German airmen were given all the fighting they required and a bit
over.

Certainly a very different picture is presented by the dismal
letters which Fritz sent home during the great Ypres offensive of
August, 1917.  In these letters he bewails the fact that one
after another of his batteries is put out of action owing to the
perfect "spotting" of the British airmen, and arrives at the sad
conclusion that Germany has lost her superiority in the air.

An account has already been given of the skill and prowess of
Captain Ball.  On his own count--and he was not the type of man
to exaggerate his prowess--he found he had destroyed fifty
machines, although actually he got the credit for forty-one. 
This slight discrepancy may be explained by the scrupulous
care which is taken to check the official returns.  The air
fighter, though morally certain of the destruction of a certain
enemy aeroplane, has to bring independent witnesses to
substantiate his claim, and when out "on his own" this is no easy
matter.  Without this check, though occasionally it acts harshly
towards the pilot, there might be a tendency to exaggerate enemy
losses, owing to the difficulty of distinguishing between an
aeroplane put out of action and one the pilot of which takes a
sensational "nose dive" to get out of danger.

One of the most striking illustrations of the growth of the
aeroplane as a fighting force is afforded by the great increase
in the heights at which they could scout, take photographs, and
fight.  In Sir John French's dispatches mention is made of
bomb-dropping from 3000 feet.  In these days the aerial
battleground has been extended to anything up to 20,000 feet. 
Indeed, so brisk has been the duel between gun and aeroplane,
that nowadays airmen have often to seek the other margin of
safety, and can defy the anti-aircraft guns only by flying so low
as just to escape the ground.  The general armament of a
"fighter" consists of a maxim firing through the propeller, and a
Lewis gun at the rear on a revolving gun-ring.

It is pleasant to record that the Allies kept well ahead of the
enemy in their use of aerial photography.  Before a great
offensive some thousands of photographs had to be taken of
enemy dispositions by means of cameras built into the aeroplanes.

Plates were found to stand the rough usage better than films, and
not for the first time in the history of mechanics the man beat
the machine, a skilful operator being found superior to the
ingenious automatic plate-fillers which had been devised.

The counter-measure to this ruthless exposure of plans was
camouflage.  As if by magic-tents, huts, dumps, guns began, as it
were, to sink into the scenery.  The magicians were men skilled
in the use of brush and paint-pot, and several leading figures in
the world of art lent their services to the military authorities
as directors of this campaign of concealment.  In this connection
it is interesting to note that both Admiralty and War Office took
measures to record the pictorial side of the Great War.  Special
commissions were given to a notable band of artists working in
their different "lines".  An abiding record of the great struggle
will be afforded by the black-and-white work of Muirhead Bone,
James M'Bey, and Charles Pears; the portraits, landscapes, and
seascapes of Sir John Lavery, Philip Connard, Norman Wilkinson,
and Augustus John, who received his commission from the Canadian
Government.

CHAPTER XL
The Atmosphere and the Barometer

For the discovery of how to find the atmospheric pressure we are
indebted to an Italian named Torricelli, a pupil of Galileo, who
carried out numerous experiments on the atmosphere toward the
close of the sixteenth century.

Torricelli argued that, as air is a fluid, if it had weight it
could be made to balance another fluid of known weight.  In his
experiments he found that if a glass tube about 3 feet in length,
open at one end only, and filled with mercury, were placed
vertically with the open end submerged in a cup of mercury, some
of the mercury in the tube descended into the cup, leaving a
column of mercury about 30 inches in height in the tube.  From
this it was deduced that the pressure of air on the surface
of the mercury in the cup forced it up the tube to the height Of
30 inches, and this was so because the weight of a column of air
from the cup to the top of the atmosphere was only equal to that
of a column of mercury of the same base and 30 inches high.

Torricelli's experiment can be easily repeated.  Take a glass
tube about 3 feet long, closed at one end and open at the other;
fill it as full as possible with mercury.  Then close the open
end with the thumb, and invert the tube in a basin of mercury so
that the open end dips beneath the surface.  The mercury in the
tube will be found to fall a short distance, and if the height of
the column from the surface of the mercury in the basin be
measured you will find it will be about 30 inches.  As the tube
is closed at the top there is no downward pressure of air at that
point, and the space above the mercury in the tube is quite
empty:  it forms a VACUUM.  This vacuum is generally known as the
TORRICELLIAN VACUUM, after the name of its discoverer.

