Next water ballast is admitted into certain other spaces in the ship's structure, these spaces being called, because of the use to which they are put, ballast tanks. Gradually, as the incoming water increases the weight of the vessel, she sinks until she is awash. Then the diving rudders are set at the right angle (a pendulum serves to show the angle at which the boat points) and down she goes. As the pressure-gauge indicates the approach to the required depth the rudder is flattened out a little until just that position is found which keeps the boat under at the desired depth.

Of course, when all hatches and openings were closed the supply of fresh air was cut off and after that the crew had to depend upon the air contained in the submarine. Also, they had to stop the engine, for without air it cannot work: nor can it work without giving off fumes, which, if admitted to the ship, would soon suffocate the crew. Just before closing up, therefore, the engine is stopped and electric motors take up the task of driving the ship.

Now suppose that, while running submerged, the commander espies, through his periscope, an unsuspecting enemy. He tries forthwith to get as close as he can. Having noted the direction of the vessel and which way she is going and as far as possible her speed, he submerges more deeply, in all probability, lest the white streak which represents the wake caused by his periscope should reveal his presence. For possibly she is one of those terrible destroyers in fair fight with which he has but a poor chance. His only safety lying in complete invisibility, he therefore submerges entirely, trusting to his calculations to lead him in the desired direction. Thus he attempts and, if he have good luck, he succeeds in getting reasonably near to his foe.

Then he must try so to man[oe]uvre that his bow shall at the right moment be pointing towards the quarry, for his torpedo tubes are in the bow and they are fixed, or nearly so at all events, so that he can only fire them in a direction nearly, if not precisely, in the direction of the centre line of his ship.

Nay, he must do even more than that. It will not do to fire the torpedo directly at the ship, for a torpedo is comparatively slow. Suppose it is capable of forty miles an hour, and the other ship is a mile away: the torpedo will take ninety seconds to reach it. And in that time it may have travelled a mile or so itself. So the submarine man has to allow for that.

Occasionally, therefore, he comes up a little for a moment in the hope of getting a sight of the enemy while not revealing his own presence. Or perhaps he may decide to risk being seen and caught, trusting to the chance of getting his own blow in first. He needs to be a most resourceful man, with clear and keen judgment and supreme self-confidence, or he can never grapple with such a task.

Supposing, then, that he succeeds in getting undetected into a favourable position, as he thinks; at the critical moment the other ship may change its course, and the whole scheme goes awry. Perhaps he then tries to follow, but that is bad, for the end of a ship is not nearly so good a target as the side and the part hit is not so vulnerable. The first torpedo may, however, so disable the vessel as to give him chance to get into position for a second and better shot.

Anyway, when he thinks he has got his best chance he lets off a torpedo, immediately diving to be safe out of harm's way for a while. Then he rises to see the result of his work. If successful he would be sure to hear the sound, for water is an excellent sound-conductor and a submarine is like a gigantic telephone ear-piece.

It must be a nerve-racking job at the best of times, for the submarine is a very vulnerable craft. A member of the crew of a German submarine captured during the war is reported to have said that out of ten submarines attacked, nine were sunk. That may or may not be true, but it is certain that a very little damage, which would hardly affect an ordinary craft, is enough to sink a submarine. That is because, in order to be able to sink at will, the reserve of buoyancy has to be very low. An ordinary surface ship has at least as much of its bulk above water as below: hence it can take on board a weight of water almost equal to, if not exceeding its own weight before it sinks. At the best a submarine has not more than 30 per cent of excess and so it sinks if water amounting to only 30 per cent of its weight gets into it. In other words, the reserve in one case is at least 100 per cent: in the other at most 30 per cent.

During the war a submarine saw and tried to track down, somewhat after the manner described, a slow, steady-going collier which plies between London and the north carrying coal for a London gas-works. Having, as it thought, got into position for discharging its torpedo it rose for a final look when (it must have been to the amazement of the crew) the collier was seen making straight for them. What they really thought no one will ever know, for the collier had the best of the encounter, the submarine was crushed beneath her blunt bows and sank, no doubt, for ever. The mere fact that a slow, clumsy, heavily-laden collier could ever thus vanquish an up-to-date submarine is eloquent testimony to their vulnerability.

Many a submarine, too, has fallen to the shells of an armed fishing trawler simply because the shells of the latter were so much quicker in action than a torpedo, coupled with the fact that one well-placed shot, by preventing a submarine from diving, renders it almost helpless.

