Pascal's Treatise on the weight of the whole mass of air forms the basis
of the modern science of Pneumatics. In order to prove that the mass of
air presses by its weight on all the bodies which it surrounds, and also
that it is elastic and compressible, he carried a balloon, half filled
with air, to the top of the Puy de Dome, a mountain about 500 toises
above Clermont, in Auvergne. It gradually inflated itself as it
ascended, and when it reached the summit, it was quite full, and swollen
as if fresh air had been blown into it; or, what is the same thing, it
swelled in proportion as the weight of the column of air which pressed
upon it was diminished. When again brought down, it became more and more
flaccid, and when it reached the bottom, it resumed its original
condition. In the nine chapters of which the Treatise consists, Pascal
shows that all the phenomena and effects hitherto ascribed to the horror
of a vacuum arise from the weight of the mass of air; and after
explaining the variable pressure of the atmosphere in different
localities, and in its different states, and the rise of water in
pumps, he calculates that the whole mass of air round our globe weighs
8,983,889,440,000,000,000 French pounds.
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