But this enthusiast was not to be daunted, and he made a direct appeal to President Lincoln, offering to prove the practicability of this means of scouting. So he took his balloon to Washington and made an ascent from the grounds of the Smithsonian Institution, while the President came out on the lawn south of the White House to watch the demonstration. In order to test him, Mr. Lincoln took off his hat, waved his handkerchief, and made other signals. Lowe observed each act through his field-glasses and reported it to the President by telegraph. Mr. Lincoln was so impressed by the demonstration that he ordered the army to use the observation balloon, and so with some reluctance the gas-bag was introduced into military service, Professor Lowe being made chief aëronautic engineer. Under Lowe's direction the observation balloons played an important part in the operations of the Union Army.
On one occasion a young German military attaché begged the privilege of making an ascent in the balloon. Permission was given and when the German officer returned to earth he was wildly enthusiastic in praise of this aërial observation post. He had had a splendid view of the enemy and could watch operations through his field-glasses which were of utmost importance. Realizing the military value of the aircraft, he returned to Germany and urged military authorities to provide themselves with captive balloons. This young officer was Count Ferdinand von Zeppelin, who was destined later to become the most famous aëronautic authority in the world and who lived to see Germany equipped with a fleet of balloons which were self-propelling and could travel over land and sea to spread German frightfulness into England. He also lived to see the virtual failure of this type of war-machine in the recent great conflict, and it was possibly because of his deep disappointment at having his huge expensive airships bested by cheap little airplanes that Count von Zeppelin died in March, 1917. However, he was spared the humiliation of seeing a fleet of Zeppelins lose their way in a fog and fall into France, one of them being captured before it could be destroyed, so that all its secrets of construction were learned by the French.
THE WEIGHT OF HYDROGEN
Before we describe the Zeppelin airships and the means by which they were eventually overcome, we must know something about the principles of balloons. Every one knows that balloons are kept up in the air by means of a very light gas, but somehow the general public fails to understand why the gas should hold it up. Some people have a notion that there is something mysterious about hydrogen gas which makes it resist the pull of gravity, and that the more hydrogen you crowd into the balloon the more weight it will lift. But hydrogen has weight and feels the pull of gravity just as air does, or water, or lead. The only reason the balloon rises is because it weighs less than the air it displaces. It is hard to think of air as having weight, but if we weigh air, hydrogen, coal-gas, or any other gas, in a vacuum, it will tip the scales just as a solid would. A thousand cubic feet of air weighs 80 pounds. In other words, the air in a room ten feet square with a ceiling ten feet high, weighs just about 80 pounds. The same amount of coal-gas weighed in a vacuum would register only 40 pounds; while an equal volume of hydrogen would weigh only 5½ pounds. But when we speak of volumes of gas we must remember that gas, unlike a liquid or a solid, can be compressed or expanded to almost any dimensions. For instance, we could easily fill our room with a ton of air if the walls would stand the pressure; or we could pump out the air, until there were but a few ounces of air left. But in one case the air would be so highly compressed that it would exert a pressure of about 375 pounds on every square inch of the wall of the room, while in the other case its pressure would be almost infinitesimal. But 80 pounds of air in a room of a thousand cubic feet would exert the same pressure as the atmosphere, or 15 pounds on every square inch. And when we say that a thousand cubic feet of hydrogen weighs only a little over 5 pounds, we are talking about hydrogen at the same pressure as the atmosphere.
Since the hydrogen is sixteen times lighter than air, naturally it will float in the air, just as a piece of wood will float in water because it is lighter than the same volume of water. If we surrounded the thousand cubic feet of hydrogen with a bag so that the gas will not diffuse into the air and mix with it, we shall have a balloon which would float in air provided the bag and the hydrogen it contains do not weigh more than eighty pounds. As we rise from the surface of the earth, the air becomes less and less dense, or, in other words, it becomes lighter, and the balloon will keep on rising through the atmosphere until it reaches a point at which its weight, gas-bag and all, is exactly the same as that of an equal volume of air.
But there are many conditions that affect the height to which the balloon will ascend. The higher we rise, the colder it is apt to become, and cold has a tendency to compress the hydrogen, collapsing the balloon and making it relatively heavier. When the sun beats upon a balloon, it heats the hydrogen, expanding it and making it relatively lighter, and if there is no room for this expansion to take place in the bag, the bag will burst. For this reason, a big safety-valve must be provided and the ordinary round balloon is open at the bottom so that the hydrogen can escape when it expands too much and the balloonist carries ballast in the form of sand which he can throw over to lighten the balloon when the gas is contracted by a sudden draft of cold air.
