The arm _h_ is close to, but insulated from a metal plate _n_, which latter is permanently connected with the conducting wire leading from the firing battery, and thus while in a state of rest is electrically charged; beyond the arm _i_ is a spring _o_, which is connected with the earth, and in such a position that when the central portion is moved round, this spring _o_ comes in contact with the arm _i_, and the plate _n_ with the arm _h_ simultaneously, and the circuit is thus completed through earth to the battery, but the current of electricity does not pass through the fuze. The arms _k_, _l_ on the opposite sides of the cylinder, and consequently insulated one from the other, are connected with the fuze, and the arm _m_ is connected with the earth. On a further pressure of the vessel on the buffer, the arm _i_ is pushed beyond the spring, and in contact therewith, and consequently the circuit by earth to the battery is broken, while the contact of the arm _h_ and plate _n_ is still retained, and the current is passed by the arm _k_ through the fuze to the arm _l_, and then to earth through the arm _m_, thus completing the electric circuit of the firing battery through the fuze, and to exploding the mine. The spring acts as a circuit breaker, and by means of an intensity coil in connection with the firing battery, the current is only passed through the fuze when at the point of greatest intensity. By detaching the firing battery, the channel defended by such submarine mines may be rendered safe. _Fuze only in Circuit at Moment of Firing it._--One of the principal objects to be gained by the employment of such an arrangement for the closing of the electric circuit in connection with submarine mines, is the prevention of premature explosion from induction which might be caused by the proximity of any atmospheric electricity, the fuze in this system being entirely cut out of circuit until the moment when it is necessary to fire it. The Austrians employed this form of circuit closing instrument during the war of 1866, and still continue to use it in connection with their coast defence by submarine mines. _McEvoy's Mercury Circuit Closer._--At Fig. 59 is represented a longitudinal section of a circuit closer of this construction. It is placed in the mine in such a manner that when undisturbed it maintains an approximately upright position.