The forward end of the torpedo is the war head filled with guncotton or trinitrotoluol. A detonator is screwed into the end of the war head to set off the main charge on contact. An air flask forms the middle portion of the torpedo. Aft of this is the depth-control mechanism, in which a diaphragm controls the diving rudder by the pressure of the water against a spring set for the desired depth. A pendulum controls the levelling mechanism and a gyroscope its direction in the horizontal plane, tending to keep it on the course by its control of the vertical rudder.] [Illustration: REAR END OF THE WHITEHEAD TORPEDO Courtesy of the Scientific American Showing compressed air engine and twin propeller with their control gear.] =Automobile Torpedoes.=--These are the projectiles which are used to destroy the enemy's ship. They are called automobile torpedoes because they will, on being ejected from the torpedo tubes, continue running in the direction in which they are aimed, from power and mechanism contained within themselves. They are wonderful pieces of mechanism and cost several thousand dollars each. They are virtually miniature submarine boats. The essential features of the automobile torpedo are the airflask, the warhead, the depth control, and steering and propelling machinery. The airflask forms the central section, which is a steel tank containing compressed air stored at high pressure; about twenty-five hundred pounds per square inch is the present practice. When the torpedo is expelled from the torpedo tube this air is automatically turned on to run the engines. It passes through reducing valves and heaters to drive either a multiple cylinder or a turbine engine, and revolves two propellers, running one clockwise and the other counterclockwise, set in tandem at the stern of the torpedo. The propellers, running in opposite directions, thus enable the torpedo to be more easily steered by the delicate automatic steering machinery. A diaphragm operated by the pressure of the water operates control mechanism which regulates the depth. An instrument called the "Obry gear" steers it in the horizontal plane. The essential feature of the "Obry gear" is a gyroscope which is started when the torpedo is ejected from the tube. It is instantly speeded up either by a powerful spring or an air turbine to about fifteen thousand revolutions per minute. The peculiarity of the gyroscope is that it has a tendency to hold the direction in which it is started. Hence, if the torpedo starts swerving either to the right or left from the direction in which it is aimed, the gyroscope causes certain valves to function which will automatically set the steering rudder to bring the torpedo back into its original course. The "Gyro" will continue this control until the torpedo has completed its course, which in some of the latest types is said to be about five miles.