The gun-cotton charge is placed in what is termed a cartridge-case, which is a wooden case similar in shape to the forward section of the torpedo, somewhat smaller, and held in place in its compartment by wooden wedges. This charge is ignited mechanically, the arrangement being as follows: Extending from the nose of the torpedo to the cartridge-case is a tube terminating in a copper case, in which is placed the priming charge and the detonating composition; within the tube is a steel rod, two feet long, fitted with a needle-point at its inner end and having its outer end screwed into a frame; this frame is capable of moving in and out, and is connected with a spiral spring which tends to force it, and consequently the steel rod, inwards, the action being similar to that of the firing-pin of breech-loading small-arms. By compressing this spiral spring, the inner end of the frame is brought forward to a catch and held retracted. If now this catch is disengaged in any manner, the rod flies back, acting like an ordinary firing-pin on the detonating composition. The extreme forward point of the torpedo, called the nose-piece, is so fitted that it is capable of being forced inwards, but in a position of rest its inner edge is just clear of the catch. This nose-piece is provided with short horizontal and vertical arms, so as to insure good contact with the body struck. The collision of the nose-piece against any resisting body forces it back and releases the firing-pin. In order to secure against accident, the nose-piece is provided with a safety-wedge and key. Before discharging the torpedo the key is removed, and after the discharge the wedge is withdrawn by the action of mechanism, and is so arranged that it may be replaced automatically and thus disarm the torpedo after its run, in case it is desired to recover it.