_McEvoy's Single Main System._--Hitherto in connection with a system of electrical submarine mines, it has been necessary to employ either a single cable between each submarine mine and the torpedo station, or a single cable, termed a "multiple cable," containing a limited number of insulated wires, leading from the station, and branching off from a junction box to each mine, by which considerable cost and complication is incurred. To remedy the above serious defects of such a system, and also to simplify the arrangement of electrical tests, Captain McEvoy has devised and patented the following apparatus; at the firing, or torpedo station, the end of the single main cable, that is, the single core cable leading to the junction box, is connected to a make and break contact apparatus, by which, by the movement of a dial or pointer around a fixed centre, a battery can be successively put in connection with the wire, and disconnected from it, in a somewhat similar manner to Wheatstone's step by step dial telegraphs. In the junction box at the opposite end of the single core main cable is an electro-magnetic apparatus for working a dial or pointer in exact unison with the aforesaid dial or pointer at the torpedo station. This junction box dial or pointer serves as a contact maker to put the wire of the main cable successively in contact with the branch wires leading to the several torpedoes, as it is caused to turn with a step by step motion by the sending of a succession of currents from the firing station. As the contact maker completes the circuit between the main cable and one of the branch wires, the current passes from the cable through the wire, and through the fuze of that particular torpedo to "earth"; but when any one or other of the torpedoes is to be exploded, the circuit between the main cable and the torpedo wire being completed, it is only necessary to send a current through the main cable of sufficient strength to ignite the fuze, and so explode the mine. The strength of the current used for giving the aforesaid step by step motion to the junction box dial or pointer is not sufficient to cause the ignition of the fuzes in the torpedoes.