Back of the war-head is the air-chamber, that contains the motive-power of this miniature submarine. The air is either packed into it by powerful pumps, on shore or shipboard, or else drawn from one of the storage flasks of compressed air, a number of which are carried on every submarine. The air-chamber of a modern torpedo is charged at a pressure of from 2000 to 2500 pounds per square inch. As the torpedo leaves the tube, a lever on its back is struck and knocked over by a little projecting piece of metal, and the starting-valve of the air-chamber is opened. But if the compressed air were allowed to reach and start the engines at once, they would begin to revolve the propellors while they were still in the air inside the tube. This would cause the screws to “race,” or spin round too rapidly and perhaps break off. So there is a “delaying-valve,” which keeps the air away from the engines till another valve-lever is swung over by the impact of the water against a little metal flap.
As the compressed air rushes through the pipe from the chamber to the engine-room, it passes through a “reducing-valve,” which keeps it from spurting at the start and lagging at the finish. By supplying the air to the engines at a reduced but uniform pressure, this device enables the torpedo to maintain the same speed throughout the run. At the same time the compressed air is heated by a small jet of burning oil, with a consequent increase in pressure, power, and speed, estimated at 30 per cent. All these devices are kept not in the air-chamber itself but in the next compartment, the balance-chamber.
Here is the famous little machine, once a close-kept secret but now known to all the world, that holds the torpedo at any desired depth. Think of a push-button, working in a tube open to the sea, with the water pressure pushing the button in and a spiral spring inside shoving it out. This push-button--called a “hydrostatic valve”--is connected by a system of levers with the two diving-planes or horizontal rudders that steer the torpedo up or down. By turning a screw, the spring can be adjusted to exert a force equal to the pressure of the water at any given depth. If the torpedo dives too deep, the increased water-pressure forces in the valve, moves the levers, raises the diving-planes, and steers the torpedo towards the surface. As the water pressure grows less, the spring forces out the valve, depresses the diving-planes, and brings the miniature submarine down to its proper depth again. When his torpedoes grew too big to be controlled by the comparatively feeble force exerted by the hydrostatic valve, Whitehead invented the “servo-motor”: an auxiliary, compressed-air engine, less than five inches long, sensitive enough to respond to the slightest movement of the valve levers but strong enough to steer the largest torpedo, exactly as the steam steering-gear moves the huge rudder of an ocean liner.