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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 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