Although a round balloon carries no engine and no propeller, it can be guided through the air to some degree. When an aëronaut wishes to go in any particular direction, he sends up his balloon by throwing out ballast or lowers it by letting out a certain amount of gas, until he reaches a level at which he finds a breeze blowing in the desired direction. Such was the airship of Civil War times, but for military purposes it was not advisable to use free balloons, because of the difficulty of controlling them. They were too liable to fall into the hands of the enemy. All that was needed was a high observation post from which the enemy could be watched, and from which observations could be reported by telegraph. The balloon was not looked upon as a fighting-machine.
ZEPPELIN'S FAILURES AND SUCCESSES
But Count Zeppelin was a man of vision. He dreamed of a real ship of the air—a machine that would sail wherever the helmsman chose, regardless of wind and weather. Many years elapsed before he actually began to work out his dreams, and then he met with failure after failure. He believed in big machines and the loss of one of his airships meant the waste of a large sum of money, but he persisted, even though he spent all his fortune, and had to go heavily in debt. Every one thought him a crank until he built his third airship and proved its worth by making a trip of 270 miles. At once the German Government was interested and saw wonderful military possibilities in the new craft. The Zeppelin was purchased by the government and money was given the inventor to further his experiments.
That was not the end of his failures. Before the war broke out, thirteen Zeppelins had been destroyed by one accident or another. Evidently the building of Zeppelin airships was not a paying undertaking, although they were used to carry passengers on short aërial voyages. But the government made up money losses and Zeppelin went on developing his airships.
Of course, he was not the only one to build airships, nor even the first to build a dirigible. The French built some large dirigibles, but they failed to see any great military advantage in ships that could sail through the air, particularly after the airplane was invented, and so it happened that when the war started the French were devoting virtually all their energies to the construction of speedy, powerful airplanes. As for the British, they did not pay much attention to airships. The idea that their isles might be attacked from the sky seemed an exceedingly remote possibility.
RIGID, SEMI-RIGID, AND FLEXIBLE BALLOONS
Count Zeppelin always held that the dirigible balloons must be rigid, so that they could be driven through the air readily and would hold their shape despite variations in the pressure of the hydrogen. The French, on the other hand, used a semi-rigid airship; that is, one in which a flexible balloon is attached to a rigid keel or body. The British clung to the idea of an entirely flexible balloon and they suspended their car from the gas-bag without any rigid framework to hold the gas-bag in shape. In every case, the balloons were kept taut or distended by means of air-bags or ballonets. These air-bags were placed inside the gas-bags and as the hydrogen expanded it would force the air out through valves, but the hydrogen itself would not escape. When the hydrogen contracted, the air-bags were pumped full of air so as to maintain the balloon in its fully distended condition. Additional supplies of compressed hydrogen were kept in metal tanks.
In the Zeppelin balloon, however, the gas was contained in separate bags which were placed in a framework of aluminum covered over with fabric. Count Zeppelin did not believe in placing all his eggs in one basket. If one of these balloons burst or was injured in any way, there was enough buoyancy in the rest of the gas-bags to hold up the airship. As the Zeppelins were enormous structures, the framework had to be made strong and light, and it was built up of a latticework of aluminum alloy. Aluminum itself was not strong enough for the purpose, but a mixture of aluminum and zinc and later another alloy known as duralumin, consisting of aluminum with three per cent of copper and one per cent of nickel, provided a very rigid framework that was exceedingly light. Duralumin is four or five times as strong as aluminum and yet weighs but little more.
The body of the Zeppelin is not a perfect circle in section, but is made up in the form of a polygon with sixteen sides, and the largest of the Zeppelins used during the war contained sixteen compartments, in each of which was placed a large hydrogen gas-bag. A super-Zeppelin, as the latest type is called, was about seventy-five feet in diameter and seven hundred and sixty feet long, or almost as long as three New York street blocks. In its gas-bags it carried two million cubic feet of hydrogen and although the whole machine with its fuel, stores, and passengers weighed close to fifty tons, it was so much lighter than the air it displaced that it had a reserve buoyancy of over ten tons.
KEEPING ENGINES CLEAR OF THE INFLAMMABLE HYDROGEN
As hydrogen is a very inflammable gas, it is extremely dangerous to have an internal-combustion engine operating very near the gas-bags. In the super-Zeppelins the engines were placed in four cars suspended from the balloon. There was one of these cars forward, and one at the stern, while near the center were two cars side by side. In the rear car there were two engines, either of which could be used to drive the propeller. By means of large steering rudders and horizontal rudders, the machine could be forced to dive or rise or turn in either direction laterally. The pilot of the Zeppelin had an elaborate operating-compartment from which he could control the rudders, and he also had control of the valves in the ballonets so that by the touch of a button he could regulate the pressure of gas in any part of the dirigible. There were nineteen men in the crew of the Zeppelin—two in the operating-compartment, and two in each of the cars containing engines, except for the one at the stern in which there were three men. The other men were placed in what was known as the "cat walk" or passageway running inside the framework under the gas-bags. These men were given various tasks and were supposed to get as much sleep as they could, so as to be ready to replace the other men at need.
