_Description of a Series of Firing Keys._--At Fig. 78 is shown a plan and section of a series of firing keys as arranged for firing several mines by observation. It consists of a strong wooden frame _a_, of a convenient form for the purpose of attaching it to the firing table by screws through the holes _b_, _b_. On this frame a series of keys _c_, _c_, _c_ are fixed at convenient intervals. These consist of a strong brass spring firmly screwed to a series of brass plates _d_, _d_, _d_ on the front of the wooden box _a_. From these latter short copper wires pass through the woodwork, and of such a length that, when required, the mine wires may be easily attached by means of binding screws, as shown at _f_. The inner end of each key is fitted with an ebonite knob (which is shown at _c_ in the section) to insulate the hand of the operator when using the key. On the frame, and directly under each of the ebonite knobs, are arranged a series of metallic points _g_, _g_, _g_, so placed that on either of the keys _c_ being pressed down, a perfect contact is made between it and its respective metallic point; _h_, _h_, _h_ are copper wires leading from the metallic points _g_, _g_, _g_ through the box, and of such a length that binding screws _f_, _f_, _f_ can be easily attached to them when necessary. A single firing key of an improved form is shown at Fig. 79. It consists of a strong wooden box _a a_, weighted at the bottom with lead in order to steady the key on the table, &c., on which it may be placed; on the inside of the bottom of the box is fixed a piece of ebonite, by which means the metallic point _b_, and the terminal of the firing key _c_, are insulated from each other; _d d'_ are two terminals at the end of the box, to which the circuit wires are attached, one of these terminals is connected in metallic circuit to the firing key at _c_, the other one to the metallic point _b_; a wooden cover _h_, fitted with a catch _k_, protects the connections of the wires; by means of a plate, and catch _e e_, the key can be rendered inactive, thus preventing the danger of a premature closing of the electric circuit; by means of a spring _s_ a break is always established between the key and the metallic point. It is immaterial to which of the two terminals _d d'_ either wire is connected. _The Morse Firing Key._--This form of key is so well known in connection with the Morse telegraph, that it is not necessary to describe it. It is usually employed in torpedo work in connection with a testing and firing table. _The Shutter Apparatus._--The shutter signalling and firing apparatus was devised to enable the firing battery current to be thrown in circuit without the aid of a personal operator, the signalling current (which is always kept in circuit) at the same instant ringing a bell, by which is known the particular mine that has been struck.