St. Landry democrat. (Opelousas, La.) 1878-1894, 1888-02-04
ABOUT BAROMETERS.
The Principles luvolved in the Changes
which the Mercury Undergoes

The Principles Involved in the Changes
which the Mercury Undergoes.
The fundamental principle Of the
construction Of the barometer is best
shown in the experiment which led
| TorricclIi to the first discovery OF the
presence OF the ain. A glass tube about
thirty-three inches long, opened at one
end. is completely filled with mercury.
and, being firmly closed by the thumb,
is inverted and placed vertically in a
cup containing mercury When the
thumb is removed the mercury sinks in
the tube till it stands. generally, about
thirty inches above the level of the
mercury in the cup, leaving in the up
per part a space free of air which re-
ceives the name of the TorricclIian
vacuum. The mercury within the
tube being less removed from the pres
sure Of the air, while that in the cup is
exposed to it. the column falls till the
pressure ut the section of the hole, in
the same plane us the surface of the
mercury in the cup, is the same within
and without the tube. We have yet 10
better index Of the pressure of the at
mosphcre than the simple mcrcurial
column of Torricelli. and in all exact
observations it is taken as the only re-
liable standard. Simple as the Earom-
eter is. its construction demands con-
siderable care and experience. 1t is Of
the first Importance that the mercury
to be used is chemically pure, other
wise its huidity is impaired and the in
side of the tube becomes coated with
impurities in such u way a.
to render correct observations i'm
possible. Mercury, as usually sold,
is not pure; and before being en-
ployed for barometers, must be shaken
we'll with highly diluted but pure nitric
acid, to remove eXtraneous metals and
oxides. In filling the tube it is essen
TALLY necessary to get the column free
from air and moisture. To effect this,
the mercury, after filing. is boiled in
the tube. so that air and moisture may
be expelled. Barometers are usually
divided into two classes-cistern barom-
eter and siphon barometer. The siphon
barometer is in many respects Il more
perfect instrument than the cistern.
The wheel barometer, originally III
vented by Hook, and generally seen
as Il parlor ornament, has little to rec
ommcnd it as a trustworthy instru-
ment. The uses of the barometer may
be classified into physical, hypsomet-
rical and meteorological. 1t is of es-
sential use in all physical researches
where the mechanical, optical, acous-
teal and chemical properties of air or
other gases are dependent on the pres
sure of the atmosphere. It's uses in
hypsometry, or the art Of measuring
the heights of mountains, is very valu-
able, but the best known use of the |
barometer is rs a meteorological instru-
ment, or as a weather glass. Opticians
have attached to certain heights of |
the barometer certain states of the
weather, and at certain points of
the scale the words Rain "Change-
able, Fair, etc, are marked; but the
connection thus instituted is very mis-
leading Those who have observed
most carefully the connection of
barometric heights with changes OF the
weather. disregard entirely the use of
these terms, and state that it is not the
actual height of the barometer at any
place. but this height as compared with
that OF surrounding regions, which in
dicates the coming weather. General
ly speaking, falling barometer indi-
Gates rain, q rising barometer fair
weather. A steady barometer foretells
a continuance of the weather at the
time; when low this is generally broken
or bad, and when high. fair. A sud-
den fall usually proceeds storm, the
violence of which is in proportion to
the barometiic gradient. An unsteady
barometer shows an unsettled state of
the weather; gradual changes, the APP
proach Of some permanent condition Of
it. The variations must also be inter-
preted with reference to the prevailing
winds. each different wind having some
peculiar rules The connection be-
tween the changes of weather and the
pressure of the atmosphere is by no
means well understood Toledo B{0t{e.