The engine cars at each side of the balloon were known as power eggs because of their general egg shape. At the center of the Zeppelin the bombs were stored, and there were electro-magnetic releasing-devices operated from the pilot's room by which the pilot could drop the bombs whenever he chose. The Zeppelin also carried machine-guns to fight off airplanes. Gasolene was stored in tanks which were placed in various parts of the machine, any one of which could feed one or all of the engines, and they were so arranged that they could be thrown overboard when the gasolene was used up, so as to lighten the load of the Zeppelin. Water ballast was used instead of sand, and alcohol was mixed with the water to keep it from freezing. The machine which came down in French territory and was captured before it could be destroyed by the pilot, found itself unable to rise because in the intense cold of the upper air the water ballast had frozen, and it could not be let out to lighten the load of the Zeppelin.
THE ZEPPELIN'S TINY ANTAGONISTS
The one thing above all others that the Zeppelin commander feared was the attack of airplanes. In the early stages of the war, it was considered unsafe for airplanes to fly by night because of the difficulty of making a landing in the dark. Later this difficulty was overcome by the use of search-lights at the landing-fields. The airplane would signal its desire to land and the search-lights would point out the proper landing-field for it. So that after the first few months of the war Zeppelins were subjected to the danger of airplane attack. Of course, on a dark night it was very difficult for an airplane to locate a Zeppelin, because the huge machine could not be seen and the throb of its engines was drowned out by the engines of the airplane itself. Nevertheless, Zeppelins were occasionally located and destroyed by airplanes.
The danger of the Zeppelin lay in the fact that it was supported by an enormous volume of very inflammable gas and the airplane needed but to set fire to this gas to cause the destruction of the giant of the air. And so the machine-guns carried by airplanes were provided with explosive, flaming bullets. A burst of flame within the gas-bag would not set the gas on fire, because there would be no air inside to feed the fire, but surrounding the gas-bag there was always a certain leakage of hydrogen which would mix with the air in the compartment and this would produce an explosive mixture which needed but the touch of fire to set it off. The Zeppelin was provided with a ventilating-system to carry off these explosive gases, but they could never be disposed of very effectively, and, as a consequence, a number of Zeppelins were destroyed by the tiny antagonists that were sent up by the British and the French. To fight off these assailants the Germans provided their Zeppelins with guns which would fire shrapnel shell. It is difficult for a Zeppelin to use machine-guns against an airplane because the latter would merely climb above the Zeppelin and would be shielded by the balloon itself. And so the Germans put a gun emplacement on top of the balloon both forward and aft. There was a deck extending along the top of the balloon which was reached by a ladder running up through the center of the airship. But it was impossible to ward off the fleet little antagonists, once the dirigible was discovered. True, a Zeppelin could make as much as seventy miles per hour, but the fastest airplanes could travel twice as fast as that.
SUSPENDING AN OBSERVER BELOW THE ZEPPELIN
One ingenious scheme that was tried was to suspend an observation car under the Zeppelin. The car was about fourteen feet long and five feet in diameter, fitted with a tail to keep it headed in the direction it was towed. It had glass windows forward and there was plenty of room in it for a man to lie at full length and make observations of things below. The car with its observer could be lowered a few thousand feet below the Zeppelin, so that the observer could watch proceedings below, while the airship remained hidden among the clouds. The observer was connected by telephone with the chart-room of the Zeppelin and could report his discoveries or even act as a pilot to direct the course of the ship.
But despite everything that could be done, the Zeppelin eventually proved a failure as a war-vessel because it was so very costly to construct and operate and could so easily be destroyed, and the Germans began to build huge airplanes with which bombing-raids could be continued.
Strange to say, however, although the Germans were ready to admit the failure of their big airship, when the war stopped the Allies were actually building machines patterned after the Zeppelin, but even larger, and expected to use them for bombing-excursions over Germany. This astonishing turn of the tables was due to the fact that America had made a contribution to aëronautics that solved the one chief drawback of the Zeppelin.
A BALLOON GAS THAT WILL NOT BURN
When we entered the war against Germany, our allies placed before us all their problems and among them was this one of the highly inflammable airship. Could we not furnish a substitute for hydrogen that would not burn? It was suggested to us that helium would do if we could produce that gas cheaply and in sufficient quantity. Now, helium has a history of its own that is exceedingly interesting.
