It is another example of the principle of resonance, which is employed so finely in making wireless telegraph apparatus selective. Every wireless apparatus is made to have a certain natural frequency of its own and it therefore picks up readily those signals which proceed from another station having the same frequency while ignoring those from others. In just the same way a reed or spring in this speed-indicator picks up and responds to impulses derived from the engine only when they are of a frequency corresponding with its own natural frequency. Hence, one spring out of the whole range responds to the vibrations of the engine while the others remain almost if not entirely unaffected. In another form, the springs are actuated electrically. A magnet, or a series of magnets, is arranged so that as the engine turns the magnets pass successively near to a coil of wire, thereby inducing currents in that wire. They form, in fact, a small dynamo or generator, generating one impulse per revolution or two or three or whatever number may be most convenient. Then the current from this is led round the coil of a long electro-magnet placed just under the free ends of all the springs. The magnet therefore gives a series of pulls, at regular intervals, and the rapidity of those pulls will depend upon the speed of the engine, while the frequency of them will be registered by the movement of one or other of the springs. This instrument can also be employed to determine the speed of aeroplane motors and, in fact, any kind of engine, especially those whose speed is very high. CHAPTER XIV ENGINES OF WAR The phrase which I have used for the title of this chapter is often given a very wide meaning which includes all kinds and varieties of devices used in warfare. In this case I am giving it its narrower sense, taking it to indicate the steam-engines and oil-engines which are employed to drive our battleships, cruisers and destroyers, our submarines and our aircraft. They are inventions of the highest importance, which have played a large part in shaping modern warfare. The type of engine almost invariably used on ships of war other than submarines is the steam turbine. Great Britain, for the most part, uses that particular kind associated with the name of the Hon. Sir C. A. Parsons, while the United States rather favour the Curtiss machine. Other nations have adopted either one of these or else something very similar. All turbines are very simple in their principle, far more so that the older type of steam-engine, called, because the essential parts of it move to and fro, the "reciprocating" steam-engine.