They are cheap, convenient, and under certain circumstances very effective. One of the objections to their use is the difficulty of mooring them securely in strong currents, as otherwise they are very liable to shift their positions. Three Confederate vessels were "hoisted by their own petards," from this cause.[I] _Brook's Torpedo._--Another form of buoyant mechanical mine is represented at Fig. 9. It was designed for the express purpose of preventing its discovery by dragging, etc., by the enemy. It consists of the torpedo case _a_, formed of copper, which is attached to a spar _b_, the lower end of which is secured to an universal joint in its anchor _c_. Five percussion or chemical fuzes _d_, _d_, _d_, are screwed into the head of the copper case. _Turtle Torpedo._--To increase the danger and uncertainty of any attempt to remove this form of buoyant mine, a turtle torpedo _A_, is attached to it by a wire _e_. This torpedo contains about 100 lbs. of gunpowder, and is exploded by means of a friction primer which passes through a watertight joint _f_, and is attached to the wire _e_. Whether this combination would prove effective, has yet to be seen, but the buoyant mine alone was considered one of the most dangerous used by the Confederates. _Singer's Mechanical Mine._--An elevation and section of this form of mechanical mine is shown at Fig. 10. It consists of an air chamber _a_, and a powder chamber _b_; in the latter is fixed a rod of iron _c_, one end of which rests in a cup formed in a lug _d_, where there is a screw by means of which the rod _c_ may be screwed against the bottom of the torpedo case, on the interior. In the cup is placed the fulminating substance. A heavy cast iron cap _A B_ rests upon the top of the case and is prevented from falling off by a low rim of tin, which enters an aperture in the cap as at _e_: a wire _f_ connects this cap with a pin _g_, which keeps a plunger _h_ at rest. The head of this plunger _h_ is directly beneath the bottom of the rod _c_, within the case; by means of a spring _i_, directly the pin _g_ is drawn out, which is done by a hostile vessel striking against the mine and knocking off the cap _A B_, the plunger _h_ is forced against the bottom of the case and drives the rod _c_ into the cup containing the fulminate, and so explodes the torpedo. The case of these mines, as used by the Confederates, was formed of tin, and they contained from 50 to 100 lbs. of powder. A safety pin _k_ is provided to prevent a premature explosion due to the pin _g_ being accidentally withdrawn. This form of submarine mine was one of the most successful and most extensively employed of all, on the part of the Confederates.