The aneroid barometer was invented by M. Vidi, of Paris. It consists
essentially of a circular box, the face of which is made of thin
elastic metal, rendered more elastic by being stamped and pressed into
concentric circular wave-like corrugations. This box is nearly
exhausted of air, and its elastic face supports the pressure of the
atmosphere, and yields to it with elastic resistance in proportion to
the amount of pressure. Thus, if the atmospheric pressure increases,
the face is pressed inward; if atmospheric pressure diminishes, the
elastic reaction of the metal moves the face outward. These movements
are communicated to an index by suitable and very delicate mechanism,
and registered in largely magnified dimensions, by the movements of
this index upon the face of the dial.
Aneroid barometers are now made of pocket size, compensated for
temperature, and with double scales, one reading the height of the
barometer column, the other the elevation obtained. I have, says Prof.
W. M. Williams, used one of these during many years, and find it a
very interesting traveling companion. It is sufficiently sensitive to
indicate the ascent from the ground floor to the upper rooms of a
three-storied house, or to enable the traveler sitting in a railway
train to tell, by watching its face, whether he is ascending or
descending an incline.
Such slight variations are more easily observed on the aneroid than on
the mercurial barometer, and therefore it is commonly stated that the
aneroid is the more sensitive instrument. This, however, is a
fallacious conclusion. It is not the superior sensitiveness of the
movements of the instrument, but the greater facility of reading them,
that gives this advantage to the aneroid, the index of which has a
needle point traveling nearly in contact with the foot of the
divisions; the readings are further aided by a needle point register
attached to a movable rim, which may be brought point to point against
the index, thus showing the slightest movement that human vision may
detect. A magnifying lens may be easily used in such a case.
It should be understood that the aneroid barometer is not an
independent instrument; it is merely a device for representing the
movements of the mercurial barometer. It is regulated by comparison
with the primary instrument, and this comparison should be renewed
from time to time, as the elastic properties of the metal may and do
vary.
An adjusting or regulating screw is attached to the back of the
instrument, and is usually movable by a watch key.
Besides this, the magnified reading of course magnifies any primary
error, and is largely dependent on the accuracy of the mechanism.
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