Some submarines, however, have a gun on the deck, so that when light they can fight like a destroyer or other lightly-armed vessel. The gun shuts down into a cavity when the vessel goes below.

The periscope, which forms such an important part of the submarine's equipment, is really very little more than a telescope. On the top there is a little mirror, or more probably a prism or three-cornered piece of glass which serves precisely the same purpose in that it reflects exactly as a mirror does. This is so placed that it throws the light from distant objects down the tube into the interior of the ship. In the tube are lenses very like those of an ordinary telescope and the light may be made to throw a picture upon a little table or screen or else can be viewed through another prism directly by the eye. In either case the periscope is just like an ordinary telescope set up vertically with a prism at the top so that it can "see" at right angles, and possibly another at the bottom so that the picture can be viewed at right angles to the direction of the tube. The latter is necessary only for the convenience of the observer, since otherwise he would have to be upon his back to look up the tube. The whole apparatus can be rotated mechanically and a scale forms a means of measuring the precise direction in which the prism or mirror is at any moment pointed. This is useful for measuring roughly the position of the "prey," and it may even be used as a rough means of getting the range.

Another feature is the gyroscope compass, to which a passing reference has already been made. It is fairly well known that an object when spinning exhibits properties quite different from those which it possesses when still. A boy's top is a familiar illustration, for while spinning it will stand perfectly steady, supported only upon a tall peg with a sharp point, a pose which it will absolutely refuse to maintain when not spinning. Now fortunately for the present purpose it so happens that one of the peculiarities of the gyroscope or spinning-wheel is this: that if mounted in a certain way it persists in placing its axis in the same plane as that in which the axis of the earth lies. If you imagine for a moment a plane or flat surface of which the earth's axis forms a part you will see that wherever that plane cuts the surface of the earth will be a line in a north and south direction. Consequently, if any horizontal object has its axis in that same plane it, too, will always point north and south. A wheel, small but heavy, is therefore mounted with its axis supported horizontally upon a little metal raft floating in a trough of mercury and driven round at a very fast speed by a small electric motor fixed in it.

Whatever its position may be to start with, this revolving wheel will in a short time slew itself round upon the supporting mercury until its own axis is in the same plane as the axis of the earth: until, in fact, its axis points due north and south. Arrived in that position, it will remain there no matter how the ship upon which it stands may turn. Since it floats freely upon mercury the motion of the ship has little effect upon it, so little indeed, that it has no difficulty in following its own peculiar bent, even if the ship be describing circles.

The advantages of this are various: two of them may be stated. First, the apparatus points to the actual geographical north and not to the magnetic north, which is a slightly different direction and one, moreover, subject to frequent variation. Second, it is absolutely unaffected by the presence of iron or other magnets, a very fruitful source of error in the magnetic compass when used upon an iron ship close to steel guns and electrical machinery. Surrounded with iron as is the compass in the interior of a submarine, the magnetic needle practically refuses to work at all, so that, although employed on other ships, it is on the submarine that the gyro-compass finds its most important field of usefulness.

The pressure-gauge or manometer, which indicates the depth, is probably not different in any respect, except in its dial, which is marked in feet-depth instead of in pounds-pressure, from the pressure-gauge used on steam boilers. It has either a little cylinder with a piston in it which the water presses upwards more or less against the force of a spring, a diaphragm which is bent more or less, or a bent tube which tries to straighten itself out as the pressure inside it increases.

The older submarines derived their power from petrol engines similar to those which drive high-power motor-cars, but nowadays these have given place to engines of the type invented by the unfortunate Diesel who, after making one of the most brilliant and successful inventions of modern times, committed suicide, apparently in the height of his success.

These engines burn cheap heavy oil in place of the costly refined petrol: they are exceedingly reliable and well-behaved, and are free from many of the troubles which affect the petrol motor. They are referred to in more detail in another chapter.

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 accumulator 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 partitions 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 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 spaces 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 circumstances 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 assume 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 assertion 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 glass 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 glass, 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 glass 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 glass 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 analogy, 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 knobs, 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 glass 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.

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.

We all know how things have progressed since then. A telegram by Marconi is as commonplace to-day as a telegram by cable. The British Government is now engaged upon a series of stations dotted about the globe in such a way that every part of the widely separated British Empire shall be in constant touch with every other part by wireless telegraphy. In other words, the range of the system has now become such that nothing further is needed.

