Part 11 (1/2)
In at least two of the preceding chapters of this book reference has been made to the speed at which a sh.e.l.l fired from a gun travels through the air. Such velocities as 3,000 feet per second have been mentioned in this connection, and some readers are sure to have wondered how such measurements could possibly be made. Possibly some sceptics have even supposed that they were not measured at all but simply estimated in some way or other. They are actually measured, however, and by very simple and ingenious means.
Needless to say, electricity plays a very important part in this wonderful achievement. In fact, without the aid of electricity it is difficult to see how it could be done at all.
People often ask how quickly electricity travels, as if when we sent a telegraph signal along a wire a little bullet, so to speak, of electricity were shot along the wire like the carriers of the pneumatic tubes in the big drapers' shops. That is quite a misconception, for in reality the circuit of wire is more like a pipe full of electricity, and when we set a current flowing what we do is to set the whole of that electricity moving at once. If we think of a circular tube full of water with a pump at one spot in the circuit, we see that as soon as the water begins to move anywhere it moves everywhere. Moreover, if it stops at one point it stops simultaneously at every other point. While practically this is the case it is theoretically not quite so, for the inertia of the water when it is suddenly started or stopped no doubt causes a slight distortion of the tube itself resulting in a very slight (quite imperceptible) r.e.t.a.r.dation of the movement of the water.
Electricity also has a property comparable to the inertia which we are familiar with in the objects around us, and there is also a property in every conductor which to a certain extent resembles the elasticity of the water-pipe, whereby it may for a moment be bulged out. In a short wire, however (up to a mile or so), particularly if the flow and return parts of the circuit be twisted together, this electrical inertia practically vanishes and consequently we may say that for all practical purposes the current starts or stops, as the case may be, at precisely the same moment in every part of the circuit.
That fact is of great value when, as in the case we are now discussing, we want to compare very exactly two events occurring very near together as to time but far apart as to place.
[Ill.u.s.tration: BOMB-THROWERS AT WORK.
Many kinds of bombs are used. One has a metal head and a handle about a foot long, with a streamer to ensure correct flight; another form resembles a brush when it is flying through the air; and a third, known as ”the egg,” is oval in form.]
We need to compare the time when the sh.e.l.l leaves the gun with the time when it pa.s.ses another point, say, one hundred yards away, and then again another point, say one hundred yards further on still. Supposing, then, a velocity of 3,000 feet per second, the time interval between the first point and the second and between the second and third will be somewhere about a tenth of a second. So we shall need a timepiece of some sort which will not only measure a tenth of a second, but will measure for us a very small _difference_ between two periods, each of which is only about a tenth of a second and which will be very nearly alike. That represents a degree of accuracy exceeding even what the astronomers, those princes of measurers, are accustomed to.
This exceedingly delicate timepiece is found in a falling weight. So long as the thing is so heavy that the air resistance is negligible, we can calculate with the greatest nicety how long a weight has taken to fall through a given distance.
Near the muzzle of the gun there is set up a frame upon which are stretched a number of wires so close together that a sh.e.l.l cannot get past without breaking at least one of them. These wires are connected together so as to form one, and through them there flows a current of electricity the action of which, through an electro-magnet in the instrument house, holds up a long lead weight.
At some distance away, say one hundred yards, there is a similar frame also electrically connected to an electro-magnet in the same instrument house. This second magnet, when energized by current from the frame, holds back a sharp point which, under the action of a spring, tends to press forward and scratch the lead weight. The third frame is likewise connected to a third magnet controlling a point similar to the other.
To commence with, current flows through all three frames so that all three magnets are energized. The gun is then fired and immediately the sh.e.l.l breaks a wire in the first frame, cutting off the current from the first magnet and allowing the weight to fall. Meanwhile, the sh.e.l.l reaches the second frame, breaking a wire there, with the result that the second magnet loses its power, lets go the point which it has been holding back and permits it to make a light scratch upon the falling weight. This action is followed almost immediately by a similar action on the part of the third magnet, resulting in a second scratch on the lead weight.
The position of these two scratches on the weight and their distance apart gives a very accurate indication of the time taken by the sh.e.l.l to pa.s.s from the first screen to the second and from the second to the third. From those times it is possible to calculate the initial velocity of the sh.e.l.l and the speed at which it will move in any part of its course. Indeed, with those two times as data, it is possible to work out all that it is necessary to know about the behaviour of the sh.e.l.l.
This is rendered practicable by the fact that the moment the wire is cut the magnet lets go, no matter what the distance of the screen from the instrument may be. But for the instantaneous action of the current, allowance of some sort would have to be made for the fact that one screen is farther than another and the whole problem would be made much more complicated.
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 s.h.i.+p are doing.
As everyone knows, there is in every s.h.i.+p (except those few which are propelled by paddles) a long steel shaft, called the tail-shaft, which runs from the engine situated somewhere near amids.h.i.+ps to the propeller at the stern. Many s.h.i.+ps, 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 ma.s.sive 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 s.p.a.ce 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.
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 gla.s.s front. Through the gla.s.s one sees a row of little white k.n.o.bs. If this be placed somewhere near the engine while it is at work immediately one of the k.n.o.bs commences to move rapidly up and down, so that it looks no longer like a k.n.o.b but is elongated into a white band. There is no visible connection between the instrument and the engine, yet the number over that particular k.n.o.b which becomes thus agitated indicates the speed of the engine.
Let us in imagination open the case and we shall find that the k.n.o.bs 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.