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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 countand he was not the type of man to exaggerate his
prowesshe 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
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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。
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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。
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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 v