William J. Claxton - The Mastery of the Air
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.
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