The British Admiralty has a few wires slung to posts on the top of the offices in London, and those few wires enable touch to be maintained with ships. As almost every intelligent newspaper reader in Great Britain knows, the Germans were in the habit, during the war, of sending news to the United States by wireless telegraphy, which news was always picked up by the Admiralty installation and circulated to the British newspapers, often to the amusement of their British readers.

The famous Emden, too, which had such a run of success until it encountered the Australian cruiser Sydney, met its end entirely through the intervention of wireless telegraphy.

These incidents give us a good idea of the usefulness of wireless in naval warfare. In military work it is used chiefly in connection with air-craft, but of that more will be said in another chapter.

CHAPTER XXI
WIRELESS TELEGRAPHY IN WAR

However these wires may be arranged and supported they are very carefully insulated from their supports, for when sending they have to be charged with current at a high voltage and need good insulation to prevent its escape, while, in receiving, the currents induced in them are so very faint that good insulation is required in order that there may not be the slightest avoidable loss.

The function of these wires, it will be understood, is to form one plate of a condenser, the earth being the other plate and the air in between the "dielectric" or insulator.

In the case of ships "the earth" is represented by the hull of the vessel. It makes a particularly good "earth" since it is in perfect contact with a vast mass of salt water, and that again is in contact with a vast area of the earth's surface. Salt water is a surprisingly good conductor of electricity.

In land stations "earth" consists of a metal plate well buried in damp ground. The whole question of conduction of electricity through the earth is very perplexing. There seems to be resistance offered to the current at the point where it enters the ground, but after that none at all. Consequently the resistance between two earth plates a few yards apart and between similar ones a thousand miles apart is about the same. Though the earth is made up mainly of what, in small quantities, are very bad conductors indeed, taking the earth as a whole it is an exceedingly good conductor. That makes it all the more important that where the current enters should be made as good a conductor as possible, and the construction and location of the earth plates is therefore very carefully considered so as to get the best results.

Wires, of course, connect the antenna to the earth, thereby forming what is called an "oscillatory circuit." The ordinary electric circuit is a complete path of wire or other good conductor around which the current can flow in a continuous stream. An oscillatory circuit is one which is incomplete, but the ends of which are so formed that they constitute the two "plates" of a condenser. In that way, according to theory, the circuit is completed between the two ends by a strain or distortion in the "Ether" between them. A continuous current will not flow in such a circuit, but an alternating, intermittent or oscillating current will flow in it in many respects as if there were no gap at all but a complete ring of wire.

At some convenient point in this oscillatory circuit are inserted the wireless instruments, one set for sending and the other set for receiving, either being brought into circuit at will by the simple movement of a switch.

In small installations the central feature of the sending apparatus is an Induction Coil operated by a suitable battery or by current from a dynamo. Connected with it is a suitable spark gap consisting of two or three metal balls well insulated and so arranged that the distance between them can be delicately adjusted. This is generally done by a screw arrangement with insulating handles, so that the operator can safely adjust them while the current is on.

The current from the battery or dynamo to the coil is controlled by a key similar to those used in ordinary telegraphy, the action being such that on depressing the key the current flows and the coil pours forth a torrent of sparks between the knobs of the spark-gap, but on letting the key up again the sparks cease. Since the sparks send out etherial waves which in turn affect the distant receiving apparatus it follows that a signal is sent whenever the key is depressed. Moreover, if the key be held down a short time a short signal is sent, but if it be kept depressed for a little longer a long signal is sent, by which means intelligible messages can be transmitted over vast distances.

Certain specified wave lengths are always used in wireless telegraphy. That is to say, the waves are sent out at a certain rate so that they follow each other at a certain distance apart. In other words, it is necessary to be able to adjust the rate at which the currents will oscillate between the antenna and earth. Every oscillatory circuit possesses two properties which are characteristic of it. These two properties are known as Capacity and Inductance. It is not necessary to explain here what these terms mean precisely. It is quite sufficient just to name them and to state that the rate at which oscillations take place in such a circuit depends upon the combined effect of these two properties. Consequently, if we can arrange things so that capacity or inductance or both can be added to a circuit at will and in any quantity within limits, we can within those limits obtain any rate of oscillation which we desire and consequently send out the message-bearing waves at any interval we like; in other words, we can adjust the wave-length at will.

