From the use of concrete it was only a short step to reinforced concrete, or, concrete braced on the inside with iron or steel rods. It is sometimes called concrete steel, ferro-concrete, and armoured concrete. If we asked an engineer the idea in using reinforced concrete he might say to us that the steel when imbedded, united so closely with the concrete as to form one single mass of very great strength. Steel rods add to the tensile strength of concrete which alone has a tremendous strength under compression. In other words, steel does not break nor stretch easily; that is, it has great tensile strength. Concrete has great strength under compression; that is, it will hold up an enormous weight without crushing. Thus, a concrete beam alone might crack on the bottom, because it has not as great tensile strength as steel. But, if we put steel rods into a concrete mould, an inch or so from the bottom, turn out a reinforced concrete beam, for instance, and place it in the building, with the reinforcement at the bottom, we use a beam in which the strength of the concrete and iron is combined. Thus, when a great weight is placed on the top of the beam the concrete resists the compression of the weight, and the reinforcement at the bottom, by its tensile strength, prevents the beam from cracking where the strain of the weight is greatest.

That is what the engineer might tell us is the theory of reinforced concrete, and the practice requires the highest engineering skill and technical knowledge, but in the simplest terms, it is concrete, braced by an imbedded skeleton of steel. In actual practice the reinforcing rods run both ways, or diagonally, just as the engineers decide it is necessary to resist the particular kind of stress that the wall or beam must withstand.

Reinforced concrete was first used, so far as known, by M. Lambot, who exhibited a small rowboat made of that material at the World's Fair in Paris, in 1855. The sides and bottom of the boat were 1-1/2 inches thick, with reinforcement of steel wires. The boat is still in use at Merval, France. F. Joseph Monier, however, is called the "father of reinforced concrete," as he took out the first patent on it in France in 1865. Monier was a gardener and had experimented with large urns for flowers and shrubs. He wanted to make his pots lighter but just as strong, so he tried making some of concrete with a wire netting imbedded in the material. But even then the world did not realize that his accomplishment was more important to mankind than a great many of the wars that had been fought, and little was done with concrete as a building material until the Germans developed it.

Reinforced concrete was not used in the United States, according to the best records, until 1875, when W. E. Ward, without having studied the subject very carefully, built himself a house of it, in Port Chester, N. Y. He made the whole thing, including foundation, outside walls, cornices, towers, and roof of reinforced concrete, placing the steel rods where his own good judgment told him they would do the most good. About this time the Ransome Cement Company began to use the material for building, and put up a great many strong and beautiful structures, still to be seen in California and elsewhere.

Finally, bit by bit, in the face of opposition of all kinds, reinforced concrete came to be recognized by architects, engineers, and builders as one of the best materials for certain kinds of work. To-day we find that most of the predictions of the early enthusiasts have been fulfilled and that the age of concrete has dawned. That it will be used even more extensively in the future, as men learn more and more about this wonderful artificial stone, is certain.

OUR BOY FRIEND AND THE SCIENTIST LOOK OVER THE FIELD OF GASOLINE ENGINES AND SEE SOME BIG IMPROVEMENTS OVER THOSE OF A FEW YEARS AGO

WHILE we are following the conversations of the scientist and his young friend about new inventions, we must not overlook some of their most interesting times in keeping abreast of the vast improvements that are being made every year—almost every day—in the inventions of a dozen years ago.

For instance, there is the gas engine. Ten years ago it was a very imperfect machine, as every boy who has heard the old jokes about "auto-go-but doesn't," "get a horse," etc., will remember.

Then there is the wireless telegraph. No invention of recent years has shown a more remarkable development than that of Guglielmo Marconi for sending messages without wires.

But these are only a few of the things that the two friends talked about. They looked into the wonderful advancement in the art of photography about which every boy knows something, and they investigated the latest achievements of science in electric lighting. Ten years is a very short time, even in this fast moving age of ours, and we shall see that many inventions made years ago are still being worked upon by the original inventors and others.

First, let us see a few of the ways the gas engine has been improved, for we are all more or less familiar with it in automobiles, motor boats, or the hundred and one other places that it has become an invaluable aid to man in carrying on the world's work.

Our young friend brought up the subject one day when he asked the scientist for a few pointers on getting better results with his motor-boat engine.

