Other Types Of Barmeters Fortelling Weather Part 4
In the previous observations, general reference has been made to
mercurial barometers of the ordinary kind; but, excepting the
construction of the instruments themselves, those observations apply to
all barometers, wheel--aneroid--or metallic--and likewise, of course, to
the sympiesometer, which is a modified barometer. But as these four
last-mentioned instruments are scarcely so familiar as the simplest form
of barometer, it may be useful to add a few words about each of them.
* * *
The WHEEL barometer has a syphon tube, 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.
* * *
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, 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. 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.
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.
METALLIC barometers (in outer shape and size like aneroids) have not
yet been tested adequately in very moist, hot, or cold air for a
sufficient time. They, as well as sympiesometers, are likewise dependent
or secondary instruments, and liable to deterioration. For limited
employment, when sufficiently compared, they may be very useful,
especially in a few cases of electrical changes not foretold or shown by
mercury.
The SYMPIESOMETER is considered to be more sensitive than the marine
barometer, falling sooner, and rising earlier: but this is partly in
consequence of the marine barometer tube being contracted, to prevent
oscillation or "pumping." In the sympiesometer a gas is used, which
presses on the confined surface of the liquid with an uniform pressure
at an equal state of temperature. The liquid is raised or depressed by
an increase or diminution in the density of the atmosphere, and change
of temperature is allowed for, by the sliding scale of the instrument
being always set to agree with the height of the mercury in the attached
thermometer, bringing the pointer on the sliding scale of the
sympiesometer to the same degree on the inverted scale (over which it
slides) as is indicated by the thermometer. The height of the fluid, as
then shown by the sliding scale, indicates the pressure of the
atmosphere.
As the instrument is delicate, great care should be taken, in carrying
or handling, to keep the top always upwards, and to exclude casual rays
of the sun, or a fire, or lamp.
Oil sympiesometers seem to be affected more than mercurial, or others,
and much more than the barometer, by lightning or electricity. That
they, and the hermetically sealed "STORM GLASSES," are influenced by
causes besides pressure and temperature, appears now to be certain.
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