Even as it is, someone may urge that the magnets themselves possess inertia and will not let go quite instantaneously, but that can be overcome by making the magnets all alike so that the inertia will affect all equally. It is only necessary to have a switch which will break all the three circuits at the same moment (quite an easy thing to arrange) and then adjust all three magnets so that when this is operated they act simultaneously. After that they can be relied upon to do their duty quite accurately. Thus by a method which in its details is quite simple is this seemingly impossible measurement taken. CHAPTER XIII SOME ADJUNCTS IN THE ENGINE ROOM Before we deal with the subject of the engines employed in warfare, it may be interesting to mention two beautiful little inventions which have been made in connection with them. Let us take first of all a contrivance which tells almost at a glance the amount of work which the engines of a ship are doing. As everyone knows, there is in every ship (except those few which are propelled by paddles) a long steel shaft, called the tail-shaft, which runs from the engine situated somewhere near amidships to the propeller at the stern. Many ships, of course, have several propellers, and then there are several shafts. Now each of these shafts is a thick strong steel rod supported at intervals in bearings. If anyone were told that, in working, that shaft became more or less twisted, he would be tempted to think he was being made fun of. Yet such is literally the case. The thick strong massive bar becomes actually twisted by the turning action of the engine at one end and the resistance of the propeller at the other. And the amount of that twisting is a measure of the work which the engine is doing. The puzzle is how to measure it while the engine is running, for of course the twist comes out of it as soon as the engine stops. A space on the shaft is selected, between two bearings, for the fixing of the apparatus. Near to each bearing there is fitted on to the shaft a metal disc with a small hole in it. On one of the bearings is fixed a lamp and on the other a telescope. When the engine is at rest and there is no twist in the shaft, all these four things--the lamp, the two holes, and the telescope--are in line. Consequently, on looking through the telescope the light is visible. But when the engine is at work and the shaft is more or less twisted one of the holes gets out of line and it becomes impossible to see the light through the telescope. A slight adjustment of the telescope, however, brings all four into line again, which adjustment can be easily made by a screw motion provided for the purpose. And the amount of adjustment that is found necessary forms a measure of the amount of the twisting which the shaft suffers and that again tells the number of horse-power which the engine is putting into its work.