In twin-screw boats there are two distinct engines, one for each propeller. Each engine, too, is coupled to a dynamo by which it can generate electric current, which is stored in large acc.u.mulator batteries until required and then withdrawn to drive the dynamos as motors while the boat is submerged, for if you feed a dynamo with current it becomes a motor.

A great deal of work is done, on the submarine, by compressed air, of which large stores are carried in strong steel cylinders. For example, the ballast is ejected from the ballast tanks, when the boat is required to rise, not by pumps but by the action of compressed air from a cylinder. The simple movement of a tap thus suffices to blow out the water in a very short time. The torpedoes, too, are given their initial push which sends them out of their tube into the water by compressed air. In other ways, too, compressed air is employed and to facilitate its use there are many tubes and valves whereby the cylinders and other apparatus are connected. Like all things human, these tubes and valves have their defects, which in this case means that they leak somewhat, but this defect is of value since the leaking air helps to keep pure and sweet the air inside the boat which, when submerged, the men have to breathe.

To what extent it is used I do not know, but it is a fact that certain chemicals, caustic soda for instance, have the power to absorb the objectionable carbonic acid which makes tightly-shut rooms seem "close"

and uncomfortable, and if something of that sort be employed, it, together with the fresh air which thus leaks in by accident, is undoubtedly enough to enable men to live under water for many hours at a stretch.

On the other hand, several instances are on record in which strong healthy young officers have, after a course of service on a submarine, been found to be suffering seriously from chest and lung trouble, brought on, no doubt, by long spells of duty in this unhealthy atmosphere.

It used to be the custom to keep some white mice on board a submarine to give warning of the impurities in the air. Being very susceptible to the smell of petrol vapour, which used to be a source of considerable danger, and also to carbonic acid, these little creatures squeaked with anxiety some time before the conditions became really dangerous, thus giving timely warning. There is an instrument, however, which will give an indication of this sort and probably it has been brought in to reinforce the mice if not actually to supplant them. This interesting little instrument, which the gasworks people use for detecting leakage, consists of a metal drum with a porous diaphragm. Normally the pressure of the atmosphere upon the diaphragm is equalled and balanced by the pressure of the air inside the drum, but if there be gas in the air this balance is upset, the diaphragm is bulged in or out and a finger is thereby moved, which movement forms a measure of the amount of gas present.

In conclusion, we may fittingly take a glance at what happens when a submarine founders. Only a few years ago this occurred with lamentable frequency, though now it is quite rare except under the actual stress of warfare. Several interesting schemes were therefore invented to give the men at least a sporting chance of getting to safety. One was to make the conning-tower detachable and water-tight, so that the men could get into it, fasten themselves in and float up to the surface. The practical difficulties in the way prevented this being a success. For example, if sufficiently detachable in an emergency it was difficult to make it sufficiently water-tight in ordinary use.

Another and better device provided the men with small helmets and jackets, like the dress of a diver very much simplified. One of these for each man was stored in an accessible place in the boat and part.i.tions were devised inside the hull itself in order that whatever happened there should be air entrapped somewhere wherein the men could live for a time and put on their helmets in safety. Then, thus provided, they could crawl out through the hatchway and float up to the surface.

Arrived there they could inflate their jackets by blowing into them, open the window of the helmet and float upon the surface in comparative safety until rescued.

This apparatus was largely installed in British submarines and a tank was built at Portsmouth where the men could actually practise with it under water.

A third device may also be mentioned. This takes the form of a buoy fitted into a recess in the boat's upper surface. Sufficient line is coiled up inside it and when the occasion arises it can be released from inside. This does not in itself save the crew but it may go a long way towards ensuring their safety by letting those above know just where the sunken craft is and guiding them in their efforts to raise it.

The torpedo, the weapon without which the submarine would be practically useless, is dealt with in another chapter. Enough has been said here to give a good general idea of these interesting craft, their fittings, their uses and the sort of life which befalls those who man them.

