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.
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.