partly filled with mercury, on
which, at the short or open end of the tube, a float moves, to which a
line is attached that moves a wheel, carrying an index.[31]

* * *

ANEROID barometers, if often compared with good mercurial columns, are
similar in their indications, and valuable; but it must be remembered
that they are not independent instruments; that they are set originally
by a barometer,[32] require adjustment occasionally, and may deteriorate
in time, though slowly.

The aneroid is quick in showing the variation of atmospheric pressure,
and to the navigator who knows the difficulty, at times, of using
barometers, this instrument is a great boon, for it can be placed
anywhere, quite out of harm's way, and is not affected by the ship's
motion, although faithfully giving indication of increased or diminished
pressure of air.[33] In ascending or descending elevations, the hand of
the aneroid may be seen to move (like the hand of a watch), showing the
height above the level of the sea, or the difference of level between
places of comparison.[34]

The principle on which it is constructed may be explained in a few
words, without going into a scientific or too minute detail of its
various parts. The weight of a column of air, which in a common
barometer acts on the mercury, in the aneroid presses on a small
circular metal box, from which nearly all air is extracted; and to this
box is connected, by nice mechanical arrangement, the hand visible over
the face of the instrument. When the atmospheric pressure is lessened on
the vacuum box, a spring acting on levers, turns the hand to the left,
and when the pressure increases, the spring is affected differently, the
hand being turned to the right. It acts in any position, but as it
_often varies several hundredths with such a change_, it should be held
uniformly, while read off.

The known expansion and contraction of metals under varying
temperatures, caused doubts as to the accuracy of the aneroid under such
changes; but they were partly removed by introducing into the vacuum box
a small portion of gas, as a compensation for the effects of heat or
cold. The gas in the box, changing it bulk on a change of temperature,
was intended to compensate for the effect on the metals of which the
aneroid is made. Besides which, a further and more, reliable
compensation has lately been effected by a combination of brass and
steel bars.[35]

METALLIC baromet