Fortunately, it is very easy to add these properties to an oscillatory circuit in a very simple manner. A certain little instrument called a "tuner" is connected up in the circuit and by the simple movement of a few handles the desired result can be obtained quickly even by an operator with but a moderate experience. He has certain graduated scales to guide him, and he is only called upon to work according to a prearranged rule in order to obtain any of the regulation wave-lengths.

As a matter of fact, the instruments are not directly inserted in the antenna circuit, the circuit that is which is formed by the aerial wires, the earth and the inter-connecting wires. Instead, the two sides of the spark-gap are connected together so as to form a separate circuit of their own, the local circuit as we might call it, and then the two circuits, the antenna circuit and the local circuit, are connected together by "induction."

A coil of wire is formed in each, and these two coils are wound together so that currents in one winding induce similar currents in the other winding, and by that means the oscillations set up by the coil in the local circuit are transformed into similar oscillations in the antenna circuit. This transformation involves certain losses, but it is found in practice to be by far the most effective arrangement. Both the circuits have to be tuned to the desired wave length, but that is done quite easily by the operation of the handles in the tuner already referred to.

It is to this coupling together of tuned circuits that Marconi's most famous patent relates. It is registered in the British Patent Office under the number 7777, and hence is known as the "four sevens" patent. It has been the subject of much litigation, which proves its exceptional importance, and it is to the fact that the Marconi Company have been able to sustain their rights under it that they owe their commanding position to-day in the realm of wireless telegraphy.

The Receiving Apparatus also consists of a separate local circuit which can be coupled when desired to the antenna circuit through a transformer. The same simple tuning arrangement is made to affect this circuit also, so that the "multiple tuner," as the instrument is called, controls all the circuits both for sending and for receiving. The oscillations caused in the antenna circuit by the action upon it of the etherial waves flowing from the distant transmitting station pass through one winding of the transformer and thereby induce similar oscillations in the local receiving circuit which are made perceptible by the receiving instrument.

Reference has already been made to the original form of receiving apparatus called the Coherer. This, however, has been very largely superseded by the Magnetic Detector of Marconi and the Crystal Detector, both of which make the signals perceivable as buzzing sounds in the telephone.

The magnetic detector owes its existence to the fact that oscillations tend to destroy magnetism in iron. It is believed that every molecule of iron is itself a tiny magnet. If that be so one would expect every piece of iron to be a magnet, which we know it is not. We can always make a piece of iron into a magnet by putting another magnet near it, but when we take the other magnet away the iron loses its power, or to be precise it almost loses it. A piece of even the best and softest iron having once been magnetized retains a little magnetic power which we call "residual" magnetism.

All this is easily explained if we remember first that a heap of tiny magnets lying higgledy-piggledy would in fact exhibit no magnetic power outside the heap. If, however, we brought a powerful magnet near them it would have the effect of pulling a lot of them into the same position, of arranging them in fact so that instead of all more or less neutralizing each other they could act together and help each other. Then the heap would become magnetic. On removing the powerful magnet, however, a lot of the little ones would be sure to fall down again into their old places and so the heap would at once lose a large part of its power, yet some would remain and so it would retain a certain amount of "residual" magnetism. If, then, you were to give the table on which the little magnets rest a good shake, the "higgledy-piggledyness" would be restored and even the "residual" magnetism would vanish.

So we believe that the little molecules lie just anyhow, wherefore they neutralize each other and the mass of iron is powerless. When another magnet comes near, however, they are more or less pulled into the right position and the iron becomes magnetized. When the magnet is removed the magnetism which it produced is largely lost, and if last of all we give the iron a smart blow with a hammer even the residual magnetism vanishes too.

Now, oscillations taking place in the neighbourhood of a piece of iron possessing residual magnetism have much the same effect as the blow of a hammer. Probably because of its rapidity an oscillating current shakes the molecules up and strews them about at random, entirely destroying any orderly arrangement of them. And Marconi used that fact in detecting oscillations.

Two little coils of wire are wound together, one inside the other. Through the centre of the innermost there runs an endless band of soft iron wire. Stretched on two rollers this band travels steadily along, the motive power being clockwork, so that it is always entering the coil at one end and leaving it at the other. As it travels it passes close to two powerful steel magnets, so that as it enters the coil it is always slightly magnetized. The oscillations are passed through one of the two concentric coils, and their action is to remove suddenly the residual magnetism in that part of the moving wire which is at the moment passing through. That sudden demagnetization then affects the second of the concentric coils, inducing currents in it, not of an oscillating nature but of an ordinary intermittent kind which can make themselves audible in a telephone which is connected with the coil.