Every now and then the moon bobs its head into our light and we have a solar eclipse. But our satellite is not big enough to cut off all the light of the big luminary and the fiery atmosphere of the sun shows us a brilliant halo all around the black disk of the moon. Long ago, astronomers analyzed this flaming atmosphere with the spectroscope, and by the different bands of light that appeared they were able to determine what gases were present in the sun's atmosphere. But there was one band of bright yellow which they could not identify. Evidently this was produced by a gas unknown on earth, and they called it "helium" or "sun" gas.
For a quarter of a century this sun gas remained a mystery; then one day, in 1895, Sir William Ramsay discovered the same band of light when studying the spectrum of the mineral cleveite. The fact that astronomers had been able to single out an element on the sun ninety million miles away before our chemists could find it right here on earth, produced a mild sensation, but the general public attached no special importance to the gas itself. It proved to be a very light substance, next to hydrogen the lightest of gases, and for years it resisted all attempts at liquefaction. Only when Onnes, the Dutch scientist, succeeded in getting it down to a temperature of 450 degrees below zero, Fahrenheit, did the gas yield to the chill and condense into a liquid. The gas would not burn; it would not combine with any other elements, and apparently it had no use on earth, and it might have remained indefinitely a lazy member of the chemical fraternity had not the great world conflict stirred us into frenzied activity in all branches of science in our effort to beat the Hun.
Because the gas had no commercial value, there was only a small amount of helium to be found in the whole world. Not a single laboratory in the United States had more than five cubic feet of it and its price ranged from $1,500 to $6,000 per cubic foot. At the lowest price it would cost $3,000,000,000 to provide gas enough for one airship of Zeppelin dimensions and it seemed absurd even to think of a helium airship.
AMERICAN CHEMISTS TO THE RESCUE
Just before the war it was discovered that there is a considerable amount of helium in the natural gas of Oklahoma, Texas, and Kansas, and Sir William Ramsey suggested that our chemists might study some method of getting helium from this source. The only way of separating it out was to liquefy the gases by subjecting them to extreme cold. All gases turn to liquid if they are cooled sufficiently, and then further cold will freeze them solid. But helium can stand more cold than any other and this fact gave the clue to its recovery from natural gas. The latter was frozen and one after another the different elements condensed into liquid, until finally only helium was left. This sounds simple, but it is a difficult matter to get such low temperature as that on a large scale and do it economically. To be of any real service in aëronautics helium would have to be reduced in cost from fifteen hundred dollars to less than ten cents per cubic foot. Several different kinds of refrigerating-machinery were tried and finally just before the war was brought to a close by the armistice we had succeeded in producing helium at the rate of eight cents per cubic foot, with the prospect of reducing its cost still further. A large plant for recovering helium was being built. The plant will have been completed before this book is published, and it will be turning out helium for peaceful instead of military airships.
The reduction in the cost of helium is really one of the most important developments of this war. By removing the fire risk from airships we can safely use these craft for aërial cruises or for quick long-distance travel over land and sea. For, even in time of peace, sailing under millions of cubic feet of hydrogen is a serious matter. Although no incendiary bullets are to be feared, there is always the danger of setting fire to the gas within the exhaust of the engines. Engines have had to be hung in cars well below the balloon proper. But with helium in the gas-bags the engines can be placed inside the balloon envelop and the propellers can operate on the center line of the car.
In the case of one Zeppelin, the hydrogen was set on fire by an electric spark produced by friction on the fabric of one of the gas-bags, and so even with the engine exhausts properly screened there is danger. The helium airship, however, would be perfectly safe from fire and passengers could smoke on deck or in their cabins within the balloon itself without any more fear of fire than they would have on shipboard. Wonderful possibilities have been opened by the production of helium on a large and economical scale, and the airship seems destined to play an important part in transportation very soon. As this book is going to press, we learn of enormous dirigibles about to be built in England for passenger service, which will have half again as great a lifting-power as the largest Zeppelins. The final chapter of the story of dirigibles is yet to be written, but in concluding this chapter it is interesting to note that the world's greatest aëronautic expert got his first inspiration from America and finally that America has now furnished the one element which was lacking to make the dirigible balloon a real success.
CHAPTER IX
Getting the Range
Every person with a good pair of eyes in his head is a range-finder. He may not know it, but he is, just the same, and the way to prove it is to try a little range-finding on a small scale.