CHAPTER XX

THE STORY OF WIRELESS TELEGRAPHY

For ages people were puzzled as to the nature of light. Pythagoras, that old Greek who invented what we now call the forty-seventh proposition of Euclid, thought that the bright body shot off streams of tiny particles which literally hit the observer in the eye. Sir Isaac Newton thought the same, but for once "the greatest scientist of all time" was wrong.

For when the Danish astronomer, Romer, discovered that light travelled at the rate of somewhere about 186,000 miles per second it dawned upon people that it was scarcely believable that particles of any kind could by any means be made to move so fast. So they set about searching for a new explanation, and they found it in the idea that light was conveyed from the bright body to the observer's eye by means of waves, and as there cannot be waves of nothing they had to imagine a something to exist in all the vacant s.p.a.ces of the universe capable of forming the waves of light. This something was called the luminiferous ether or light-bearing ether. We can neither see, feel, taste nor hear it. Our senses tell us nothing about it. Indeed, if it does really exist it must be so very different from anything that we do know by our senses that one is often tempted to doubt its existence. Still, it explains so many things which are otherwise unexplainable and enables us so correctly to reason from one phenomenon to another that our reason forces us to accept it as a fact, at all events until something better comes along.

This wave theory in regard to light was finally set at rest by the curious discovery about a century ago by Dr. Thomas Young of London that if two lots of light were brought together in a certain way they produced darkness.

Now if a ray of light were a stream of particles, two such rays would inevitably and always, if added together, produce a doubly brilliant light, and under no conceivable circ.u.mstances could they do anything else. But two lots of waves can, and do, under the proper conditions, neutralize each other so as to produce rest.

This mutual action upon each other of two sets of waves can be very simply exhibited by two violin strings tuned to _nearly but not quite the same note_. If you have a violin handy, try it and you will find that when either string is plucked separately it gives a steady continuous sound, but if both be plucked at the same time they give a throbbing sound. That is because, periodically, as one string is coming up the other is going down, so that they neutralize each other, while at other times, owing to the fact that one is vibrating faster than its fellow, both are rising and falling together. When neutralizing each other there is a momentary silence, while in between the silences come the times when both are acting together and therefore producing a specially loud sound. And so as the vibrations of the faster keep gaining upon those of the slower string one hears a continual crescendo and then diminuendo repeated over and over again. So two sets of sound waves sometimes produce silence.

And in like manner two sets of light waves can be made so to "interfere"

(that is the technical term) that together they produce darkness.

So for a hundred years or more people have, generally speaking, accepted the idea that light consists of waves in a medium called The Ether. Heat also is brought to us from the sun and from any distant hot body by similar means, the difference between light waves and heat waves being simply in their wave length or the distance apart. The different colours of light, too, are to be accounted for by different wave lengths.

You have of course seen how a magnet can act upon a piece of iron at a distance. You may, too, have tried the experiment of jerking a magnet past a piece of wire, thereby generating an electric current in the wire. Both those things need, for explanation, that we a.s.sume the existence of a something invisible and undetectable by our senses between the magnet and the iron and between the magnet and the wire, by which the action of one is conveyed to the other. So people imagined another Ether capable of acting like a link between the magnet and the iron and between the magnet and the wire.

Now just about half a century ago a celebrated professor of Cambridge University brought all these facts about light, heat, magnetism and electricity together and by skilful reasoning showed that but one Ether sufficed to explain all these things. He showed how magnetic and electric forces acting together could produce waves like those of light and heat. And finally he demonstrated by figures that waves so formed would necessarily travel at the very speed at which light and heat are known to move.

This is known as the electro-magnetic theory of light. And not content with showing the nature of things already known, Professor Clerk-Maxwell added a prophecy that there were other waves in existence of longer wave length, which no one then knew how to make or to detect if made.

Following up this prophecy many investigators sought these waves, and the first to find them was Professor Hertz of Carlsruhe in Germany.