This arrangement, then, causes the oscillations, which will not operate a telephone, to produce other currents of a different nature which will.

The reason why oscillations have no effect in a telephone is no doubt because they change so rapidly, at rates, as has been mentioned already, of the order of a million per second. The telephone diaphragm, light and delicate though it is, is far too gross and heavy to respond to such rapidly changing impulses as that. In the magnetic detector the difficulty is overcome by making them change the magnetic condition of some iron wire which change in turn produces currents capable of operating a telephone. The Crystal Detector achieves the same result in another way.

There are certain substances, of which carborundum is a notable example, which conduct electricity more readily in one direction than the other. Most of these substances are crystalline in their nature, and hence the detector in which they are used gets its name. Carborundum, by the way, is a sort of artificial diamond produced in the electric furnace and largely used as a grinding material in place of emery.

It is easy to see that by passing an oscillating current, which is a very rapidly alternating current, through one of these one-direction conductors one half of each oscillation is more or less stopped. Oscillations, again, are surgings to and fro: the crystal tends to let the "tos" go through and to stop the "fros." That does not quite explain all that happens. It is not fully understood. The fact remains, however, that by putting a crystal in series with the telephone the oscillations become directly audible. The term "in series with" means that both crystal and telephone are inserted in the local receiving circuit so that the currents in that circuit pass through both in succession.

The resistance of the crystal being very great, a special telephone is needed for use with it. It is quite an ordinary telephone, however, except in that it is wound with a great many turns of very fine wire and is therefore called a high-resistance telephone.

Whichever of these detectors be used, then, the operator sits, with his telephone clipped on to his head, and with his tuner set for that wave length at which his station is scheduled to work, listening for signals. He may go for hours without being called up, and in the meantime he may hear many signals intended for others. He knows they are not for him, since every message is preceded by a code signal indicating to whom it is addressed.

Under the conditions of warfare there is far more listening than there is sending, but when a station wishes to send the operator just switches over, cutting out his receiving apparatus and bringing his transmitting instruments into operation, and, having adjusted his tuner for the wave length of the station to which he desires to communicate, he flings out his message.

In war-time, too, there is much listening for the signals of the enemy, which is the reason why as few messages are sent out as possible. In this case the man sits with his telephone on his head carefully changing his tuner from time to time in the endeavour to catch any message in any wave-length which may be travelling about. This searching the ether for a chance message of the enemy must be at times a very wearisome job, but it must be varied with very exciting intervals.

On aircraft it is clear that no earth connection is possible. The antenna in that case usually hangs vertically down from the machine or airship. Under these conditions the valuable effect of the earth connection is of course lost. As will be remembered, the earth-connected apparatus sends forth waves which cling more or less to the neighbourhood of the earth's surface, while those from the non-earthed apparatus as used by aircraft tend to fly in all directions. The latter apparatus is in fact almost precisely similar to that which Hertz used in his first experiments. Hence the range is comparatively poor under these conditions, but it is good enough for very valuable work in warfare. Communication between airman and artillery by this means has revolutionized the handling of large guns in the field.

To save the airman from the accidental catching of his aerial wire in a tree or on a building there is sometimes fitted a contrivance of the nature of wire-cutters so that he can at any moment cut himself free from it.

So far we have dealt almost exclusively with the naval and aerial use of this wonderful invention. It is employed, though in a lesser degree, in land warfare. In such cases the aerial may be merely a wire thrown on to and caught up on a high tree. More elaborate devices are used, however, such as a high telescopic tower similar to the tall fire-escape ladders of the fire-brigades. Anyone who has seen the ladders rush up to a burning building and commence to erect themselves almost before they have stopped will realise how valuable such a machine must be for forming a temporary and easily movable wireless antenna. The power which causes the tall tower to extend itself erect in a few seconds is compressed air carried in cylinders upon the machine, while the power which takes it from place to place is a petrol motor, and since the latter can be made to re-charge the storage cylinders it is clear that in it we have a marvellously convenient adjunct to the wireless apparatus.

But apart from such carefully prepared devices the men of the Royal Engineers are past masters in the art of rigging up, according to the conditions of the moment, all sorts of makeshift apparatus whereby signalling over quite long ranges can be carried on by "wireless." Such improvisations, could they be recorded, would constitute war inventions of a high order.