Use the top of a table for your field of operations, and pick out some spot within easy reach of your hand for the target whose range you wish to find. The target may be a penny or a small circle drawn on a piece of white paper. Take a pencil in your hand and imagine it is a shell which you are going to land on the target. It is not quite fair to have a bird's-eye view of the field, so get down on your knees and bring your eyes within a few inches of the top of the table. Now close one eye and making your hand describe an arc through the air, like the arc that a shell would describe, see how nearly you can bring the pencil-point down on the center of the target. Do it slowly, so that your eye may guide the hand throughout its course. You will be surprised to find out how far you come short, or overreach the mark. You will have actually to grope for the target. If by any chance you should score a hit on the first try, you may be sure that it is an accident.
Have a friend move the target around to a different position, and try again. Evidently, with one eye you are not a good range-finder; but now use two eyes and you will score a hit every time. Not only can you land the pencil on the penny, but you will be able to bring it down on the very center of the target.
The explanation of this is that when you bring your eyes to bear upon any object that is near by, they have to be turned in slightly, so that both of them shall be aimed directly at that object. The nearer the object, the more they are turned in, and the farther the object, the more nearly parallel are the eyes. Long experience has taught you to gage the distance of an object by the feel of the eyes—that is, by the effort your muscles have to make to pull the eyes to a focus—and in this way the eyes give you the range of an object. You do not know what the distance is in feet or inches, but you can tell when the pencil-point has moved out until it is at the same focus as the target.
The experiment can be tried on a larger scale with the end of a fishing-rod, but here you will probably have to use a larger target. However, there is a limit to which you can gage the range. At a distance of, say, fifteen or twenty feet, a variation of a few inches beyond or this side of the target makes scarcely any change in the focus of the eyes. That is because the eyes are so close together. If they were farther apart, they could tell the range at much greater distances.
SPREADING THE EYES FAR APART
Now the ordinary range-finder, used in the army and in the navy, is an arrangement for spreading the eyes apart to a considerable distance. Of course the eyes are not actually spread, but their vision is. The range-finder is really a double telescope. The barrel is not pointed at an object, but it is held at right angles to it. You look into the instrument at the middle of the barrel and out of it at the two ends. A system of mirrors or prisms makes this possible. The range-finder may be a yard or more in length, which is equivalent to spreading your eyes a yard or more apart. Now, the prisms or object-glasses at the ends of the tube are adjustable, so that they will turn in until they focus directly on the target whose range you wish to find, and the angle through which these glasses are turned gives a measure of the distance of the target. The whole thing is calculated out so that the distance in feet, yards, or meters, or whatever the measure may be, is registered on a scale in the range-finder. Ordinarily only one eye is used to look through the range-finder, because the system of mirrors is set to divide the sight of that one eye and make it serve the purposes of two. That leaves the other eye free to read the scale, which comes automatically into view as the range-finder is adjusted for the different ranges.
On the battle-ships enormous range-finders are used. Some of them are twenty feet long. With the eyes spread as far apart as that and with a microscope to read the scale, you can imagine how accurately the range can be found, even when the target is miles away. But on land such big range-finders cannot conveniently be used; they are too bulky. When it is necessary to get the range of a very distant object, two observers are used who are stationed several hundred yards apart. These observers have telescopes which they bear upon the object, and the angle through which they have to turn the telescope is reported by telephone to the battery, where, by a rapid calculation, it is possible to estimate the exact position of the target. Then the gun is moved up or down, to the right or to the left, according to the calculation. The observers have to creep as near to the enemy as possible and they must be up high enough to command a good view of the target. Sometimes they are placed on top of telegraph poles or hidden up a tall tree, or in a church steeple.
GETTING THE OBSERVER OFF THE GROUND
This was the method of getting the range in previous wars and it was used to a considerable extent in the war we have just been through. But the great European conflict brought out wonderful improvements in all branches of fighting; and range-finding was absolutely revolutionized, because shelling was done at greater ranges than ever before, but chiefly because the war was carried up into the sky.
A bird's-eye observation is much more accurate than any that can be obtained from the ground. Even before this war, some observations were taken by sending a man up in a kite, particularly a kite towed from a ship, and even as far back as the Civil War captive balloons were used to raise an observer to a good height above the ground. They were the ordinary round balloons, but the observation balloon of to-day is a very different-looking object. It is a sausage-shaped gas-bag that is held on a slant to the wind like a kite, so that the wind helps to hold it up. To keep it head-on to the wind, there is a big air-bag that curls around the lower end of the sausage. This acts like a rudder, and steadies the balloon. Some balloons have a tail consisting of a series of cone-shaped cups strung on a cable. A kite balloon will ride steadily in a wind that would dash a common round balloon in all directions. Observers in these kite balloons are provided with telephone instruments by which they can communicate instantly with the battery whose fire they are directing. But a kite balloon is a helpless object; it cannot fight the enemy. The hydrogen gas that holds it up will burn furiously if set on fire. In the war an enemy airplane had merely to drop a bomb upon it or fire an incendiary bullet into it, and the balloon would go up in smoke. Nothing could save it, once it took fire, and all the observers could do was to jump for their lives as soon as they saw the enemy close by. They always had parachutes strapped to them, so they could leap without an instant's delay in case of sudden danger. At the very first approach of an enemy airplane, the kite balloon had to be hauled down or it would surely be destroyed, and so kite balloons were not very dependable observation stations for the side which did not control the air.
