But it is also necessary to know how fast the engine is working. There are many devices which will tell this, of which the speedometer on a motor-car is a familiar example. Most of those work on the centrifugal principle, the instrument actually measuring not the speed but the centrifugal force resulting from the speed, which amounts to the same thing. There is one instrument, however, which operates on quite a different principle, because of which it is specially interesting. It consists of a nice-looking wooden box with a glass front. Through the glass one sees a row of little white knobs. If this be placed somewhere near the engine while it is at work immediately one of the knobs commences to move rapidly up and down, so that it looks no longer like a knob but is elongated into a white band. There is no visible connection between the instrument and the engine, yet the number over that particular knob which becomes thus agitated indicates the speed of the engine. Let us in imagination open the case and we shall find that the knobs are attached to the ends of a number of light steel springs set in a row. The springs are all precisely alike except for their length, in which respect no two are alike. Indeed, as you proceed from one side of the instrument to the other each succeeding one is a little longer than the previous one. Now a spring has a certain speed at which it naturally vibrates and other things being equal that speed depends upon its length. You can, of course, force any spring to vibrate at any speed if you care to take the trouble, but each one has its own natural speed at which it will vibrate under very slight provocation. Every engine is, of course, made to run as smoothly as possible. All revolving or reciprocating parts are for this reason carefully balanced and in turbines the whole moving part, since it is round and symmetrical, naturally approaches a condition of perfect balance. Hence every engine ought to run perfectly smoothly. As a matter of fact, however, no engine ever does. There are certain limitations to man's skill and at the high speed of a fast-running engine, such as is to be found on a destroyer, for example, some little irregularity is sure to make itself felt by a slight vibration in the floor. It may be hardly perceptible to the senses, but to a spring whose natural frequency happens to be just that same speed or nearly so, it will be very apparent and in a few seconds that spring will be responding quite vigorously.