The star. (Reynoldsville, Pa.) 1892-1946, 1907-09-11
READING THE GLASS,
Use and Construction of the Mer-
curial Barometer.
ITS VALUE ON BOARD SHIP.
The Significance of a Fall of the Glass
Varies Considerably According to
Latitude--Fluctuatio of the Ba-
rometer l- the Tropics.

varies CONSIDERABLY ACCORDING TO
Latitude--Fluctuation of the Ba-
rometer lr the Tropics.
How's the glass? This is a question
which is asked daily by thousands of
people all over the world ana frequent
ly for very trivial reasons. To the
navigator the fluctuations of the ba-
rometer are always of interest and not
infrequently of great moment. Hence
it is customary in the navy to record
the reading at least every four hours
and in storny weather at much shorter
intervals. The style of barometer
which is supplied for use in the navy
is that known to makers and physicists
as the "cistern" barometer. 1t consists
of glass tube about thirty y-thre :
inches in length and about one-third
inch internal diameter.
Any one who has seen a barometer
of this type Is aware that the glass
tube contains n column of mercury
reaching within short distance of the
top, but 1t is not as plainly apparent to
the ordinary and uninstructed observer
why this tall and narrow column thus
remains in the tube The two condi
tions essential to the construction of
mercurial barometer are, first, that
there shall be as perfect vacuum as
possible in the upper end of the tube,
which is closed, and, secondly, that t."
lower end, which is open, shall be kept
Immersed in a basin or cistern of mer-
curry.
These ends are accomplished In q
simple fashion. first discovered by an
italian scientist named Torricelli as
far back as the year 1913. The tube is
held perpendicular, with the closed end
downward, and mercury is poured in
until it is quite full. Then the finger is
placed over the opening and the tube
is turned over, end for end: the open
end is immersed in cistern of mer-
curry, and the finger or stopper is re-
moved. The mercury then sinks a
short distance in the tube, leaving a
space at the top which must obviously
be completely devoid of air. The pres
sure which the atmosphere exerts upon
the surface Of the mercury in the els
tern maintains the column in the tube.
and any variations in the air pressure
will be accompanied by correspond
ing rise or fall Of the mercury in the
tgba
Ihe cistern must, of course, be of the
N0nllest dimensions possible or the in
cfT l,n nI, In tl,n
marine barometer 1t consists of cup
about one and one-half inches in di-
ameter The necessary limitation of
the cistern causes certain irregularities.
which, however. do not practically af
feet the efficiency of the instrument as
a weather glass, and in barometers
which are designed for more accurate
readings the difficulty is overcome by
an ingenious device which there Ia not
space to describe here.
1t should be noted that mercury is
used in the construction Of barometers
simply because 1t Is by far the heaviest
of liquids. A barometer can be made
equally well, for instance, with water
but this would entail having tube
nearly forty feet In length, for when
the mercury stands at thirty inches the
water barometer will stand at about
thirty-four feet, cumbrous sort of In.
strument for the navigator to carry
about with him.
The glass tube and cistern thus com,
bined are attached to a wooden mount.
ing or placed in a metal case, with
openings to show the upper part of the
tube, and on the case scale is cut
showing inches and tenths and hun.
dredths Of an inch, or in marking q
more accurate instrument. something
ike the two hundred and fiftieth part
Of an inch. Barometers are usually
scaled from twenty-sevcn to thirty-one
inches, this being the practical limit Of
the fluctuations due to weather. and,
indeed, when the glass gets down TO
twenty-eight inches il is time to look
about you.
The significance of fall of the ba
rometer, however. varies considerably
according to the latitude. In Great
Britain, says a writer in the Navy
Auu AIuly, IL uiS l.Iitu IbtTuJItS udI1
m inch or more during the night, and
yet there may be no very alarming
gale Of wind, but such fall in the
tropics would be the signal for the
navigator to prepare for cyclone. the
fluctuations of the glass in these lati-
tudes being usually confined to slight
and regular rise and fall at certain
hours. In recent cyclone in Japan
the fali during one hour was three
tenths and the rise during the next
hour no less than six-tenths. or more
than half an inch.
1t is indeed with regard to these
terrific storms that the barometer is
Of the utmost service to the navigator.
The behavior of the glass. in conjunc-
fion with the shifting of the wind, will
Indicate to the experienced seaman the
position Of the dreaded vortex and the
approximate course upon which the
storm is travelling.
, > Eiasro