As stated in the preceding chapter, just before the fighting came to an end, our army was preparing to use balloons that were not afraid of flaming bullets, because they were to be filled with a gas that would not burn.
MAKING MAPS WITH A CAMERA
Because airplanes filled the sky with eyes, everything that the army did near the front had to be carefully hidden from the winged scouts. Batteries were concealed in the woods, or under canopies where the woods were shot to pieces, or they were placed in dugouts so that they could not be located. Such targets could seldom be found with a kite balloon. It was the task of airplane observers to search out these hidden batteries. The eye alone was not depended upon to find them. Large cameras were used with telescopic lenses which would bring the surface of the earth near while the airplane flew at a safe height. These were often motion-picture cameras which would automatically make an exposure every second, or every few seconds.
When the machine returned from a photographing-expedition, the films were developed and printed, and then pieced together to form a photographic map. The map was scrutinized very carefully for any evidence of a hidden battery or for any suspicious enemy object. As the enemy was always careful to disguise its work, the camera had to be fitted with color-screens which would enable it to pick out details that would not be evident to the eye. As new photographic maps were made from day to day, they were carefully compared one with the other so that it might be seen if there was the slightest change in them which would indicate some enemy activity. As soon as a suspicious spot was discovered, its position was noted on a large-scale military map and the guns were trained upon it.
CORRECTING THE AIM
It is one thing to know where the target is and another to get the shell to drop upon it. In the firing of a shell a distance of ten or twenty miles, the slightest variation in the gun will make a difference of many yards in the point where the shell lands. Not only that, but the direction of the wind and the density of the air have a part to play in the journey of the shell. If the shell traveled through a vacuum, it would be a much simpler matter to score a hit by the map alone. But even then there would be some differences, because a gun has to be "warmed up" before it will fire according to calculation. That is why it is necessary to have observers, or "spotters" as they are called, to see where the shell actually do land and tell the gun-pointers whether to elevate or depress the gun, and how much to "traverse" it—that is, move it sideways. This would not be a very difficult matter if there were only one gun firing, but when a large number of guns are being used, as was almost invariably the case in the war, the spotter had to know which shell belonged to the gun he was directing.
One of the most important inventions of the war was the wireless telephone, which airplanes used and which were brought to such perfection that the pilot of an airplane could talk to a station on the earth without any difficulty, from a distance of ten miles; and in some cases he could reach a range of fifty miles. With the wireless telephone, the observer could communicate instantly with the gun-pointer, and tell him when to fire. Usually thirty seconds were allowed after the signal sent by the observer before the gun was fired, and on the instant of firing, a signal was sent to the man in the airplane to be on the lookout for the shell. Knowing the position of the target, the gun-pointer would know how long it would take the shell to travel through the air, and he would keep the man in the airplane posted, warning him at ten seconds, five seconds, and so forth, before the shell was due to land.
In order to keep the eyes fresh for observation and not to have them distracted by other sights, the observer usually gazed into space until just before the instant the shell was to land. Then he would look for the column of smoke produced by the explosion of the shell and report back to the battery how far wide of the mark the shell had landed. A number of shell would be fired at regular intervals, say four or five per minute, so that the observer would know which shell belonged to the gun in question.
There are different kinds of shell. Some will explode on the instant of contact with the earth. These are meant to spread destruction over the surface. There are other shell which will explode a little more slowly and these penetrate the ground to some extent before going off; while a third type has a delayed action and is intended to be buried deep in the ground before exploding, so as to destroy dugouts and underground positions. The bursts of smoke from the delayed-action shell and the semi-delayed-action shell rise in a slender vertical column and are not so easily seen from the sky. The instantaneous shell, however, produces a broad burst of smoke which can be spotted much more readily, and this enables the man in the airplane to determine the position of the shell with greater accuracy. For this reason, instantaneous shell were usually used for spotting-purposes, and after the gun had found its target, other shell were used suited to the character of the work that was to be done.
