Torpedo is an appliance for exploding a charge of gun-cotton or dynamite near a vessel, and may be fixed - in which case it is usually known as a submarine mine - or may be provided with some means for propelling it in any direction. A torpedo of the former class consists of an iron vessel containing from 30 to 500 lbs. of explosive, either moored so as to float some 10 feet below the surface, or resting on the bottom, A mine may be arranged to explode when struck by a ship, or may be controlled electrically from the shore. In the latter case the firing key which closes the circuit and explodes the charge, must be pressed when the ship is over the mine, and to ascertain when this moment arrives two sights or telescopes are arranged so that the lines of vision of two observers cross at a point over the mine; when the ship is sighted by both observers it is known to be in the desired position. In some cases a current from a battery on shore is taken by an insulated wire to the mine, and the circuit is automatically closed when the mine is struck by a vessel, while the safety of friendly ships may be secured by disconnecting the battery on shore. Of the numerous types of locomotive torpedoes, the Whitehead is the only one of any great importance. It is practically a submarine boat, shaped like a cigar, and from 12 to 19 feet in length. It is propelled by a three-cylinder engine, supplied with compressed air from a chamber provided for that purpose. The nose of the torpedo contains a charge of from 30 to 100 lbs. of gun-cotton, and is provided with a mechanical arrangement for exploding it on contact with the side of it ship. By means of a secret piece of mechanism, two horizontal rudders adjust themselves so that the torpedo, maintains a fixed distance below the surface; it can travel some 600 yards at a speed of 24 knots.