During the early days of submarine defensive warfare, the latter method alone was used, owing to the absence of anything like a practicable form of self-acting apparatus; but within the last few years, the former has almost entirely superseded the latter method, except in very exceptional cases; this revolution being due to the vast improvements that have been, and still are being effected in the system of firing electrical submarine mines automatically. _Use of Circuit Closers._--Electrical submarine mines may by means of an apparatus, termed a _circuit closer_, be rendered self-acting; that is to say, by the action of a vessel coming in contact with such an apparatus, which may be either within the mine itself, or within a buoy attached to the mine, the electric circuit is closed, and the mine in connection with the circuit closer so struck, exploded. The essential feature of such a mode of closing the electric circuit is, that electrical submarine mines may be rendered either active or harmless, at the will of the operator, which is effected by the putting in, or taking out of a plug, by which means the firing current is either thrown in, or out of the circuit. _Circuit closers._--Many different forms of circuit closers have been devised, among which the following seem the most suitable and are those generally used:-- 1.--Mathieson's inertia circuit closer. 2.--Mathieson's spiral spring circuit closer. 3.--Austrian self-acting circuit closer. 4.--McEvoy's mercury circuit closer. 5.--McEvoy's weight magneto circuit closer. _Mathieson's Circuit Closer._--This form of circuit closer has been adopted by the English government in connection with their system of defence by electrical submarine mines. The details of this apparatus are shown at Pl. xiii. Fig. 53, _a_ is a gun-metal dome screwed on to a metal base _b_, its foot resting on a gutta percha washer _c_, so as to exclude any water; _d_ is a cap screwed on to the top of the dome, and made watertight by the leather washer _e_; _f_ is a guard cap screwed into the cap _d_, this is to keep the spindle of the circuit closer steady during transport, and would be removed when the apparatus is prepared for service; _g_ is the ebonite base plug through which pass the insulated wires _E_ and _L_; _h_ is an hexagonal collar, working in the metal base plate _b_, by means of which, and the brass collar _i_, and the leather washer _k_, the base plug is secured, and water is excluded from the interior of the circuit closer; _l_, _l_, _l_ are brass columns supporting a circular ebonite piece _m_; _n_ is a metal bridge screwed on to the base plate _b_, into which is screwed the spindle _p_, both of which are prevented from moving after being screwed up by the set screws _r_ and _s_.