MINIATURE BATTLE-FIELDS
Observation of shell-fire from an airplane called for a great deal of experience, and our spotters were given training on a miniature scale before they undertook to do spotting from the air. A scaffolding was erected in the training-quarters over a large picture of a typical bit of enemy territory. Men were posted at the top of this scaffolding so that they could get a bird's-eye view of the territory represented on the map, and they were connected by telephone or telegraph with men below who represented the batteries. The instructor would flash a little electric light here and there on the miniature battle-field, and the observers had to locate these flashes and tell instantly how far they were from certain targets. This taught them to be keen and quick and to judge distance accurately. Airplane observing was difficult and dangerous, and often impossible. On cloudy days the observer might be unable to fly at a safe height without being lost in the clouds. Then dependence had to be placed upon observers stationed at vantage-points near the enemy, or in kite balloons.
SPOTTING BY SOUND
When there is no way of seeing the work of a gun, it is still possible to correct the aim, because the shell can be made to do its own spotting. Every time a shell lands, it immediately announces the fact with a loud report. That report is really a message which the shell sends out in all directions with a speed of nearly 800 miles per hour—1,142 feet per second, to be exact. This sound-message is picked up by a recorder at several different receiving-stations. Of course it reaches the nearest station a fraction of a second before it arrives at the next nearest one. The distance of each station from the target is known by careful measurement on the map, and the time it takes for sound to travel from the target to each station is accurately worked out. If the sound arrives at each station on schedule time, the shell has scored a hit; but if it reaches one station a trifle ahead of time and lags behind at another, that is evidence that the shell has missed the target and a careful measure of the distance in time shows how far and in what direction it is wide of the mark. In this way it was possible to come within fifty or even twenty-five yards of the target.
This sound-method was also used to locate an enemy battery. It was often well nigh impossible to locate a battery in any other way. With the use of smokeless powder, there is nothing to betray the position of the gun, except the flash at the instant of discharge, and even the flash was hidden by screens from the view of an airplane. Aside from this, when an airplane came near enough actually to see one of these guns, the gun would stop firing until the airplane had been driven off. But a big gun has a big voice, and it is impossible to silence it. Often a gun whose position has remained a secret for a long time was discovered because the gun itself "peached."
The main trouble with sound-spotting was that there were usually so many shell and guns going off at the same time that it was difficult if not impossible to distinguish one from another. Sometimes the voice of a hidden gun was purposely drowned by the noise of a lot of other guns. After all, the main responsibility for good shooting had to fall on observers who could actually see the target, and when we think of the splendid work of our soldiers in the war, we must not forget to give full credit to the tireless men whose duty it was to watch, to the men on wings who dared the fierce battle-planes of the enemy, to the men afloat high in the sky who must leap at a moment's notice from under a blazing mass of hydrogen, and finally to the men who crept out to perilous vantage-points at risk of instant death, in order to make the fire of their batteries tell.
CHAPTER X
Talking in the Sky
In one field of war invention the United States held almost a monopoly and the progress Americans achieved was epoch-making.
Before the war, an aviator when on the wing was both deaf and dumb. He could communicate with other airplanes or with the ground only by signal or, for short distances, by radiotelegraphy, but he could not even carry on conversation with a fellow passenger in the machine without a speaking-tube fitted to mouth and ears so as to cut out the terrific roar of his own engine. Now the range of his voice has been so extended that he can chat with fellow aviators miles away. This remarkable achievement and many others in the field of radio-communication hinge upon a delicate electrical device invented by Deforest in 1906 and known as the "audion." For years this instrument was used by radiotelegraphers without a real appreciation of its marvelous possibilities, and, as a matter of fact, in its earlier crude form it was not capable of performing the wonders it has achieved since it was taken over and developed by the engineers of the Bell Telephone System.
THE AUDION
Although the audion is familiar to all amateur radio-operators, we shall have to give a brief outline of its construction and operation for the benefit of those who have not had the opportunity to dabble in wireless telegraphy.
The audion is a small glass bulb from which the air is exhausted to a high degree of vacuum. The bulb contains three elements. One is a tiny filament which is heated to incandescence by a battery, so that it emits negatively charged electrons. The filament is at one side of the bulb and at the opposite side there is a metal plate. When the plate and the filament are connected with opposite poles of a battery, there is a flow of current between them, but because only negative electrons are emitted by the filament, the current will flow only in one direction—that is, from the plate to the filament. If the audion be placed in the circuit of an alternating-current generator, it will let through only the current running in one direction. Thus it will "rectify" the current or convert alternating current into direct current.
But the most important part of the audion, the part for which Deforest is responsible, is the third element, which is a grid or flat coil of platinum wire placed between the filament and the plate. This grid furnishes a very delicate control of the strength of the electric current between plate and filament. The slightest change in electric power in the grid will produce large changes of power in the current flowing through the audion. This makes it possible to magnify or amplify very feeble electric waves, and the extent to which the amplifying can be carried is virtually limitless, because a series of audions can be used, the current passing through the first being connected with the grid of the next, and so on.
