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Mr。 Coxwell pulling the valve line with his teeth； as all his limbs were
disabled。
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THE MASTERY OF THE AIR
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
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THE MASTERY OF THE AIR
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 portableit
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 poundsand that it is
exceptionally accurate and strong。 Some idea of the labour involved in its
construction may be gathered from the fact that this small and
insignificant…looking instrument； fitted in its aluminium case； costs over
L8。
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THE MASTERY OF THE AIR
CHAPTER XLII How an Airman finds his Way
In the early days of aviation we frequently heard of an aviator losing
his way； and being compelled to descend some miles from his required
destination。 There are on record various instances where airmen have lost
their way when flying over the sea； and have drifted so far from land that
they have been drowned。 One of the most notable of such disasters was
that which occurred to Mr。 Hamel in 1914； when he was trying to cross the
English Channel。 It is presumed that this unfortunate pilot lost his bearings
in a fog； and that an； accident to his machine； or a shortage of petrol；
caused him to fall in the sea。
There are several reasons why air pilots go out of their course； even
though they are supplied with most efficient compasses。 One cause of
misdirection is the prevalence of a strong side wind。 Suppose； for example；
an airman intended to fly from Harwich to Amsterdam。 A glance at the
map will show that the latter place is almost due east of Harwich。 We will
assume that when the pilot leaves Earth at Harwich the wind is blowing to
the east； that is； behind his back。
Now； however strong a wind may be； and in whatever direction it
blows； it always appears to be blowing full in a pilot's face。 Of course this
is due to the fact that the rush of the machine through the air 〃makes a
wind〃； as we say。 Much the same sort of thing is experienced on a bicycle；
when out cycling we very generally seem to have a 〃head〃 wind。
Suppose during his journey a very strong side wind sprang；up over the
North Sea。 The pilot would still keep steering his craft due east； and it
must be remembered that when well out at sea there would be no familiar
landmarks to guide him； so that he would have to rely solely on his
compass。 It is highly probable that he would not feel the change of wind at
all； but it is even more probable that when land was ultimately reached he
would be dozens of miles from his required landing…place。
Quite recently Mr。 Alexander Gross； the well…known maker of aviation
instruments； who is even more famous for his excellent aviation maps；
claims to have produced an anti…drift aero…compass； which has been
specially designed for use on aeroplanes。 The chief advantages of this
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compass are that the dial is absolutely steady； the needle is extremely
sensitive and shows accurately the most minute change of course； the anti…
drift arrangement checks the slightest deviation from the straight course；
and it is fitted with a revolving sighting arrangement which is of great
importance in the adjustment of the instrument。
Before the airman leaves Earth he sets his compass to the course to be
steered； and during the flight he has only to see that the two boldly…marked
north pointson the dial and on the outer ringcoincide to know that he is
keeping his course。 The north points are luminous； so that they are clearly
visible at night。
It is quite possible that if some of our early aviators had carried such a
highly…efficient compass as this； their lives might have been saved； for
they would not have gone so far astray in their course。 The anti…drift
compass has been adopted by various Governments； and it now forms part
of the equipment of the Austrian military aeroplane。
When undertaking cross…country flights over strange land an airman
finds his way by a specially…prepared map which is spread out before him
in an aluminium map case。 From the illustration here given of an aviator's
map； you will see that it differs in many respects from the ordinary map。
Most British aviation maps are made and supplied by Mr Alexander Gross；
of the firm of 〃Geographia〃； London。
Many airmen seem to find their way instinctively； so to speak； and
some are much better in picking out landmarks； and recognizing the
country generally； than others。 This is the case even with pedestrians； who
have the guidance of sign…posts； street names； and so on to assist them。
However accurately some people are directed； they appear to have the
greatest difficulty in finding their way； while others； more fortunate；
reme