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,