A submarine is a surface ship which can be submerged and driven ahead at a steady depth-line. She is built strongly in order to resist the water-pressure when she is deep down. She is propelled on the surface by (usually) heavy-oil Diesel engines. When submerged she cannot use these, as they consume air, so she has an electric battery and motors for use under water. The battery is charged when on the surface at her convenience or by favour of the enemy. The Diesel engines are used for this purpose, and they recharge the batteries through the motors, using the latter as dynamos. In old pictures of projected submarines of the seventeenth century one sees that the principle of "trimming down" for submersion was known, and that our present-day system of tanks was intended to be substituted by pig-skins which fitted into cylindrical hollows in the hull. These skins were emptied by screwing out a ram from inside (on the idea of old-fashioned printing presses) which squashed the skins. Nowadays a boat is all tanks along her lower half, the upper half being living space and battery room, etc. These tanks are flooded through valves in order to destroy the boat's buoyancy. The water is ejected from them either by pumps or by the use of compressed air, the latter taking the place of the old seventeenth-century screw-press idea. When the tanks have flooded until the boat's buoyancy is all gone--_i.e._, until you could press her down or lift her up with one hand--she is "trimmed," and by going ahead and working the bow and stern hydroplanes you can keep whatever depth-line is required. When submerged, a look-out is kept through a periscope--a tube about thirty feet long which has lenses all the way up, is watertight, and has an eyepiece like an ordinary telescope at its lower end. To dive, a boat opens her vents, puts "dive" helm on, and goes under with her motors running. The flooding valves are kept open to save time; in surface trim a boat is "hanging on the vents"--_i.e._, if you open the vents (upper valves of the tanks) the water enters and she goes down; until the vents are open the water cannot enter beyond a certain point; when they do open the air in the tanks can get away and the tanks fill up with a rush.