The principal use of the submarine is in war. It is a diving torpedo-boat, and acts under cover of water, as the light artillery on land is secured behind intrenchments. The weapon used by the submarine is the torpedo. The torpedo is itself a small submarine able to propel itself, and if started in the water toward a certain object, to go under water straight to the mark. It carries a heavy charge either of guncotton or dynamite, which explodes when the torpedo strikes a solid object, such as a battle-ship. The first torpedo was intended to be steered from the shore by means of long tiller-ropes, and to be propelled by a steam-engine or by clockwork. The Whitehead fish torpedo, invented in 1866, is self-steering. At the head of the torpedo is a pointed steel firing-pin. When the torpedo strikes a ship or any rigid object this steel pin is driven against a detonator cap which is in the centre of the charge of dynamite. The blow causes the cap to explode, and the explosion of the cap explodes the dynamite. The torpedo is so arranged that it cannot explode until it is about thirty yards away from the ship from which it is fired. The steel pin cannot strike the cap until a small "collar" has been revolved off by a propeller fan, and this requires a distance of about thirty yards. The screw propeller is driven by compressed air. A valve which is worked by the pressure of the water keeps the torpedo at any depth for which the valve is set. The torpedo contains many ingenious devices for bringing it quickly to the required depth and keeping it straight in its course. One of these devices is the gyroscope, which will be described under the head of "spinning tops." Whitehead torpedoes are capable of running at a speed of over thirty-seven miles an hour for a range of two thousand yards and hitting the mark aimed at almost as accurately as a gun. The submarine boat carries a number of torpedoes, and has one torpedo-tube near the forward end from which to fire the torpedoes. It would be very difficult for one submarine to fight another submarine, for the submarine when completely submerged is blind. It could not see in the water to find its enemy. The torpedo-boat-destroyer is able to destroy a submarine by means of torpedoes, shells full of high explosives, or quick-firing guns. Advantage must be taken of the moment when the submarine comes to the surface to get a view of her enemy. One of the great enemies of the submarine will probably be the air-ship, for while the submarine when under water cannot be seen from a ship on the surface, it can, under favorable conditions, be seen from a certain height in the air.