"We will look it over together," said the man. "Of course you know that every gasoline engine has its own peculiarities, and crankinesses, so it's hard to tell just what's the matter with one until you see it. I don't know very much about them; I wish I knew more, but I have been talking with my automobile friends a good deal lately about the new motor invented by Charles Y. Knight."

"Oh, I know," replied the boy, "it is called the 'Silent Knight' motor because it doesn't make any noise, and it is used on a great many high-priced automobiles."

"That's it. If you like we will go and have one of these engines explained to us. At any rate the automobile man can tell you more about your motor-boat engine than I can."

The expedition was made shortly after the conversation. "You understand, of course," said the scientist on the way, "that the Knight motor represents only one of the many, many improvements in the gas engine, but it is what we call a fundamental improvement, as it is a development in the main idea of the gasoline motor, rather than merely an improvement of one of the parts. Most of the evolution of gas engines has consisted merely of the improvement and perfection of the various parts for more power, and more all around efficiency.

"You remember what you found out about gasoline motors in general when we were spending so much time talking about aeroplanes. The high speed motor, as we know it now, was invented, you know, by Gottlieb Daimler, a German inventor, in 1885, and with the ordinary four-cycle engine it takes four trips, or two round trips of the piston rod, to exert one push on the crankshaft of the engine. In other words, the explosion drives down the piston giving the power, and on its return trip the piston forces out the burned fumes. On the next downward stroke the fresh vapour is sucked into the cylinder and on the fourth trip, or second upward trip, the gas is compressed for the explosion. The carbureter on your motor-boat engine, and all others, as you know, is the device that mixes the gasoline with air and converts it into a highly explosive gas, and the sparking system is the electrical device that ignites the gas in the cylinders for each explosion which makes the 'pop, pop, pop' so familiar with all gasoline engines.

"In the old gas engines the ignition was derived from a few dry-cell batteries and some sort of a transformer coil, whereas nowadays the magneto takes care of this work. As you know there are many kinds of magnetos, and inventors have spent years working out better and better ones. Also, in the old style motors the carbureter was more or less of a makeshift, with a drip feed arrangement, and a hand regulating shutter for admitting the air. Now a special automatic device regulates this, so that it is no longer a toss up whether the gas is mixed in the proper quantities or not. Then, too, the oiling systems have been improved, so that the function is done automatically. In short, the motor has been made a perfectly reliable servant instead of a very capricious plaything.

"All these improvements made no fundamental change in the valves through which the gas was admitted to the cylinders, and the exhausted vapours expelled—and from your own experience you know that you are just about as apt to have trouble with your valves as with any other part of your machine.

"It is in these valves that the Knight motor departs from the usual style, and by this it eliminates the well-known 'pop, pop, pop' by which gas engines have been known all over the world."

As they looked over the engine, an expert in gasoline motors explained all the parts of the "Silent Knight" and showed the scientist and his boy friend just how the machine worked.

He said that the only big difference between the Knight motor and other standard makes of engines is that the Knight substitutes for the intake and exhaust valves an entirely new device composed of two cylinders, one within the other, sliding upon each other so as to regulate the flow of gas and the exhaust of fumes.

Early in his career as an inventor, while living in his home city of Chicago, Knight decided that gasoline engines had entirely too many parts—that they were too complicated—and he set about trying to simplify them. For one thing, he made a careful study of valves, and collected a specimen of every kind known to mechanics. The sliding locomotive valve seemed to him to hold the greatest possibilities for his work, and he began a series of experiments with sliding valves until he finally brought out his first engine in 1902.

Strange as it may seem, Knight's work was not recognized in his own country until after he had gone to Europe, where his engine was taken up by some of the biggest automobile manufacturers of England, France, Germany, Belgium, and Italy. After that it was taken up in the United States, and only now is coming to be generally known. The inventor now lives in England, where he was first successful, and he is still at work on improvements of his engine.

The motor expert went on to explain that the advantage of the Knight motor lay in the fact that the two sleeves or cylinders, which go to make up the combustion chamber or engine cylinder, sliding up and down upon one another, give a silent, vibrationless movement, as against the noisy action of the old style poppet or spring valve motors.

