t round upon
itself at its lower end; the reading is taken of the difference in the
levels of the mercury in the two limbs. The "aneroid" barometer (from the
Gr. [Greek: a-] privative, and [Greek: neros], wet) employs no liquid, but
depends upon the changes in volume experienced by an exhausted metallic
chamber under varying pressures. "Baroscopes" simply indicate variations in
the atmospheric pressure, without supplying quantitative data. "Barographs"
are barometers which automatically record any variations in pressure.

[Sidenote: Historical.]

Philosophers prior to Galileo had endeavoured to explain the action of a
suction pump by postulating a principle that "Nature abhorred a vacuum."
When Galileo observed that a common suction pump could not raise water to a
greater height than about 32 ft. he considered that the "abhorrence" was
limited to 32 ft., and commended the matter to the attention of his pupil
Evangelista Torricelli. Torricelli perceived a ready explanation of the
observed phenomenon if only it could be proved that the atmosphere had
weight, and the pressure which it exerted was equal to that of a 32-ft.
column of water. He proved this to be the correct explanation by reasoning
as follows:--If the atmosphere supports 32 feet of water, then it should
also support a column of about 2-1/2 ft. of mercury, for this liquid is
about 13-1/2 times heavier than water. This he proved in the following
manner. He selected a glass tube about a quarter of an inch in diameter and
4 ft. long, and hermetically sealed one of its ends; he then filled it with
mercury and, applying his finger to the open end, inverted it in a basin
containing mercury. The mercury instantly sank to nearly 30 in. above the
surface of the mercury in the basin, leaving in the top of the tube an
apparent vacuum, which is now called the _Torricellian vacuum_; this
experiment is sometimes known as the _Torricellian experiment_.
Torricelli's views rapidly gained ground, notwithstanding the objections of
certain philosophers. Valuable confirmation was afforded by the variation
of the barometric column at different elevations. Rene Descartes and Blaise
Pascal predicted a fall in the height when the barometer was carried to the
top of a mountain, since, the pressure of the atmosphere being diminished,
it necessarily followed that the column of mercury sustained by the
atmosphere would be diminished also. This was experimentally observed by
Pascal's broth