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.
[Illustration: (C) Underwood & Underwood
British Anti-aircraft Section getting the Range of an Enemy Aviator]
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.
[Illustration: (C) Kadel & Herbert
A British Aviator making Observations over the German Lines]
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.