The warhead is the forward portion of the torpedo and contains usually wet gun-cotton, which is a safe high explosive and can be exploded only by a detonating charge of the more sensitive explosives. This detonating charge is placed in a tube screwed into the forward end of the torpedo. Extending out from the forward end of the tube is a small propeller, the purpose of which is to set the firing mechanism after the torpedo has run a certain distance from the vessel from which it has been fired. This is a safety device to prevent the torpedo from being exploded near its own ship. The torpedo running through the water causes the propeller to revolve, which turns a shaft. After the shaft makes a certain number of revolutions it sets a firing pin, and then if it hits an object it will explode. Many modern torpedoes are loaded with trinitrotoluol. This is a much more powerful explosive. According to experts, the explosion of two hundred and fifty pounds of T-N-T, as it is called, will destroy any battleship ever built. [Illustration: RAPID-FIRING GUNS Courtesy of the Scientific American Rapid-fire disappearing guns may be quickly elevated above armored turret when the submarine rises to the surface.] =Divers' Compartment.=--Some submarines are fitted with a divers' compartment, from which compartment mines may be planted, either when on the surface or when submerged. This compartment is fitted with a door which opens outwardly in the bottom of the boat. It is shut off from the living and machinery rooms of the vessel by an air lock and heavy pressure-resisting doors. The divers' door may be opened when the vessel is submerged and navigating on the bottom, and _no water will come into the vessel when the door is opened_. This is accomplished in the following manner: The members of the crew who wish to go outside the vessel first go into the diving compartment. They close the door which shuts them off from other parts of the vessel. They then turn compressed air gradually into the compartment until the air pressure in the compartment equals the water pressure outside. If the depth is one hundred feet the air pressure in the compartment would need to be 43.4 pounds per square inch; if the depth is two hundred feet, twice that, or 86.8 pounds per square inch, etc. When the air pressure in the compartment equals the water pressure outside, at any depth, the door in the bottom may be opened and the water will not rise up into the compartment, because the air pressure keeps it out. Tests have been made which show that it is safe for divers to go out from compartments of this kind in depths up to two hundred and seventy-five feet. [Illustration: DIVING COMPARTMENT