not, while a vacuum is always maintained to assist the various other engines with which the boat is fitted. In this respect it should be mentioned that all the engines are specially designed with such valve arrangements as will make the utmost use of the vacuum, it having been found that while the boat is running beneath the surface as much power can be developed below the atmospheric line as above it. The boiler, B, is of the ordinary marine return-tube type, with two furnaces, and the heating surface is about seven hundred and fifty square feet. The tanks at each end of the boat contain about fifteen tons each, and there is a third of seven tons capacity at the bottom of the central compartment for regulating buoyancy. The coal is stored around the hot-water cistern as well as at the sides of the boiler and over the central ballast tank. Three men and the captain can efficiently work this boat, although she may carry a crew of seven, who could remain in her for over seven hours beneath the water without experiencing any difficulty in respiration. No attempt is made as in some systems to purify the atmosphere by chemical means, as it is said to be quite unnecessary.