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.