Fortunately for his position in the minds of English people he died before the War, so that his name is not sullied by the stupidities of which German professors in more recent days have been guilty. On the contrary, his writings show him to have been a kindly, modest, genial soul, and particularly gratifying is his generous a.s.sertion in one of his books that had he not himself discovered these waves he is certain Sir Oliver Lodge would have done so. He seemed quite anxious to share the credit of his discovery with his "English colleague" as he called him.

Let us see then how these "Hertzian waves" are produced. In the year 1748 a Dutch experimenter named Cuneus thought he would try to electrify water. He got a gla.s.s flask and filled it with water into which he let drop one end of a chain connected to an old-fashioned frictional electrical machine. Thus he stood with the flask in his hand while a friend worked the machine. After a short time the friend stopped and Cuneus took hold of the chain to lift it out, when to his astonishment he received a shock which knocked him over, broke his flask and sent him to bed to recover.

Unwittingly Cuneus had invented what became known thereafter as a Leyden jar, Leyden being the town in which he lived. It consisted, you will notice, of two conductors, the water and his hand, with an insulator, the gla.s.s, in between.

To understand or rather to give ourselves a useful working explanation of how such an apparatus comes to be charged we must first imagine that everything contains a certain normal amount of electricity which we can by certain means add to or take away from at will. When we add some to anything we say we have given it a positive charge: when we subtract some we say that we have imparted a negative charge. Clearly, if we add some to one thing we must first obtain it from something else, and if we take some away from one thing we must do something with what we have taken, and so we add it to something else. Therefore whenever we charge anything positively we must charge something else negatively and vice versa.

Now the ease with which we can thus charge two bodies seems to depend upon their nearness to each other, so that the easiest things to charge are two plates of metal separated by the thinnest possible insulator.

Modern Leyden jars are usually formed of a thin gla.s.s jar with a lining inside and out of tinfoil.

The Leyden jar is, however, only one form of the piece of electrical apparatus known as an electrical condenser, and many other forms exist.

For example, a flat sheet of gla.s.s with foil above and below, or several such piled one on top of another. An eminent electrician whom I know has recently made some of two tin patty pans put bottom to bottom, nearly but not quite touching, the whole being enclosed in a solid block of paraffin wax. And I might describe many other forms, but whatever they may be every one is essentially two conductors with an insulator between.

Now when a condenser has been charged its charges remain for a considerable time unless they be given a chance to escape. Suppose you have a charged condenser and that you take a wire and with it touch simultaneously both the conductors, the surplus on one "plate" will rush through the wire and make good the deficiency upon the other; it will thus in an instant become discharged.

Now several scientific men had suggested, before Hertz's time, that when that occurred something else happened too. They thought that the charge did not simply rush from one plate to the other instantly, but that it oscillated to and fro for a period; that the surplus rushing round overshot the mark, so to speak, and not only made up the deficiency but caused a surplus on the opposite plate, after which this new surplus rushed back again through the wire, doing the same thing, though to a less and less degree, several times over before a condition of perfect rest was reached. To use a simple a.n.a.logy, it was thought that the surplus swung to and fro like the swinging of a pendulum. We know that a pendulum swings because of its inertia, and electricity possesses a property very like inertia which, it was thought, would cause it to behave in the same way.

The Ether waves travel at the rate of 186,000 miles per second, so that if, as was thought, a sudden current of electricity gives rise to a wave, currents which succeed each other at the rate of one per second would produce waves 186,000 miles apart. A hundred currents per second would give a wave length of 1860 miles. A thousand per second would give 186 miles. But a thousand succeeding currents per second are difficult to produce, and 186 miles is so very much greater than the tiny fraction of an inch, which is the length of the light and heat waves, that Hertz had to find some way of making currents succeed each other faster even than a thousand times per second.

So he thought of these oscillating currents which were supposed to occur when a condenser was discharged, and he rigged up a condenser with an induction coil and a spark gap in a way which he thought would do what he wanted.

