Tables for
the convenience of calculation are given in several scientific works,
and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15.
Briefly, however, we may state, that between 0 deg. and 32 deg., 34
thousandths of an inch must be allowed for depression or contraction,
and between 32 deg. and 52 deg. 33 thousandths. The weight of the atmosphere
is not only affected by rarefaction, but by currents of air, which
give it a sudden density or rarity. Those who have ascended mountains
have experienced both these changes.
A common experiment to prove the weight of air is that of the
Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a
merchant of that city. It is a part of every complete philosophical
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.
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