"But," interrupted the boy, "there are lots of other engines that run without making a noise nowadays."

"That is true," the man answered, "but most of them run quietly only when at low speed, or stationary. When they begin to hit the high places the noise of the poppet valves is very noticeable. A few years ago, when most engine builders were satisfied to make motors that would run, regardless of noise, they paid no attention to some of the finer mechanical problems, but since they have become more skilful, they are cutting down on the noise. But, as I say, the explosions are plainly heard when these engines are running at high speed. With the 'Silent Knight' the only noise is that of the fan and magneto, whether at low speed or the very fastest the motor can run. There can be no noise, for there is nothing for the sleeves to strike against."

The expert then went on to explain the motor in detail. The combustion chambers of the four or six-cylinder "Silent Knight," he explained, are made up of two concentric cylinders or sleeves, or, in other words, one cylinder within another. There is only the smallest fraction of an inch between them, and as they are well oiled by an automatic lubricating device they slide up and down upon each other with perfect ease. Of course the sleeves, which are made of Swedish iron, a very fine material for cylinder construction, are machined down inside and out so that they are perfectly smooth to run upon each other.

The two sleeves which go to make up one cylinder work up and down upon each other by means of a small connecting rod affixed to the bottom of each sleeve connected to an eccentric rod, which is driven by a noiseless chain from the engine shaft.

The most important features are the slots cut in each side, and close to the upper end of each sleeve, so that, as the sleeves move upon one another the slot in the right-hand side of the inner one will pass the slot of the right-hand side of the outer sleeve, and also the same with the left-hand side.

Then when the left-hand slots of the outer sleeve open upon, or come into register with the left-hand slots of the inner sleeve, a passage into the cylinder is opened for the new gas to enter. When a charge of gas has been drawn into the cylinder, one sleeve rises while the other falls, so that the openings are separated and the passage is tightly closed. The compression stroke then begins with the piston rising to the top. At this juncture the igniting spark explodes the compressed gas and the downward or power stroke takes place. During the upward compression stroke and the downward impulse stroke the slots have been closed, allowing no opportunity for the gas to escape. When the explosion has taken place and the piston has been driven to the bottom of its stroke, the right-hand openings in the inner sleeve and those of the outer sleeve come together, providing a passage for the exhausted gases to escape with the fourth or exhaust stroke. Thus it is plain that the motor is of the four-cycle type and it should not be confounded with two-cycle motors.

As the expert explained the motion he showed how it was carried out on an engine from which the casing had been partly removed. The careful mechanical adjustment of the eccentric shaft, which operated the connecting rods that pull the sleeves of the cylinder up and down so that the openings for the entrance of the fresh gas and the expulsion of the exploded fumes come together at just the proper second, was what took the boy's eye.

In connection with this the scientist handed the boy a magazine to read. It was a copy of the Motor Age in which an expert said editorially:

The friends of the Knight motor claim that it is simpler than the ordinary types of engines, having about one third less parts, that it is economic, powerful, and, as previously pointed out, runs silently. Beside these advantages, there are claimed for it many technical virtues that we need not enter into here.

The lubricating system of the Knight motors is another interesting point, as it serves to illustrate one more way in which the gasoline engine has been improved upon of late years. The manner of oiling used is known as the "movable dam" system. Located transversely beneath the six connecting rods are six oil troughs hinged on a shaft connected with the throttle. With the opening and closing of the throttle these troughs are automatically raised and lowered. When the throttle is opened, which raises the troughs, the points on the ends of the connecting rods dip deep into the oil and create a splashing of oil on the lower ends of the sliding sleeves. These sleeves are grooved circularly on their outer surfaces in order to distribute the oil evenly, while toward the lower ends holes are drilled to allow for the passage of oil.

When the motor is throttled down, which lowers the troughs, the points barely dip into the oil and a corresponding less amount of oil is splashed. An oil pump keeps the troughs constantly overflowing.

The motor is cooled by a complete system of water jackets, and it is fitted with a double ignition system, each independent of the other.

Of course in the adoption of the sliding sleeve type, mushroom valves, cams, cam rollers, cam shafts, valve springs, and train of front engine gears all are eliminated, the sliding parts fulfilling their various functions.

