The torpedo is, in its simplest form, simply some arrangement of a powerful explosive to be set off beneath or against the bottom of a ship, and shatter or sink it. The idea is as old as gunpowder, but it is only in recent times that it has been made effective,—how effective we do not yet know. Torpedoes are used in two ways: one is by fixing the torpedo beneath the water, either to be exploded by means of a percussion-cap when the ship runs against it, or from the shore by means of electricity. Such arrangements as this, called submarine mines, are regarded as a most important means of defending harbors against hostile attack. During our Civil War they were extensively used by the Confederates, and were sometimes successful, as when one destroyed the monitor _Tecumseh_ in Mobile harbor, during Farragut’s famous attack there in 1864. [Illustration: THE MONITOR “TECUMSEH” SUNK BY A TORPEDO AT MOBILE, 1864.] The former class, for which the word _torpedoes_ is now reserved, includes explosive agents which are to be placed or sent against a ship’s bottom at sea and exploded there. Various devices of that kind, also, have been used for a long time in naval warfare. The Confederates tried hard to destroy several Northern vessels in the blockading squadron by devising very small, half-submerged boats, towing torpedoes astern, or else projecting on a long spar from their bows; and now and then they succeeded, as when one of the latter kind was made to sink the _Housatonic_ off Charleston. [Illustration: THE SEARCH-LIGHT REVEALING THE TORPEDO-BOAT.] Then there have been invented, during the past fifty years, several cigar-shaped machines, which, by means of a chemical or compressed-air engine or clockwork, or some other application of power that might keep motive machinery within them going long enough, could be launched from shore or from another vessel and sent under water against a hostile ship. At first these were made to glide along just beneath the surface, carrying little flags that could be seen, and trailing two electric wires, enabling a person, by means of electric currents, to direct their flight; but latterly ingenuity has devised such an arrangement of rudders and self-acting balances within the torpedo’s mechanism that it will continue perfectly straight upon the course it is aimed for, swerving neither right nor left, up nor down, and will explode the instant it touches an object hard enough to jar the delicate cap of fulminate in its snout. This latter kind, called the automobile (self-moving) torpedo, is now almost exclusively used, and some modification of the Whitehead is most popular. It is cigar-shaped, and about twelve feet in length; the forward third is filled with gun-cotton—in quantity sufficiently powerful, if accurately applied, to ruin almost instantly the greatest battle-ship afloat. [Illustration: A SELF-MOVING TORPEDO ON ITS WAY TO ATTACK A MAN-OF-WAR.]