It consists of a metal tube _a_ into which the cup _b_ of vulcanite, or other insulating material is fixed. The cup is contracted at some distance from the top by the perforated plug _c_, which is also of insulating material; _d_ is a metal pin fixed into the bottom of the cup _b_, it is connected with the wire _e_, which is insulated and passes to the battery; _f_ is a metal plug closing the tube _a_ and the cup _b_ at the top; _g_ is a wire attached to the plug _f_, and passing from it to an earth connection. The cup _b_ is filled with mercury up to the level of the plug _c_. By the contact of a passing vessel the instrument would be tilted sufficiently to cause the mercury to flow into contact with the metal plug _f_, thus completing the electric circuit and exploding the mine. This form of circuit closer, though not generally adopted, would, on account of its being less liable to derangement by the motion of the waves, or by the explosion of an adjacent or counter mine, seem to fulfil the many requirements of a circuit closer for general service. _McEvoy's Weight Magneto Circuit Closer._--This form of circuit closer, which is shown in section and plan at Figs. 60 and 61, is one of the most important improvements that has ever been effected in such apparatus, and bids fair to become universally adopted. A heavy metal conical shaped weight _a_ (Fig. 60), hollowed out in its base and working in a ball and socket joint _b_, rests on a solid brass base _c_, and is so arranged that on the apparatus being struck, the weight _a_ will fall over, pivoting on one of its supports _d_, _d_; _e_ is a band of india rubber, encircling the weight _a_, for the purpose of preventing a jar on its falling against the sides of the brass cylinder _f_, which contains the weight _a_ and joint _b_. A brass rod _g_, connected to the ball and socket joint, passes through the base _c_, through a strong spiral spring _h_ (which latter rests on an adjusting screw _k_), through a piece of ebonite _l_, which supports the bobbins and core _m_, _m_^{1}; then between these bobbins _m_, _m_^{1} through an armature _n_, which is pivoted at _p_; and lastly through a slight spiral spring _o_, which is kept in position by the adjusting screw _i_. The armature _n_ is fitted with a small piece of brass _r_, so arranged that when it (the armature) is in the position shown in Fig. 60, this piece of brass _r_ does not make contact with the two strips of metal, _s_, _s_, between which it, _r_, works; but when the armature _n_ is in contact with the cores of the bobbins _m_, _m_^{1}, then the piece of brass _r_ makes contact with the metal strips _s_ _s_, and so makes a short circuit for the electric current. An ordinary telephone _t_, Fig. 61, in which some small shot, bells, &c., are placed, is fixed to the top of the brass cylinder _f_. _Action of Circuit Closer._--The action of this apparatus is as follows:--