The _air chamber_ is made of the finest compressed steel, or of phosphor-bronze, a third of an inch thick. When ready for action this chamber has to bear a pressure of 1350 lbs. to the square inch. So severe is the compression that in the largest-sized torpedoes the air in this chamber weighs no less than 63 lbs. The air is forced in by very powerful pumps of a special design. Aft of this chamber is that containing the stop-valve and steering-gear. The stop-valve is a species of air-tap sealing the air chamber until the torpedo is to be discharged. The valve is so arranged that it is impossible to insert the torpedo into the firing-tube before the valve has been opened, and so brought the air chamber into communication with the starting-valve, which does not admit air to the engines till after the projectile has left the tube. The _steering apparatus_ is undoubtedly the most ingenious of the many clever contrivances packed into a Whitehead torpedo. Its function is to keep the torpedo on an even keel at a depth determined before the discharge. This is effected by means of two agencies, a swinging weight, and a valve which is driven in by water pressure as the torpedo sinks. When the torpedo points head downwards the weight swings forward, and by means of connecting levers brings the horizontal rudders up. As the torpedo rises the weight becomes vertical and the rudder horizontal. This device only insures that the torpedo shall travel horizontally. The valve makes it keep its proper depth by working in conjunction with the pendulum. The principle, which is too complicated for full description, is, put briefly, a tendency of the valve to correct the pendulum whenever the latter swings too far. Lest the pendulum should be violently shaken by the discharge there is a special controlling gear which keeps the rudders fixed until the torpedo has proceeded a certain distance, when the steering mechanism is released. The steering-gear does not work directly on the rudder. Mr. Whitehead found in his earlier experiments that the pull exerted by the weight and valve was not sufficient to move the rudders against the pressure of the screws. He therefore introduced a beautiful little auxiliary engine, called the servo-motor, which is to the torpedo what the steam steering-gear is to a ship. The servo-motor, situated in the _engine-room_, is only four inches long, but the power it exerts by means of compressed air is so great that a pressure of half an ounce exerted by the steering-gear produces a pull of 160 lbs. on the rudders.