_McEvoy's Mechanical Mines._--Captain McEvoy has also devised a plan, whereby a mechanical mine of the foregoing form may be placed in a state of safety, even after it has been rendered active. In the place of the aforesaid papier maché wad at _p_, Fig. 14, he uses a plunger which fits into the cavity _p_, of the heavy weight _d_. This plunger is always kept in a position clear of the weight by means of a spiral spring, unless it is desired to render the mine inactive when the plunger is forced into the aforesaid cavity and kept there by means of a pin inserted above it. Above this there is another plunger, acted on by a spiral spring sufficiently powerful to enable it to force the previous mentioned plunger into the safety position; this upper plunger is rendered inactive by means of a pin. The mine being placed in position, that pin which is keeping the lower plunger inserted in the cavity _p_, of the weight _d_, is withdrawn and the mine rendered active. To the pin of the upper plunger is attached a line which is anchored some distance from the mine in a known position. Then to render the mine inactive for the purpose of picking it up, etc., it is only necessary to raise the aforesaid line, and draw out the pin of the upper plunger, which by means of the strong spiral spring will force the lower plunger into the safety position, and render the mine inactive. Whether this invention is a practicable one or not, remains to be proved, but anyhow it is a step in the right direction. [Illustration: EXTEMPORE MECHANICAL MINE, MECHANICAL PRIMERS. PLATE 4] _Abel's Mechanical Primer._--This is shown in section and elevation at Fig. 15 (A and B). _a_, _a_, is the powder chamber in which the priming charge is placed; _b_ is a screw plug to close the chamber; _c_ is a flexible india rubber tube; _d_, _d_, are screw bands; _e_ is a glass tube containing oil of vitriol enclosed in a lead tube; _f_ which contains the explosive mixture; _g_, an eye at the head of the primer to receive the firing line; _h_, _h_ are segmental guards; _i_ is the guard ring; and _j_ the safety screw pin. This apparatus is screwed into a socket in the upper part of the torpedo case, as shown at Fig. 15 (C). _Mode of Action._--When placed in position, to render the primer ready for action, the guard ring, _i_, is pulled off, first having removed the safety pin _j_, when the segmental guards _h_, _h_, will fall away, leaving the india rubber tube _c_, _c_, exposed. A sufficient strain being brought on the rope secured to the ring _g_, the lead tube _f_ bends, causing the fracture of the glass tube _e_, thus igniting the priming charge and exploding the mine. A submarine mine so fitted may be fired at will, by bringing a line, from the ring _g_, to the shore, or it may be made self-acting by connecting two of them together, etc.