The Submarine Torpedo Boat
Its Characteristics And Modern Development

SUBMARINE MINES 199 exploding a percussion cap, closes an electric circuit. There is however another break in the electric circuit which must be closed before the mine can be exploded; this is effected by leading an electric cable down through the anchor and thence to a shore sta- tion where the circuit is broken by a switch. Unless this switch is closed the mine cannot be fired. In this way if no ships of the enemy are in the vicinity the switch at the shore sta- tion can be left open and all friendly ship- ping can pass in and out with absolute safety. Another form of elec- tro-contact mine has the firing mechanism and the bursting charge in separate cases; the contact buoy contain- Diagram showing depth regulation of contact mines When the mine is dropped overboard, the drop-weight sinks first, as in figure on the left, and causes the drum of the anchor- weight cable to unwind until the drop- weight reaches the bottom, as in central figure. This causes the drum to stop un- winding, and the anchor-weight then pulls the mine below the surface, the depth of the drop-weight cable length, as in the fig- ure on the right. ing the firing mechanism is held a few feet below the sur- face and connected to the mine proper by cable. The mine itself, which is several feet below the buoy, is anchored as before, and is in electrical connection with a shore sta- tion. There are to this type of mine two distinct and separate circuits; a signal circuit leading from the contact