CHAPTER XXII
MILITARY TELEGRAPHY

In view of the fact that soldiers speak of their home-land, for which they are enduring all manner of risk and hardship, and to which they are longing to return, by the contemptuous-sounding name of "Blighty," we might expect that what they call a buzzer has nothing whatever to do with making sound, but in this case the name describes the thing very aptly. Its sole purpose and intent is to make buzzing sounds of either long or short duration.

Perhaps the simplest way in which I can describe this useful and interesting invention is by telling you how you can make one for yourself. It is nothing more than an electric-bell mechanism connected up in a certain way.

As most people know, an electric bell contains a magnet made of two round pieces of iron placed parallel and yoked together at one end by means of a third piece of iron, generally flat, while on to each round piece is threaded a bobbin of insulated wire. The iron becomes a magnet when, and only when, current flows through the wire.

Near the free ends of the round pieces, or the poles of the magnet, to use the orthodox term, is placed another little piece of iron called the armature, carried upon a light spring. When the current flows in the wire the armature is pulled towards the poles against the force of the spring, but when the current ceases the magnet lets go and the armature, urged by the spring, swings back again.

Behind the armature is a little post through which passes a screw tipped with platinum, and in operation this screw is advanced until its point touches a small plate of platinum carried by the armature. Connection for the current is made to this "contact screw" whence it passes to the armature, through the spring to the wire upon the magnet, through that and away. On completing the circuit, then, as when you push the button at the front door, current flows and energizes the magnet. A moment later, however, the armature moves, breaks the contact with the screw and stops the current. Then the magnet lets go and the armature springs back, making contact once more and setting the current flowing again. These actions repeat themselves over and over again quite automatically, and the hammer which is attached to the armature vibrates accordingly.

That is the ordinary familiar electric bell. Cut off the hammer and you have a buzzer with which excellent telegraph signals can be sent.

So much for the sending apparatus. The receiving device is simply an ordinary telephone receiver. There is sometimes a little confusion in people's minds because of this. A telephone is used, but it is used as a telegraph instrument. The sounds heard in it are not speech but long and short buzzing sounds which, being interpreted according to the code of Morse, deliver up their message.

Now the telephone, by which term is always meant the receiver (the sending part of the telephone apparatus being a "microphone"), is one of the most remarkable pieces of electrical apparatus which the mind of man has ever conceived. It is astonishingly robust. With ordinary care you cannot damage it. There is no need whatever to keep it wrapped in cotton wool or even to keep it in a case. Without harm you can put it loose in your pocket. Within reason you may even drop it a few times without harm. Its cost is only a few shillings. Yet its sensitiveness is simply astounding. It will detect the existence of currents so small that any other type of instrument to deal with them has to be extremely delicate and costly.

It consists of a magnet fitted into a little brass case with a little piece of soft iron fixed on each pole, while each of these "pole-pieces" is surrounded by a tiny coil of wire. The lid of the box is a disc of thin sheet-iron, and things are so proportioned that the pole pieces nearly but not quite touch this sheet-iron "diaphragm."

An outer cover, generally of ebonite, serves to catch the sound-waves caused by any movement of the diaphragm and convey them to the ear.

The action of the permanent magnet tends to pull the diaphragm inwards—to bulge it in slightly—so that it is in a state of very unstable equilibrium. Because of this instability a very tiny current flowing through the coils and either adding to or subtracting from the strength of the magnet is sufficient either to draw it still closer or to let it recede a little. Whether it approaches or recedes depends upon the direction of the current through the coils and makes no difference to the sound. The movement of the diaphragm is great or small according as the current is strong or weak: any variation in the current causes a perfectly corresponding movement in the diaphragm. Even those very small and very complex changes in air-pressure which give us the sensation of sound are very faithfully followed by this simple bit of sheet iron, so that the sounds are faithfully reproduced for our benefit. At the moment, however, we are not dealing with speech but with buzzing sounds, which are very simple, being merely a rapid succession of "ticks."

The telephone, it must be remembered, takes no notice of a steady current, except when it starts and stops. But each time that occurs it gives a tick. Hence, if we start and stop a current very rapidly, or to use another term, make it rapidly intermittent, we get a rapid succession of ticks, and if rapid enough they form a humming, buzzing, or singing sound. If very fast you can get a positive shriek. The precise character of the sound depends entirely upon the rapidity of the intermittency.

