FIG. 49. HOW THE MINE ADJUSTS ITSELF.
Mines of this kind are made about like contact mines in that they contain an explosive charge and a detonator, but they are much larger and far more powerful, for they have to be laid on the bed of the harbor, and instead of being fired by a pin striking a percussion cap they are fired by an electric spark.
From each mine laid in the harbor a pair of insulated copper wires run to the shore, and this electric circuit connects with two shore stations at a distance apart and which with the mine form a triangle. The electric circuit has a key in it at each shore station, and to explode the mine both of these keys must be pressed down at the same time.
FIG. 50. HOW THE ELECTRIC MINE WORKS.
Each shore station has a telescope fixed in position so that its line of sight (shown by the dotted lines in [Fig. 50]) passes directly over the mine. Now when an enemy ship approaches, each observer will see it when it sails on or across his line of sight and he will press his key. But it is necessary that both observers see the ship at the same time that it is at the point where their lines of sight cross, and consequently directly over the mine, and when this happens they will both press their keys, the circuit will be closed, the electric current will fire the mine, and the ship will change her course to one straight up and speed with all possible haste to the port whence no ship ever clears again.