There is not room here to explain the Induction Coil, indeed it is so well known that it will be quite sufficient to state that it is an apparatus which takes steady current from a battery and gives back instead a lot of little spurts or splashes of current at a rate of, say, fifty or one hundred splashes per second, according as we adjust the little vibrating spring which forms a part of the coil. We can so connect this to a condenser that each splash will charge it up; and we can combine with it a spark-gap, that is to say, a gap between two k.n.o.bs, so that every time it is charged it immediately discharges again through this gap. Thus we may have, say, one hundred splashes per second, and each splash is followed by several oscillations across the air-gap, the oscillations taking place at the rate of perhaps a million per second. Each series of oscillations is called a "train."

Now a million per second gives a wave-length somewhere about what Hertz wanted, so he arranged his apparatus as just described.

For a condenser he used two metal plates a little distance apart, the air between forming the insulating material. He set up his apparatus in a large room, and having started the coil he moved about with a nearly complete hoop of wire, the ends of which nearly touched. Working in darkness he found after a while that sometimes he could see little sparks, very small but just visible across the gap between the ends of the bent wire. Those sparks only occurred when the coil was in action, and so he knew that the one was the result of the other's work. By careful painstaking experiment he found that the sparks were unquestionably caused by waves, and that the waves moved with the same speed as light, also that they could be reflected and refracted just on precisely the same principles as those which control light. Moreover, he measured the wave-length.

At first sight it seems incredible that anyone could measure the distance apart of waves which travel at such a speed as 186,000 miles per second, but fortunately, by a special application of "interference,"

it is possible to make the waves stand still and tamely submit to measurement. An example of this can be seen by simply tapping a gla.s.s of water, when the ripples being reflected off the sides interfere with each other and become stationary. Stationary waves are half the wave-length of the original waves, and by using this method Hertz was able to make a measurement which at first sight seems beyond the bounds of possibility.

Thus Hertz discovered how to make the waves which Clerk-Maxwell had predicted and also how to detect them when made.

It was not long before the idea arose of using these waves for signalling to a distance. Many experiments were made but with no very striking success until 1896 when Marconi first came to England.

Hertz had noticed that the farther apart he placed the plates of his condenser the farther could he get his tell-tale spark, so Marconi saw that the plates of his condenser, too, must be far apart. He also found that the earth could be used as one of the plates, that in fact there was a great advantage in so using it. So, one plate having to be the earth itself and the other removed as far as possible from it, the tall masts of the wireless antenna came into being.

[Ill.u.s.tration: LISTENING FOR THE ENEMY.

Special sensitive cylinders are sunk into the ground to which the usual telephonic apparatus is fixed. This enables the sappers to detect any underground operations by the enemy.]

When Marconi came to England he was taken under the kindly wing of Sir William Preece, the veteran engineer of the Post Office, and the facilities which Sir William was able to give no doubt helped largely in his subsequent rapid progress. After a few experiments in London he got to work across the Channel, sending messages from the North Foreland Lighthouse to Wimereux on the coast of France, including congratulatory messages between the French authorities and good Queen Victoria.

A little later he was signalling from Niton in the Isle of Wight to the mainland and to the far west at the Lizard. The first wireless telegram which was actually paid for was sent by Lord Kelvin, the father of cable telegraphy, from Niton to the mainland, whence it was transmitted by land wires to Sir George Stokes. This incident, so interesting because of its marking a stage in the history of this great invention, also because of the persons concerned, occurred in 1898.

But Marconi was quickly increasing the range of his apparatus far beyond anything already mentioned. He journeyed in the Italian warship _Carlo Alberto_ as far north as Cronstadt and as far east as Italy, keeping in communication with England all the time. Then he crossed the Atlantic, again keeping up communication with England the greater part of the journey.

Raising his wires to a great height by means of kites he was soon able to signal from Nova Scotia to the great station just previously built at Poldhu in Cornwall, and then wireless telegraphy from land to land across the great ocean became an accomplished fact.