Before Mr. Knight ever achieved success with his motor it was subjected to some of the severest tests on record in the whole automobile industry. In France, Germany, and England, it was only accepted by the leading manufacturers after being tried out for periods extending over several months of the hardest kind of usage. Now, that it has proven itself a practical success, automobile men declare that the sliding valve principle, never before applied to gas engines until Knight began work, will undoubtedly have a lasting effect on the whole industry.

The compact little two-cycle motors represent another big fundamental development in the field of gas engines. There are many different makes of two-cycle motors, of course, and all have their various merits. They are used in practically all the work for which gas engines are employed, including automobiles, motor boats, and aeroplanes. It will not be necessary to describe these engines further than to say that the name describes the fundamental difference between them and the four-cycle motors. Instead of the piston making four strokes for every explosion—that is, an, upward stroke to clean out the burnt vapours, a downward stroke to suck in the fresh gas, an upward stroke to compress it, and finally the downward explosion or power stroke, all this work is done in two strokes.

For the general development of the gasoline engine, it is only necessary for a boy to look about him. Everywhere motors built on the same ideas as laid down in earlier inventions, but improved in every detail, are in use. Not only do we see them on fine pleasure automobiles, motor boats, and aeroplanes, but on our biggest trucks, fire engines, and in business establishments where light machinery is to be run.

THE SCIENTIST TALKS OF AMATEUR WIRELESS OPERATORS—THE GREAT DEVELOPMENT OF WIRELESS THAT HAS ENABLED IT TO SAVE ABOUT THREE THOUSAND LIVES—LONG DISTANCE WORK OF THE MODERN INSTRUMENTS

WHILE the inspiring stories of Jack Binns of the steamship Republic, and of J. G. Phillips and Harold S. Bride of the ill-fated Titanic are fresh in our minds, it is not necessary to say that within the last few years the wireless telegraph has established itself as indispensable to the safe navigation of the seas. The story of its development is a marvellous one when we think that it was only in December of 1901 that Marconi received the first signal ever transmitted across the Atlantic Ocean without wires. Now, as every boy knows, all the big steamships are equipped with wireless, all the governments of the world operate their own stations to communicate with their warships, at sea, and thousands upon thousands of boy amateurs operate their own little plants with complete success.

More wonderful still is the story when we think that by the use of this invention a total of about three thousand persons have been saved from death in shipwrecks. Nowhere in the pages of all history are there any more thrilling stories of heroism and devotion to duty than those of the men who, in the face of death themselves, have stuck by their keys sending out over the waves the "C. Q. D." and the "S. O. S." signals, which as every boy knows are the wireless calls for help.

The scientist and his boy friend never tired of talking of these things, for the young man was one of the many amateurs who had mastered the art, so that many a night as he sat at his receiver he caught the messages of steamships far out on the broad Atlantic, and heard the Navy Yard station transmitting orders to Uncle Sam's ships at sea.

One day shortly after the Titanic disaster the boy said to his friend: "I saw by the paper to-day that they are talking of passing a law to prevent the amateur wireless operators from working. I don't think they ought to do that. I'm sure most amateurs never interfere with any signals, as was said they did in connection with the messages to and from ships that went to the rescue of the Titanic."

"So long as the amateurs do not have powerful sending apparatus," answered the scientist, "I don't think they will make any serious trouble, for it makes no confusion to have them 'listening in' on the passing radiographs. Of course with a powerful sender a mischievous person could work irreparable damage by sending fake messages of one kind or another. In fact there have been several instances of messages that were thought to be fakes, but I am sure no boy with the intelligence to rig up a wireless outfit, would be so lacking in understanding of his responsibilities as to try to confuse traffic.

"But it would be a shame to stop the amateurs altogether," he continued, "for, no matter what the companies may say, the wireless telegraph is still in an experimental stage, and we must look to the bright boys who are studying it now, for its greatest development. The marvellous strides in improving the apparatus, and solving the mysteries of electro-magnetic currents, that have been made in the last dozen years, should be eclipsed in the next decade, if young men with some practical experience and a desire to get at the real scientific basis of the art, work at it."

"What are some of the main improvements of the last few years?" asked the boy.

