When the fuze of any one or other of the torpedoes is exploded by the passing of a strong firing current through it, the wire leading from the box _C_ to this torpedo is simultaneously cut off from electrical connection with the contact pin _E_ to which it was previously connected, and this pin is put to earth through a resistance either somewhat greater or less than the resistances in the torpedoes, so that the firing of one or more of the torpedoes does not interfere with the power of being able to turn the pointer _F_ of the apparatus _C_ in unison with the pointer of the dial _b_. Afterwards the operator at the firing point can ascertain which of the torpedoes has been fired by passing the pointer of the dial _b_ to each of the divisions of the dial in succession, and ascertaining by the galvanometer a the resistance of the circuit through each of the torpedoes, so that he at once ascertains which torpedo has been put to earth through the greater or less resistance. The cutting off of the wire _D_ from its contact _E_ when a strong current is passed through it may be effected by the wire being coiled around an iron core forming an electro magnet, which when a strong current is passed through the wire is of sufficient strength to shift the position of a contact apparatus and then effect the required alterations in the connections, but which is not of sufficient strength to effect any change when the weaker currents used for the signalling and testing operations are passed through the wire. It will be evident that with the above described apparatus any one or other of the torpedoes can if desired be exploded by the operator at the firing point whenever he desires to do so. To effect this he would by turning the handle _a_ bring the pointer of the dial _b_ opposite to the division of this dial; that would indicate that the cable had been brought into electrical communication with the torpedo required to be exploded, and then when it is ascertained by previously adjusted sight points that the vessel is above the torpedo, he can fire the torpedo by passing a strong firing current to the cable. In this way the apparatus can be used for firing any one or other of a group of sunken torpedoes, or if the torpedoes are buoyant ones, they need not be fitted with apparatus for putting the wire from their fuze directly to earth whenever the torpedo is struck by a passing vessel. The same arrangement of apparatus can also be used for firing any one or other of a number of mines or torpedoes on land and for separately testing the firing mechanism of each mine whenever desired. Captain McEvoy's single main system will shortly undergo a series of experiments under the supervision of the English torpedo authorities at Chatham, which will most probably result in its adoption by the English government, and also by the principal continental powers. TABLE[Y] SHOWING THE VALUE OF THE FRACTIONS A AND B FOR EVERY HALF DEGREE.