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 barome er, 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.