Now, just as long as the torpedo moves straight ahead in its course, the gyro has no effect on it, but the moment the torpedo tries to swerve from one side to the other, or shift out of its course, the gyro pulls it back by acting on the steering rudder; it does this by regulating the air supply of a compressed air motor, called the servo-motor, and this in turn works the rudder by means of a series of levers. In this way the torpedo is automatically kept to its course.
The Hydrostatic, or Water Pressure, Control.—The water pressure control is one of the devices which keeps the torpedo at the right depth in the water.
It consists of a steel cylinder with a piston working in it. A cylinder which is open at both ends is set in the hull of the torpedo so that one end opens into the water outside. A spring presses against the piston on the inside of the hull and the water presses on the piston on the outside.
The spring can be adjusted by a screw to just balance the pressure of the water at any depth. A small valve rod is fixed to the piston, and this opens a valve which lets compressed air into a motor when the pressure of the water is greater or lesser than the pressure of the spring and this in turn works the diving rudder as shown in [Fig. 40].
Suppose, for example, that the captain wants the torpedo to travel at a depth of exactly 15 feet during its whole course. The spring is adjusted to offset the pressure of the water at this depth, and the torpedo is launched. Should it for any reason try to go deeper than 15 feet or to rise above 15 feet, the spring or the water presses on the piston and closes or opens the air valve of the motor, when it moves the diving rudder up or down until the torpedo is brought back to its right depth.
FIG. 40. HOW THE WATER PRESSURE CONTROL ACTS.
The Automatic Pendulum Control.—The water pressure control, however, is not enough to make the torpedo stick to an absolutely constant depth, and so to help it a pendulum control is also used.
This control consists of a pendulum which swings to and fro from fore to aft as the torpedo dives and rises, and it also starts the compressed air motor which operates the diving rudder, and this brings the torpedo back to its proper depth. It is shown in [Fig. 41].
The Engine That Drives the Torpedo.—Next comes the engine that drives the torpedo, and it, too, is run by compressed air.