TALKING FROM NEW YORK TO SAN FRANCISCO
There is a limit to which telephone conversations can be carried on over a wire, unless there is some way of adding fresh energy along the line. For years all sorts of experiments were tried with mechanical devices which would receive a telephone message and send it on with a fresh relay of current. But these devices distorted the message so that it was unintelligible. The range of wire telephony was greatly increased by the use of certain coils invented by Pupin, which were placed in the line at intervals; but still there was a limit to which conversation could be carried on by wire and it looked as if it would never be possible to telephone from one end of this big country of ours to the other. But the audion supplied a wonderfully efficient relay and one day we awoke to hear San Francisco calling, "Hello," to New York.
Used as a relay, the improved audion made it possible to pick up very faint wireless-telegraph messages and in that way increased the range of radio outfits. Messages could be received from great distances without any extensive or elaborate aërials, and the audion could be used at the sending-station to magnify the signals transmitted and send them forth with far greater power.
Having improved the audion and used it successfully for long-distance telephone conversation over wires, the telephone company began to experiment with wireless telephony. They believed that it might be possible to use radiotelephony in places where wires could not be laid. For instance, it might be possible to talk across the Atlantic.
But before we go farther, just a word of explanation concerning radiotelegraphy and radiotelephony for the benefit of those who have not even an elementary knowledge of the subject.
SIMPLE EXPLANATION OF RADIOTELEGRAPHY
Suppose we should set up two stakes in a pond of water, at some distance from each other, and around each we set a ring-shaped cork float. If we should move one of these floats up and down on its stake, it would produce ripples in the water which would spread out in all directions and finally would reach the opposite stake and cause the float there to bob up and down in exactly the same way as did the float moved by hand. In wireless telegraphy the two stakes are represented by antennæ or aërials and the cork floats are electric charges which are sent oscillating up and down the antennæ. The oscillations produced at one aërial will set up electro-magnetic waves which will spread out in all directions in the ether until they reach a receiving-aërial, and there they will produce electric oscillations similar to the ones at the transmitting-antenna.
Telegraph signals are sent by the breaking up of the oscillations at the transmitting-station into long and short trains of oscillations corresponding to the dots and dashes of ordinary wire telegraphy. In other words, while the sending-key is held down for a dash, there will be a long series of oscillations in the antenna, and for the dot a short series, and these short and long trains of waves will spread out to the receiving-aërial where they will reproduce the same series of oscillations. But only a small part of the energy will act on the receiving-aërial because the waves like those on the pond spread in all directions and grow rapidly weaker. Hence the advantage of an extremely delicate instrument like the audion to amplify the signals received.
The oscillations used in wireless telegraphy these days are very rapid, usually entirely too rapid, to affect an ordinary telephone receiver, and if they did they would produce a note of such high pitch that it could not be heard. So it is customary to interrupt the oscillations, breaking them up into short trains of waves, and these successive trains produce a note of low enough pitch to be heard in the telephone receiver. Of course the interruptions are of such high frequency that in the sending of a dot-and-dash message each dot is made up of a great many of the short trains of waves.
Now in radiotelephony it is not necessary to break up the oscillations, but they are allowed to run continuously at very high speed and act as carriers for other waves produced by speaking into the transmitter; that is, a single speech-wave would be made up of a large number of smaller waves. To make wireless telephony a success it was necessary to find some way of making perfectly uniform carrier-waves, and then of loading on them waves of speech. Of course, the latter are not sound-waves, because they are not waves of air, but they are electro-magnetic waves corresponding exactly to the sound-waves of air and at the receiving-end they affect the telephone receiver in the same way that it is affected by the electric waves which are sent over telephone wires. The telephone engineers found that the audion could be used to regulate the carrier-waves and also to superpose the speech-waves upon them, and at the receiving-station the audion was used to pick up these waves, no matter how feeble they might be, and amplify them so that they could be heard in a telephone receiver.
TALKING WITHOUT WIRES
Attempts at long-distance talking without wires were made from Montauk Point, on the tip of Long Island, to Wilmington, Delaware, and they were successful. This was in 1915. The apparatus was still further improved and then the experiment was tried of talking from the big Arlington station near Washington to Darien, on the Isthmus of Panama. This was a distance of twenty-one hundred miles, and speech was actually transmitted through space over that great distance. That having proved successful, the next attempt was to talk from Arlington to Mare Island and San Diego, on the Pacific Coast, a distance of over twenty-five hundred miles. This proved a success, too, and it was found possible even to talk as far as Honolulu.