This ingenious apparatus was called into existence owing to the fact that the mechanism of the balance chamber was unable, through its feeble power, to work the horizontal rudders of the faster Whiteheads direct. The servo-motor is, then, the air engine from which is derived the power to move the diving rudders. It is only about 4 inches long, but so great is its power that with only half-an-ounce pressure on the slide valve the piston is capable of lifting 180 lbs. It consists of a cylinder, a piston, and a cylindrical slide valve. Its balance mechanism acts on the slide valve of the servo-motor, and this acts on the piston, and the motion of the piston is transmitted to the diving rudders by means of a rod and a system of levers. 6. THE BUOYANCY CHAMBER. Abaft the engine room is the buoyancy chamber which gives the necessary buoyancy to the torpedo: to guard against the collapse of the chamber flat steel rings are fitted into it for support. In the “Tail,” the rearmost compartments of the torpedo, are carried the bevel wheel mechanism, the vertical and horizontal rudders and the propellers, and the counter mechanism for adjusting the length of run. THE GYROSCOPE. From the foregoing description of the many devices employed to enable the Whitehead to accomplish the tasks for which it is intended, it might be thought that everything that science could imagine has been done to ensure its efficiency. There still, however, remained one great drawback to the efficiency of the torpedo, and this was its deflection from right to left, which was often so serious as to prevent it from striking the object at which it was aimed. The hydrostatic valve and the pendulum were sufficient to keep the torpedo at the required depth without diverging from her true vertical course, but it was apt to swerve from its course in a right or left direction either by reason of the blow it received on striking the water, by dents on its shell, by air leakage, or other causes. An error of only one degree in its course means a lateral error of nearly 50 feet at 800 yards, and it was in order to prevent the deflection of the Whitehead out of the line of fire that the principle of the gyroscope has been applied to the torpedo. In addition to her pair of ordinary vertical rudders, which may be set to any angle up to 20 degrees by means of a clamping screw, the torpedo carries a pair of movable vertical rudders placed in recesses in the vertical fins and controlled by the gyroscope through a servo-motor. The ordinary vertical rudders are usually discarded if the latter are carried. [Illustration: _Photos by_] [_West & Co._ THE FIRING OF A WHITEHEAD TORPEDO. ]