Now it is easy to see that the current passed through an electric-bell mechanism is intermittent. It is the very nature of the apparatus to make the current intermittent. It is by so doing that it works. Therefore, if we pass the same current which works a bell through a telephone we get a buzzing or humming sound according to the speed of interruption.

The vibration of the armature itself also causes a humming sound of a similar note or tone to that heard in the telephone, but it must be clearly understood that these two sounds are quite different. One is the result of mechanical motion, the other is the result of electrical action producing motion in the diaphragm of the telephone. When you listen in the telephone it is not that you hear the sound of the bell mechanism, you hear another sound altogether, although, since both have the same origin, both have the same note or tone.

Take any old bell, then, which you may happen to have or be able to procure and an old telephone such as can be bought for a shilling or so at a second-hand shop, and these together with a pocket-lamp battery can be formed into a military field telegraph.

The way to connect these up is to run a wire from one of the copper strips on the battery to one of the terminal screws on the bell, a second wire from the other screw on the bell to one of the flexible wires of the telephone, which may be a mile away if you like, a third wire returning from the other flexible wire of the telephone back to the battery. To send signals all you have to do is to touch the return wire upon the second strip of the battery for short or long intervals, thereby making the dot-and-dash signals. Or a simple form of key can easily be contrived for the purpose.

Every time you complete the circuit the buzzer will buzz, in other words, it will permit an intermittent current to pass round the circuit and a buzzing or humming sound will be heard in the telephone, no matter how far away it may be.

This arrangement, however, involves two wires between the two stations, and in practice only one is usual. This could be arranged by running the third wire from the telephone not back to the sending station but to a peg driven into the earth, connecting the second pole of the battery in like manner to an earth pin at the sending end. Thus the return wire would be done away with and the earth utilized instead. To do that, unfortunately, you would need to increase very greatly the power of your battery, for although the path through the earth itself offers practically no resistance at all to the current, the actual places where the current passes to earth and from earth, especially if they be simply temporary pegs driven into the ground, offer very considerable resistance, so that in order to get enough current through the buzzer to make it work would need a powerful battery. There is another way, however, by which that difficulty can be overcome quite easily.

Probably all my readers know something of the induction or shocking coil, wherein intermittent currents in one part of the coil induce intermittent currents of a somewhat different kind in another part of the coil. Few people realize, however, that the same effect can be attained, within limits, in a single coil such as the winding upon the magnet of an electric bell.

Watch a bell at work and you will notice a bright spark at the place where the contact is made and broken. That spark is due to a sudden rush of current which takes place in the coil when the original current is stopped, in other words, when the contact is broken. It is as if the coil gives a rather vicious "kick" every time the current is stopped. There is not much electricity in this "kick" current, but it is very forceful, and it is that force which makes it actually jump across the gap after contact has been broken, thereby causing the spark.

Now we can capture most of that energy and make it go a long distance through wire and through earth carrying our messages for us. To do this we need to make a new connection on the bell at the place where the spring is fixed. Then we can make two circuits. One is between the two terminal screws of the buzzer, in which circuit we must include the battery and the key. That circuit will be just as it would be if we were fixing the buzzer to announce our visitors at the front door.

The second circuit is different: lead one wire from the new connection just made and take it to a pin driven into the ground. If the ground is just a shade moist a wire meat-skewer will answer admirably. Then lead a second wire from that one of the two terminal screws which is connected directly to the winding of the magnet (not to that one which is connected to the contact screw) and lead it away to your distant station.

At the other station connect the single wire to the telephone as before and the other "end" of the telephone to a pin in the earth. You will find that the "kicks" from the coil will traverse wire and earth-return quite easily, while there will be no difficulty about working the bell, for the small battery will do that quite well. In fact, after cutting the hammer off and so converting a bell into a buzzer, I have got quite good results with one-third of a pocket-lamp battery. The little flat batteries so familiar to us all if divested of their outer covering will be found to consist of three little dry cells any one of which is quite capable of sending messages in the way described as far as any amateur is likely to want to send.

To be able to send and receive at either end it is only necessary to connect both telephones and both coils "in series." That is to say, connect one end of the coil to the long wire and the other to one wire of the telephone, the other wire of the telephone being connected to earth. If this be done at both ends signals can be sent and received both ways.

Many young readers, scouts, members of cadet corps and the like, will find great pleasure and interest in constructing and working this apparatus, besides which it shows precisely what the official "buzzer" is like.

