The Submarine Torpedo Boat
Its Characteristics And Modern Development

200 THE SUBMARINE TORPEDO BOAT buoy, and a firing circuit leading to the mine itself. When the firing pin of the contact buoy is struck the signal cir- cuit is closed causing a bell to ring or a lamp to light at the shore station. To discharge the mine the observer at the keyboard of the shore station has then only to close the switch in the firing circuit of the mine which corre- sponds to the signal given. Where the depth of the water is too great or the current too swift to make practicable the use of these mines ground mines are resorted to. The ground mines are very heavy and contain exceedingly heavy charges of explosive, the amount depending upon the depth at which they are placed. A method commonly employed in firing ground mines is to have two shore stations in electrical connection with the mine and a break in the circuit at each station. These two breaks must be closed simultaneously in order to explode the mine. To effect the simultaneous closing of the breaks, each station is provided with a telescope mounted upon a swivel base which is constructed so as to practically constitute a selective switch. The switch points of the base are arranged to close the break in that circuit leading to the particular mine upon which the tele- scope is directly bearing at that instant. To explode a mine the observers at each shore station have therefore only to keep their telescopes bearing upon the approaching ship and when the ship is directly over any mine each tele- scope sighting upon it will have simultaneously closed the breaks and the firing circuit completed. Should a ship not pass directly over a mine the two breaks cannot be closed at the same time thus preventing the useless explo- sion of a valuable mine.