It consists of a barrel _a_, which is thoroughly coated inside and out with hot pitch, etc., to make it watertight, a block of wood _b_, secured to the top of the cask _a_, and having a recess cut in it to receive a round shot _c_, also a hole through which a strop _d_, is passed, and another hole to receive a toggle _e_. At the bottom of the cask on the inside, is fixed a wooden frame work _f, f_, to the top of which two ordinary gun friction tubes are fixed _g_, _g_. A piece of wood _h_, is secured to the bottom of the cask on the outside, bored with two holes, one to receive a thin iron rod _i_, the other for the safety pin _k_. Wires _x_, _x_, secure the gun tubes _g_, _g_, to one end of the iron rod _i_, the other end of which is connected by means of a rope lanyard to the shot _c_. Weights are slung beneath the barrel to keep the mine upright. The principle of action of this form of mechanical mine is precisely similar to that of Captain McEvoy's improved Singer's mine, and need not, therefore, be described. _McEvoy's Mechanical Primer_.--A sectional view of this apparatus is represented by Fig. 14. It consists of two brass tubes fitting accurately one within the other, of which _a_, _a_, is the inner one. To this inner tube are affixed two brass diaphragms _b_, _b_. A brass spindle _c_, carries a weight _d_, which is regulated by a spring, _e_. A locking rod, _f_, moves in a ball and socket joint at _g_. A hammer _h_, which is shown in Fig. 14, at full cock, is kept in that position by the rod _f_. A vessel, striking the mine, in which this apparatus is placed causes the weight, _d_, to cant over, allowing the rod, _f_, to be forced upwards by means of the spring _e_, and so frees the hammer _h_, which falls on a nipple _i_, on which is placed the percussion substance, and so explodes the mine. _McEvoy's Papier Maché Safety Plug._--To prevent a premature explosion during transport, etc., of a mine in which this apparatus is placed, a plug of papier maché, which is soluble in water, is inserted in the two spaces _p_, _p_, by which the spindle _c_, is prevented from moving to one side or the other. The use of a papier maché, instead of a cement plug for the purposes of safety, is a great improvement, as by the simple process of pressure, any period of time that it is necessary should elapse before the complete destruction of the plug, can be readily and certainly obtained, which when a cement plug, formed of different ingredients is used, is not always the case.