. It consists of brass caps, which, when joined together, fit
tightly and become a globe. The air within being exhausted, it will be
found difficult to separate them. If the superficies be 100 square
inches and the height of the mercury be 30 inches, the atmosphere will
press on these hemispheres with a weight of 1,475 lbs, requiring the
efforts of seven or eight powerful men to tear them asunder. One of
these instruments, of the diameter of a German ell, required the
strength of 24 horses to separate it. The experiment was publicly made
in 1650 at the Imperial Diet at Rendsborg, in the presence of the
Emperor Ferdinand III. and a large number of princes and nobles, much
to their astonishment.

As compared with water, the air (the barometer indicating 30 deg., and the
thermometer 55 deg.) is 833 times lighter.

It is this weight of the atmosphere which counterbalances that of a
column of mercury 29 inches in height, and a column of water 32 to 34
feet in height.

The old quaint notion of Nature's abhorring a vacuum was found to be
practically only an assertion that the air had weight. The ordinary
pump, commonly called the suction-pump, is constructed on this
principle. The weight of the atmosphere at the level of the sea is
found to be the same all over the world.

* * * * *

We find the atmosphere with another characteristic,--Elasticity.

However it may be compressed, air returns, on liberation, to its
original volume, and while thus perfectly elastic it is also the most
compressible of bodies. This elasticity arises from the repulsive
force of its particles, and is always equal to the compressive force
which it balances. A glass vessel full of air, placed under a receiver
and then exhausted by the air-pump, will burst into atoms. Water, on
the other hand, is almost the reverse. Twenty cubic inches, introduced
into a cannon whose sides are three inches thick, cannot be compressed
into nineteen inches without bursting it. This non-elastic property of
water, with another, that of communicating, when under the action of
any force, an equal pressure in all directions, led to the invention
of the hydraulic press.

The elasticity of the air enables fishes to rise and sink in water,
through the action of the air-bladder.

The sudden compression of air liberates its latent heat, and produces
fire. On this principle the pneumatic tinder-box is constructed.

Brockhaus says that air has