Next the air chamber comes the balance, or secret chamber, although the secret is now universally known. Here is contained the mechanism for automatically transmitting to the horizontal rudders the movements necessary for keeping the torpedo at a uniform and pre-arranged depth below the surface during its run. It consists of a hydrostatic valve and a pendulum whose combined movements are transmitted to an air cylinder called a “servo-motor,” placed in the engine room. The hydrostatic valve is kept in its place by a spring that is forced in by the pressure of the water when the torpedo goes below a certain depth to which the valve has previously been adjusted. If the pressure be less than that of the set depth the opposite action takes place. This valve is connected with the servo-motor, which in its turn acts on the horizontal rudders. The pendulum consists of a heavy iron weight curved to correspond with the circular section of the torpedo and suspended by the pivoted steel rods or arms. It swings in a fore and aft direction and is connected by rods to the rudder for a certain distance after the discharge of the torpedo. A controlling gear is provided which keeps the rudders fixed. It will thus be seen that by the combined actions of the hydrostatic valve and the pendulum the Whitehead, after leaving the tube, is brought to the proper depth very rapidly and is held at this depth throughout her run. Both these devices are necessary, as the torpedo has a great tendency to run down an inclined plane at great speed, and this requires to be checked. In addition the balance chamber contains various valves (the stop valve, the charging valve, the starting valve, the delay action valve and the reducing valve) through which the air passes on its way to the engines from the air chamber. 5. THE ENGINE ROOM. Inside the engine room are the propelling engines and the servo-motor. The engines are of the single-acting three-cylinder Brotherhood type. The compressed air, after leaving the air reservoir, passes through the main pipe to the pressure-reducing valve. In the latest pattern 18 inch Whitehead the indicated horse power is 56. The torpedo is started by means of a trigger which projects a little beyond the casing of the torpedo, and which automatically opens the starting valve when the torpedo is fired, the trigger just before leaving the tube is caught by a catch in the tube which draws it back when the catch releases itself. THE SERVO-MOTOR.