For answer, the scientist and the boy made a journey down to the steamship docks, and visited the wireless cabins of several of the big transatlantic liners. They also went to the Brooklyn Navy Yard, where there is a wireless school, that turns out Navy operators after a thorough course in all the various branches of the art. While on vacations to the seashore, the youth had visited some of the big high-power stations that send and receive messages to and from the ships at sea.

In talking to the operators and electricians the boy learned much about the wide extent to which wireless is used nowadays. The law passed by Congress in the United States in 1911, making it necessary for every passenger steamer sailing from American ports with fifty or more passengers, to carry a wireless outfit capable of working at least 100 miles, in charge of a licensed operator, capable of transmitting 20 or more words a minute, did a great deal to increase the use of wireless. Also, not only the actions of one government but the concerted action of all the civilized nations represented at the various international wireless conferences have brought it to the official notice of the whole world.

Thus it has become a commercial reality on the sea, and the Great Lakes, and also it has become a big factor in war. All of the nations, besides having their warships equipped with wireless, now have wireless squads in the army, and have small compact apparatus that can be transported in small wagons, or even on horses' backs. These portable army wireless outfits are very valuable for the communication between detachments of an army, particularly in places where there are few disturbing elements to intercept the electro-magnetic waves.

In the recent campaign in Tripoli, in the war between Italy and Turkey, the wireless was extensively used by the Italian army in the field, and it was found that the messages radiated over the desert just about as well as over the sea. Of course as will be seen later, it is not meant here to convey the idea that wireless cannot be sent over the land, for the electro-magnetic waves travel through the ether in every direction, and as the ether fills the whole universe, mountains, buildings, or water just as well as the air, the waves are thought to go through obstacles as well as over water. The difficulty in sending over land, is that there are various electrical disturbances that intercept and confuse the wireless waves. In other words, wireless works through mere physical obstructions without any difficulty, just so long as certain little known electrical disturbances do not interfere. Just think of the thousands and thousands of wireless messages that are passing through the ether every hour of the day and night. And yet the scientists really know very little about the laws that govern them!

One of the instances of the strange antics of wireless was told to the boy by an operator who had been in charge of the wireless outfit on a Hudson River boat. He said that he and the operators on the other boats were able to communicate with a station on shore until they had passed the Poughkeepsie bridge, and the great steel and stone structure stretched between the boat and the station. Immediately communication stopped short and all efforts failed to get any response. A series of experiments proved that the obstruction was at the bridge, but whether it was some electrical property in the bridge itself, or in the hills on each side of the bridge, they have never been able to find out, and the land station was finally discontinued.

This is just an instance of what the scientists do not know about wireless, but it shows the many boy amateurs that there are still worlds for them to conquer in scientific research.

The central principle upon which the wireless telegraph works now is the same as it was when Marconi, through his marvellous invention, first received a signal from the other side of the Atlantic Ocean, but the inventors have learned much more about the details of the theory and it is in the improvement of devices for applying these laws of electricity that the development has been, rather than in the discovery of new theories. Nikola Tesla's invention for the wireless transmission of power by earth waves is a revolutionary departure from the usual wireless practice, but as we saw in the earlier chapter on this subject the Tesla invention has not yet been put in practical operation.

Though Guglielmo Marconi did not discover the laws of electricity upon which his invention is based, to him belongs all the credit for making use of the discoveries of the scientists of his day, and working out from them a practical system of wireless communication.

As many boys know, the wireless telegraph is possible through the radiation of electric waves. For instance, if a stone is thrown into a pool waves are started out in every direction from the point where the water is disturbed. The water does not move except up and down, and yet the waves pass on until they reach the side of the pool, or their force is expended.

The scientists before Marconi found out that when an electric spark was made to jump between two magnetic poles it started electric waves in every direction, much like the stone thrown into the pool, except at a speed that is reckoned at 186,000 miles per second.

Prof. Amos Dolbear, of Tufts College, Massachusetts, first made use of these waves in 1880, and a few years later Doctor Hertz, conducting experiments along the same lines, discovered them. Since that time these waves have been called Hertzian waves.

For many years scientists had understood that electrical waves or vibrations travelled through the ether in a copper wire, and that gave us telegraphy by wires, but it was a new thing to think of the waves travelling in every direction through space without wires. These early investigators found out that they could detect these waves by a device called a Hertzian loop, which was simply a copper wire bent into a hoop with the two ends close together but not touching. A spark would appear between the ends of the wire when the electric waves were sent out.

