apparatus. 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°, and the
thermometer 55°) 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