=The Purpose of the Firing-Key.= (_Plate XVI._)—The full power of the electro-magnets of the D. E. machine will be reached soonest and will be greatest when the two binding-screws are joined by a piece of metal of practically no resistance as, for instance, a short wire. If this short-circuit is kept closed until the moment of firing and is at that, moment replaced by the circuit containing the detonator, we will have the machine working with its magnets fully excited in the circuit in which useful work is to be done. In order to accomplish this change of circuit, without allowing the magnetism of the machine to fall, the second circuit must be completed before the first is broken. If we had no more convenient method we could take advantage of this property of the machine by connecting the two ends of the fuze-circuit to the two binding-screws of the machine, and laying a piece of metal across the two binding-screws. When the crank is turned a strong current is generated, the magnets reach their full strength and, on removing the piece of metal, the machine is thrown upon the fuze-circuit with its magnets strongly excited, generating sufficient current to fire the detonator. The firing-key furnishes a convenient method for making this change of circuit and also a means for testing the continuity of the fuze-circuit at any time before firing. When the firing-key is connected to the machine by wires between the binding-screws of the latter and those marked B, B, of the former and the binding-screws, marked T, T, of the former, are joined by a wire, there are three circuits which may be closed or broken by manipulation of the keys T and F of the firing-key. (_Plate XVII._) =The Short-Circuit.=—The current follows the path shown in Fig. 1. =The Test-Circuit.=—When the key T is pressed, the short-circuit is broken and the current follows the path shown in Fig. 2. =The Firing-Circuit.=—When the keys F and T are pressed, the current follows the path shown in Fig. 3. =To Test the Firing-Key.=—Connect as above and place the firing-key about ten or twelve feet from the machine, and so that the compass-needle points in the direction of the length of the box; ship the crank and turn it rapidly; if it turn hard the short-circuit is in good condition; then press the key T; if the crank turn easily and the compass-needle be deflected, the test-circuit is complete; then press the key F (the key T being already down); if the crank turn hard and the needle be no longer deflected, the firing-circuit is complete. FARMER‘S DYNAMO-ELECTRIC MACHINE. =Pattern C.—Plate XVI.= This machine, intended for use in boats, has less power than the large machine, and may generally be considered as having an electro-motive force of eight volts, and a resistance of four ohms, and to be capable of firing eight to ten detonators in series, or two to three arranged in as many branches, or a single detonator through 1500 feet of such cable as is now issued.