Marconi began his work where these scientists left off, as a very young man on his father's farm in Italy, but soon went to England, of which country his mother was a native, and placed the results of his experiments before the government authorities. Continuing his labors he soon had his wireless apparatus worked out in the form in which it first became known to the world.

It consisted of a transmitter, receiving machine or detector, and a set of antennæ or aerial wires from which the electrical waves were sent. For his transmitter, he created a spark between the two brass knobs on the ends of two thick brass wires by closing and opening an electrical circuit with a key, very much like, but somewhat larger than the regulation telegraph key. The space between the knobs was called the spark gap. For a dash he would hold down his key and make a large spark, and for a dot he would release his key quickly and make only a short one. Thus, he could send the regular Morse or Continental telegraphic codes of dots and dashes. These impulses were transmitted by wires to the aerial wires, or antennæ. The impulses left the antennæ as electro-magnetic waves, and went forth in all directions, only to be caught on the antennæ of another station aboard a ship or on land.

Here is where the receiver did its work, and the problem was a far more difficult one than the working out of the transmitter, for the waves as received were too weak in themselves to register a dot or a dash. In Marconi's first instruments he used a device called the "coherer." This was a glass tube about as big around as a lead pencil, and perhaps two inches long. It was plugged at each end with silver, and the narrow space between the plugs was filled with finely powdered fragments of nickel and silver, which possess the strange property of being alternately very good and very bad electrical conductors. The waves in Marconi's first experiments were received on a suspended kite wire, exactly similar to the wire used in the transmitter, but they were so weak that they could not of themselves operate an ordinary telegraph instrument. They possessed strength enough, however, to draw the little particles of silver and nickel in the coherer together in a continuous metal path. In other words, they made these particles "cohere," and the moment they cohered they became a good conductor for electricity, and a current from a battery near at hand rushed through the connection, operated the Morse instrument, and caused it to print a dot or a dash; then a little tapper, actuated by the same current, struck against the coherer, the particles of metal were broken apart, becoming a poor conductor, and cutting off the current from the home battery.

In Marconi's early experiments there was little or no attempt at tuning the instruments for waves of certain lengths, but this art has been developed to a high state in modern wireless telegraphy and we shall see how the operator tunes his instruments to talk to any one special station.

The distinguishing feature of the modern wireless transmitter, now familiar to every boy who has ever taken a trip aboard a large ship, or attended an electrical show, as it was in the old days, is the "crack, crack, cr-r-r-ack, crack" of the spark as it flickers between the brass knobs of the instrument, as the operator pounds away at his key. In some of the great high-power land stations, where long distance work is done the crash of the spark is like that of thunder, the flame is as big around as a man's wrist and of such intensity that it could not be looked at with unshaded eyes. On ships where the crash is too loud it has become necessary to cover the spark gap with a wooden muffler so as to deaden the noise.

While the simple spark gap of the early Marconi instruments was enough to send out the Hertzian waves, the modern transmitter is a marvel of electrical construction utilizing as it does the latest discoveries in electrical apparatus.

The most noticeable difference in the sending apparatus is in the arrangement of the two wires between which the spark flies. In the early instruments the wires were set in a horizontal line, and connected to an induction coil, but in the later ones the oscillator was turned up lengthwise with the spark gap between the vertical wings.

The different position of the spark gap is a change only in form, and not in principle. In the Marconi apparatus used nowadays the current comes from a dynamo of more than 110 volts, direct current. The two terminals of the circuit are connected with an induction coil, and from there to the two ends of the wires, making the terminals of the spark gap. The upper wire runs from the spark gap to the aerial, and the lower runs through a battery of Leyden jars, through a high tension transformer (as does the other side of the circuit), and thence to the ground. Aboard ship the ground connection is simply made by attaching a wire to the hull of the ship, which is in connection with the water, the best possible earth connection.

There are, of course, a great many different kinds of transmitters, but they are all worked out on the same general principle—a spark gap which creates electrical oscillations that are sent into the ether from the aerials.

In some modern stations an alternating current is used at more than 100 volts and is stepped up through a transformer to about 30,000 volts. This high power current then charges a condenser consisting of a battery of Leyden jars.