Although beautifully made, of course, the army instrument is essentially just that and little more. It has an additional feature, however, namely, a microphone, so that when desired it can be used as a speaking telephone for transmitting verbal messages. It also has the bottom of the case made of a brass plate so that earth pins are often unnecessary, the case dumped down upon the ground being a good enough "earth."

Buzzers are not used for very long lines: forty miles is about the limit, and usually the distances are very much less. That is because long lines rather object to rapidly changing currents flowing through them. Why, you say, what currents could change more rapidly than telephone currents carrying speech, yet they go for hundreds of miles? True, but in that case there are two wires, flow and return, twisted together all the way, under which conditions they interact upon each other in such a manner as to abolish the difficulty to which I am referring. Buzzers and indeed all the telegraph circuits consist of one wire and the earth, which is quite different.

Another objection to the buzzer is that it is apt to interfere with others. For instance, if two buzzer sets are at work anywhere near each other and the wires run parallel for a distance they will be able to hear each other's signals as well as their own. If two such sets are earthed near together the same thing happens, the signals of one are picked up by the other, a very annoying state of affairs for the operators.

Right at the front, however, amid the rough and tumble of the actual fighting, the buzzer is supreme. The wire used is sometimes plain copper enamelled: more often, however, it is a mixture of steel and copper strands twisted together and covered with a strong insulating covering. This is carried on reels in properly fitted carts which can advance at a gallop, paying out the wire as they go. The inner end of the wire is connected to the axle of the reel in such a way that a telegraphist in the cart is in communication all the time with the starting-point, the wheels of the cart providing him with an earth connection.

When laying these wires another interesting little device is often used—an earth plate on the operator's heel. Thus, while carrying the wire along, laying it as he goes, he can still be in communication with the starting-point every time he puts his heel to the ground.

For the longer lines away back from the fighting the methods employed are just the same as those of peace. "Sounder" instruments are used, Wheatstone automatic machines, duplex and quadruplex systems, whereby two and four messages are sent simultaneously over the same wire, indeed all the contrivances and refinements of the home telegraph office are to be found in the field telegraph offices. But it would hardly be fitting to describe them here. Some information on the subject will be found in "The Romance of Submarine Engineering," where their application to cable telegraphy is dealt with.

A genuine speciality of warfare, however, is the methods by which makeshift arrangements can be set up, such as sending telegraph messages over a telephone wire without interfering with the latter.

Imagine that A and B are the two wires of a telephone circuit running (for the sake of simplicity) from north to south. At the south end I connect a telegraph set to both wires while you, we will imagine, do the same at the north end. You and I can then signal to each other without the telephone man hearing us at all. To him the two wires are flow and return, to us they are both "flow," the earth being our return. Thus our signals never reach his instruments at all. But when we each connect to both his wires, do we not "short-circuit" or connect them to each other, thereby destroying his circuit? No, we are too cunning for that. We first connect the two wires A and B together with a coil of closely wound wire, having, in scientific language, much "inductance," and telephone currents shun a coil of that sort. Then we make our connection to the centre of that coil so that our currents go to A through half the coil and to B through the other half. This enables us to use the apparatus without interfering with the other fellow at all. For this, by the way, we must use ordinary telegraph instruments. We cannot employ a buzzer, for these coils which we use to obstruct the passage of the other man's telephone currents would also obstruct the changing currents from a buzzer. The slow, steady currents of the ordinary telegraph pass quite easily, however.

Again, suppose you and I want to communicate by buzzer and there is already a wire laid passing both of us but in use already for ordinary telegraphy. We only need to add a "condenser" to our apparatus and we can manage all right. As a matter of fact, the service instruments generally have condensers partly for this very purpose. Each of us then connects his instrument to the wire and to earth, after which we can signal to each other while the telegraphist is unaware of the fact. The reason that is possible is the reverse of what we saw just now. There we had a coil which obstructed buzzer or telephone currents but passed ordinary telegraph currents. Here we use condensers which will pass our buzzer currents but not the ordinary telegraph currents.

Thus the soldier telegraphist is up to many dodges whereby he can save time or save material, both of which may be precious. As in bridge building and other branches, he needs to be quick to adapt himself to circumstances, to utilize to the full any opportunities which may present themselves. But his principles are quite simple and do not differ in any way from those of peace. It is only in applying them that the differences arise.