To keep a torpedo on course toward its target is a job. Maintaining the proper compass course with a gyroscope is only part of the problem. The torpedo must travel at the proper depth so that it will neither pass under the target ship nor hop out of the water on the way.

As figure 10-3 shows, the torpedo contains an air-filled chamber sealed with a thin, flexible metal plate, or diaphragm. This diaphragm can bend upward or downward against the spring. You determine the spring tension by setting the depth-adjusting knob.

Suppose the torpedo starts to dive below the selected depth. The water, which enters the torpedo and surrounds the chamber, exerts an increased pressure on the diaphragm and causes it to bend down. If you follow the lever system, you can see that the pendulum will push forward. Notice that a valve rod connects the

pendulum to the piston of the depth engine. As the piston moves to the left, low-pressure air from the torpedos air supply enters the depth engine to the right of the piston and pushes it to the left. You must use a depth engine because the diaphragm is not strong enough to move the rudders.

The piston of the depth engine connects to the horizontal rudders as shown. When the piston moves to the left, the rudder turns upward and the torpedo begins to rise to the proper depth. If the nose goes up, the pendulum swings backward and keeps the rudder from elevating the torpedo too rapidly. As long as the torpedo runs at the selected depth, the pressure on the chamber remains constant and the rudders do not change from their horizontal position.

As the diver goes deeper, the air pressure increases to meet that of the water. In coming up, the pressure on the air is gradually reduced. If brought up too rapidly, the diver gets the "bends." That is, the air that was dissolved in the blood begins to come out of solution and form bubbles in the veins. Any sudden release in the pressure on a fluid results in the freeing of some gases that are dissolved in the fluid. You have seen this happen when you suddenly relieve the pressure on a bottle of pop by removing the cap. The careful matching of hydrostatic pressure on the diver by air pressure in the diving suit is essential if diving is to be done at all.

In the side of the tube is a second opening (B) that does not face the direction in which the ship is moving. Opening B passes through the outer wall of the double-wall tube, but not through the inner wall. The only pressure affecting opening B is the hydrostatic

pressure. This pressure transmits through the outer tube (shaded in the drawing) to the right-hand arm of the manometer.

When the ship is dead in the water, the pressure through both openings A and B is the same, and the mercury in each arm of the manometer stands at the same level. However, as soon as the ship begins to move, additional pressure develops at opening A, and the mercury pushes down in the left-hand arm and up into the right-hand arm of the tube. The faster the ship goes, the greater this additional pressure becomes, and the greater the difference will be between the levels of the mercury in the two arms of the manometer. You can read the speed of the ship directly from the calibrated scale on the manometer.

Since air is also a fluid, the airspeed of an aircraft can be found by a similar device. You have probably seen the thin tube sticking out from the nose or the leading edge of a wing of the plane. Flyers call this tube a pitot tube. Its basic principle is the same as that of the pitometer log.