_Mathieson's Spiral Spring Circuit Closer._--A sectional elevation of this form of circuit closer is shown at Fig. 57. It consists of a brass base _a_, provided with a grooved flange for carrying a gutta percha washer, and it has also an hexagonal projection for the purpose of screwing the circuit closer into the gun-metal mouth of its air-tight cylinder, or buoy; _b_ is a brass dome enclosing the apparatus for the purpose of protecting it from injury, and also by means of india rubber washers to prevent an ingress of water, should the circuit closer case become injured, and leak; _c_ is a brass collar to which the brass contact springs _i_, _i_ are attached, and which are regulated by the set screws _j_, _j_; a brass spiral spring _d_ carries a metal rod _e_, which supports a brass ball _f_, surrounded by an india rubber band _h_. A contact disc _g_ is secured to the base of the spindle _e_, but insulated from it by an ebonite boss; _k_ is an ebonite base plug with two channels in it, through which the wires _m_, _m^{1}_ pass. _An Improvement on the Inertia Circuit Closer._--This instrument is a vast improvement on the inertia apparatus previously described, being more simple and more certain in its action, a desideratum in all circuit closers; but notwithstanding, up to the present time Mathieson's inertia apparatus has been used by our government, to the exclusion of all other instruments of a similar nature, some of which were proved to be far superior when subjected to the crucial test of actual practice. _Austrian Self-acting Circuit Closer._--This form of circuit closing apparatus, which is purely a self-acting one, that is to say, a mine so fitted cannot be fired at will, is shown at Fig. 58. It consists of several buffers _a_, _a_, _a_, which by means of strong springs are held in position, their heads projecting outside the torpedo case _b_; on being pressed in by the contact of a passing vessel, the ends of these buffers would be forced against a ratchet wheel _c_, which is also kept in position by means of a spring. Several strong pieces of wood _d_, _d_ within the case keep the buffers and their attached arms in the proper direction, and also afford rigidity to the torpedo case. The brass ratchet wheel _c_ being put in motion carries round with it a central arrangement _e_, the lower part of which is shown at Fig. 58, _A_. This portion consists of a cylinder of brass _f_ divided into two parts insulated one from the other by a piece of ebonite _g_; on one side of this cylinder there are three arms of brass, _h_, _i_, and _k_, and on the other there are two arms, _l_ and _m_, all of which are insulated from each other. [Illustration: AUSTRIAN CIRCUIT CLOSER, MERCURY CIRCUIT CLOSER. PLATE XIV]