Suppose, now, a hole be bored through the top of the tube above
the column of mercury, the mercury will immediately fall in the
tube until it stands at the same level as the mercury in the
basin, because the upward pressure of air through the liquid in
the basin would be counterbalanced by the downward pressure of
the air at the top, and the mercury would fall by its own weight.

A few years later Professor Boyle proposed to use the instrument
to measure the height of mountains.  He argued that, since the
pressure of the atmosphere balanced a column of mercury 30 inches
high, it followed that if one could find the weight of the
mercury column one would also find the weight of a column of air
standing on a base of the same size, and stretching away
indefinitely into space.  It was found that a column of mercury
in a tube having a sectional area of 1 square inch, and a height
of 30 inches, weighed 15 pounds; therefore the weight of the
atmosphere, or air pressure, at sea-level is about 15 pounds to
the square inch.  The ordinary mercury barometer is essentially a
Torricellian tube graduated so that the varying heights of the
mercury column can be used as a measure of the varying
atmospheric pressure due to change of weather or due to
alteration of altitude.  If we take a mercury barometer up a hill
we will observe that the mercury falls.  The weight of atmosphere
being less as we ascend, the column of mercury supported becomes
smaller.

Although the atmosphere has been proved to be over 200 miles
high, it has by no means the same density throughout.  Like all
gases, air is subject to the law that the density increases
directly as the pressure, and thus the densest and heaviest
layers are those nearest the sea-level, because the air near the
earth's surface has to support the pressure of all the air above
it.  As airmen rise into the highest portions of the atmosphere
the height of the column of air above them decreases, and it
follows that, having a shorter column of air to support, those
portions are less dense than those lower down.  So rare does the
atmosphere become, when great altitudes are reached, that at a
height of seven miles breathing is well-nigh impossible, and at
far lower altitudes than this airmen have to be supported by
inhalations of oxygen.

One of the greatest altitudes was reached by two famous
balloonists, Messrs. Coxwell and Glaisher.  They were over seven
miles in the air when the latter fell unconscious, and the plucky
aeronauts were only saved by Mr. Coxwell pulling the valve line
with his teeth, as all his limbs were disabled.

CHAPTER XLI
How an Airman Knows what Height he Reaches

One of the first questions the visitor to an aerodrome, when
watching the altitude tests, asks is:  "How is it known that the
airman has risen to a height of so many feet?"  Does he guess at
the distance he is above the earth?

If this were so, then it is very evident that there would be
great difficulty in awarding a prize to a number of competitors
each trying to ascend higher than his rivals.

No; the pilot does not guess at his flying height, but he finds
it by a height-recording instrument called the BAROGRAPH.

In the last chapter we saw how the ordinary mercurial barometer
can be used to ascertain fairly accurately the height of
mountains.  But the airman does not take a mercurial barometer
up with him.  There is for his use another form of barometer much
more suited to his purpose, namely, the barograph, which is
really a development of the aneroid barometer.

The aneroid barometer (Gr. a, not; neros, moist) is so called
because it requires neither mercury, glycerine, water, nor any
other liquid in its construction.    It consists essentially of a
small, flat, metallic box made of elastic metal, and from which
the air has been partially exhausted.  In the interior there is
an ingenious arrangement of springs and levers, which respond to
atmospheric pressure, and the depression or elevation of the
surface is registered by an index on the dial.  As the pressure
of the atmosphere increases, the sides of the box are squeezed in
by the weight of the air, while with a decrease of pressure they
are pressed out again by the springs.  By means of a suitable
adjustment the pointer on the dial responds to these movements. 
It is moved in one direction for increase of air pressure, and in
the opposite for decreased pressure.  The positions of the
figures on the dial are originally obtained by numerous
comparisons with a standard mercurial barometer, and the scale is
graduated to correspond with the mercurial barometer.

From the illustration here given you will notice the pointer and
scale of the "A. G" aero-barograph, which is used by many of
our leading airmen, and which, as we have said, is a development
of the aneroid barometer.  The need of a self-registering scale
to a pilot who is competing in an altitude test, or who is trying
to establish a height record, is self-evident.  He need not
interfere with the instrument in the slightest; it records and
tells its own story.  There is in use a pocket barograph which
weighs only 1 pound, and registers up to 4000 feet.

It is claimed for the "A. G." barograph that it is the most
precise instrument of its kind.  Its advantages are that it is
quite portable--it measures only 6 1/4 inches in length, 3 1/2
inches in width, and 2 1/2 inches in depth, with a total weight
of only 14 pounds--and that it is exceptionally accurate and