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. There is also a heavy pendulum, swinging fore and aft and attached to the diving-planes, that checks any sudden up-or-down movement of the torpedo by inclining the planes and restoring the horizontal position. Next comes the engine-room, with its three-cylinder motor, capable of developing from thirty-five to fifty-five horse-power. The exhaust air from the engine passes out through the stern in a constant stream of bubbles, leaving a broad white streak on the surface of the water as the torpedo speeds to its mark.