When the operator presses his key he establishes a connection, which immediately sets up electrical waves oscillating at a rate of anywhere from 100,000 to 2,500,000 per second. These oscillations are carried to the antennæ where they pass into the ether and spread in all directions to be caught on the aerials of all stations within range.

One of the improvements in wireless transmission which makes long distance work possible aboard ships is the use of what the engineers call "coupled circuits." The arrangement consists in connecting the aerial to an induction coil, and connecting the latter with a ground wire. Another coil is placed close to this and is connected with the spark gap, and a condenser. The period of oscillation of the antennæ circuit, and of the spark gap circuit are timed to be exactly the same. The two circuits are then called "coupled circuits," for while they are coupled together by induction only, the oscillation or spark gap circuit increases its capacity, and at the same time has a small spark gap.

With these new devices for increasing the power of the oscillations, or in other words throwing a bigger stone into the pond, the electrical waves are sent out with far greater force, just as the water waves are sent farther in the pond, and will reach stations at a greater distance.

"Crash, bang," goes the oscillator, and in less time than it takes to think it the oscillations have reached the antennæ of ships hundreds or thousands of miles away, or even those of another land station on the other side of the Atlantic Ocean.

The next thing is to understand the apparatus used for receiving the faint electric waves transmitted through the ether, for the modern instruments are far different from the old style "coherer" explained before. As with the spark gaps, there are many different styles of receiving devices, all known by the general name of "detectors," as they detect the faint electro-magnetic waves radiating through the ether.

Some of the latest Marconi experiments show a return to the "coherer" idea, very greatly improved upon, but the full details of the device have not been made public.

One of the detecting devices used by the Marconi system, after the old-style "coherer" was done away with, was very simple indeed in comparison to the cohering and tapping machines. It was made up of a small glass tube wound with copper wire. One end of this made the ground connection, and the other end led to the aerial, and also to an earth connection through a tuning inductance coil. Then another coil was wound around the first one on the glass tube and connected with the head telephone receivers which have taken the place of the Morse dot and dash printing instrument in all the modern wireless instruments. Two magnets were placed just above the glass tube, and a flexible iron wire was made to move through it by means of a pair of rollers a little way from each end. When the electro-magnetic waves reached the aerial and made oscillations in the first coil about the glass tube, the magnetic intensity of the iron wire band was disturbed and the glass tube became an oscillation transformer, setting up currents in the coil leading to the telephone receivers. The impulses were manifested by ticks, just the length of the dots and dashes being sent out by the operator perhaps thousands of miles away.

Another form of detector is the "electrolytic" which consists of a very fine platinum wire about one ten-thousandth of an inch in diameter, which dips into a platinum cup filled with nitric acid. When the invisible electro-magnetic waves impinge upon the wires of the receiving station, and cause electrical surges to take place in those wires, they in turn affect the detector, giving an exact reproduction of the note of the transmitting spark at the distant station.

This device has since been replaced by one of another type, equally sensitive and much better suited for general work on account of its greater stability and freedom from atmospheric disturbances. This detector consists simply of a crystal of carborundum supported between two brass points. When connected to the antennæ it is affected by the oscillations caused by distant transmitting stations as previously stated. These wireless signals are reproduced in telephone receivers.

Another frequently used detector known as the Audion is composed of a small incandescent lamp with filaments of carbon, tantalum, or preferably tungsten, and one or more sheets or wings of platinum secured near the filaments. The lamp is lighted by a set of home batteries, and is connected with a ground wire, the aerial, and the telephone receivers. The tungsten filament and the platinum wing act as two electrodes, and the faint electric oscillations received on the antennæ and transmitted to the platinum plate are supposed to affect the discharge of negatively electrified particles, or ions, between the two electrodes. This affects the flow of the battery current, and consequently registers the oscillations in the telephone receivers.

By diligent study of the subject the wireless experts also have learned that the arrangement of the aerials is of great importance, because much depends upon the send-off received by the electrical oscillations. In Marconi's early experiments he used a single wire attached to a kite, then changed to a single wire stretched from the top of a high mast. Later, the system of stretching the wires horizontally between two masts, as we see them so often aboard passenger steamships, and at land stations, came into general use. The old idea that the height of the aerial wires had something to do with the efficiency of the apparatus has passed, for science showed that the electro-magnetic waves travelled in all directions irrespective of land, water, mountains, or buildings. Whether, in sending messages across the ocean, they actually pass through the globe, or follow the curve of the surface, is more than the most careful wireless students have been able to tell.

Another of the big improvements in wireless is in the tuning of the instruments to certain wave lengths or rates of vibrations, and in controlling the wave lengths by the sender. Science has established that these waves usually vary from a few feet up to 12,000 feet or more. The ordinary wave lengths for ships is between 1,000 feet and 1,800 feet, but on the biggest land stations and the transatlantic liners the full 12,000 feet is used. Even greater lengths of waves are used by the big Marconi stations transmitting messages between Clifden, on the west coast of Ireland, and Glace Bay, Nova Scotia. The reason for this is that with the same power messages can be sent greater distances with long wave lengths than with shorter ones.

The wave length is controlled by an apparatus called the "helix," which may be seen in the picture of the wireless outfit. It looks like a drum wound with a spiral of copper tubing, and although it looks simple it presents some of the greatest problems in connection with wireless.

On the receiving end is the instrument called the tuner, by which the operator can adjust his detector to the wave lengths being sent out by the station with which he wishes to talk. There are various kinds of "tuners," all more or less complicated. The device corresponds to the telephone exchange or the telegraph switch-board. Of course a good receiving apparatus can be tuned so that the operator can listen to any messages going through the ether, within range, but all messages that are intended to be secret are sent in code, just as all wire and cable messages that are secret are sent in code.

In line with the advent of wireless telegraphy it is fitting that we should have the wireless telephone. While this instrument is still in the experimental stage, some very promising results have been obtained. There are several experimental wireless telephone stations in New York City, but the best results are obtained when some one keeps up a steady conversation, so it is far easier to connect the reproducer of a phonograph to the transmitter of the wireless telephone. It is surprising how distinctly this music or speech is received. In fact the ship operators nearing New York are often entertained by strains of music from these wireless telephones. The wireless telephones employ what are known as undamped oscillations created by electric arcs, and it is very easy to "tune out" such vibrations for musical effects.

Just as we have the motion-picture "newspaper," we have the wireless newspaper published aboard the big transatlantic liners every day. The news is sent out from certain land stations at certain times in the day and night, and every ship within range copies it, and publishes it just as our regular daily papers are published. Of course, the paper is small, but it usually contains most of the important news of the day, the big sporting items, such as baseball scores, and the stock quotations.

In the United States the great station at Wellfleet, Cape Cod, Mass., sends out the press matter each night from dispatches prepared in the main offices of the big American press associations. Ships as far as 1,600 miles distant frequently receive this news matter, and by the time the ocean-going editor is ready to get out his next day's edition he is in touch with the wireless press station on the other side, and is receiving the world's news from the English coast.

As our young friend found out when he was gathering up all the information he could about aeroplanes, some success has been made in the equipment of the fliers with wireless. The project offers some serious difficulties, however, as on an aeroplane there is no place for long aerials. Experiments have been tried with long trailing wires, but these are dangerous to the aeroplane, and to use the wires of the machine for antennæ endangers the operator to electric shocks. One scheme tried by several aviators in the United States with some success has been the stringing of aerials in the rear framework.

The problem of equipping balloons and airships with wireless is much simpler because it allows of long trailing wires to act as the antennæ. Most boys will remember the success of the wireless apparatus that was set up on the America at the time Walter Wellman made his famous attempt to cross the Atlantic in his airship.

That wireless will take its place as one of the great forces in civilization is the idea of Guglielmo Marconi, the inventor of the wireless telegraph, expressed when he was in New York in the spring of 1912.

"I believe," he said, "that in the near future a wireless message will be sent from New York completely round the globe with no relaying, and will be received by an instrument located in the same office as the transmitter, in perhaps even less time than Shakespeare's forty minutes.

"Most messages across the Atlantic will probably go by wireless at a comparatively early date. In time of war wireless connections will be invaluable. The enemy can cut cables and telegraph wires; but it is difficult seriously to damage the wireless service. The British Empire has realized this, and is already equipping many of its outposts with wireless stations."