Bismarck daily tribune. [volume] (Bismarck, Dakota [N.D.]) 1881-1916, 1907-06-12
READING THE GLASS.
Use and Construction Of the Mer-
curial Barometer.
ITS VALUE ON BOARD SHIP.

ITS VALUE ON BOARD SHIP.
The Significance Of Fall Of the Glass
Varies Considerably According to
Latitude-FIuctuations Of the 8a-
roneter ID the Tropics.
How's the glass7 This Is a question
which is asked daily by thousands of
people all over the world and frequent
iy for very trivial reasons. To the
navigator the fluctuations of the ba
rometer are always Of interest and not
infreqnently Of great moment. Hence
1t is customary in the navy to record
the reading at least every four hours
and In stormy weather at much shorter
intervals. The style Of barometer
which is supplied for use In the navy
sis that known to makers and physicists
as the ''cisterns barometer It consists
Of q glass tube about thirty-three
inches In length and about one-third
inch internal diameter.
Any one who has seen barometer
of this type Is aware that the glass
tube contains a column Of mercury
reaching within a short distance Of the
top, but It 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-
tons essential to the consTuct don Of
a mercurial barometer are, first that
there shall be as perfect d vacuum as.
possible .in the upper end Of the tube.
which is closed, and, secondly. that the
lower end, which is open, shall be kept
Immersed In basin or cistern Of mer-
Curry. s:
These ends are accomplished In
simple fashion, first discovered by an
! italian scientist named Torricelli, as
far back as the year 1643. The tube is
held perpendicula with the closed end
downward, and mercury is poured in
until 1t 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 III a 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
1 will be accompanied by correspond
foII IT,,
4 tube. IN III 81 (. Il. B sse+4@a
i The cistern must, Of course, be Of the
4 smallest dimensions possible or the in
strument would be too clumsy. In the
j marine barometer 1t consists Of a cup
& about one and one-half inches in d.
i ameter. The necessary limitation of
i the cistern causes certain Irregularities,
which, however. do not practically af-
feet the efficiency Of the instrument as
1 a weather glass, and in barometers
which are designed for more accurate
readings the difficulty is overcome by
an ingenious device which there is not
I space to describe here.
i It should be noted that mercury is
used in the construction Of barometers
simply because it is by far the heaviest
Of liquids. a barometer can be made
equally well, for instance, with water
4 but this would entail having a tube
4 nearly forty feet in length, for when
4 the mercury stands at thirty inches the
4 water barometer will stand at about
i thirty -four feet, cumbrous sort of in.
strument for the navigator to carry
about with him. r.
i The glass tube and cistern thus com
bined are attached to a wooden mount
i ing or placed in a metal case, with
| openings to show the upper part of the
i tube, and on the case a scale is cut
4 showing inches and tenths and hun
y dredths Of an inch, or, In marking 9
more accurate instrument, something
like the two hundred and fiftieth part
Of an inch. Barometers are usually
scaled from twenty seven to thirty-one
inches, this being the practical limit OF
the fluctuations due to weather, and
indeed, when the glass gets down tx
twenty-eight inches it is time to lool
about you.
The significance Of a fall Of the ba
rometer, however, varies considerably
/ according to the latitude. In Great
Britain, says a writer In the Navy
and Army, it has fallen perhaps hall
an inch or more during the night, anG
yet there may be no very alarming
gale Of wind. but such a fall in ths
tropics would be the signal for the
navigator to prepare for cyclone, the
U fluctuations of the glass In these lati
tudes being usually confined to a Sigh
and regular rise and fall at certain
hours. In a recent cyclone in Japar
the fall during one hour was three
tenths and the rise during the mex
hour no less than six-tenths. or mors
than half an inch.
1t Is indeed with regard to these
terrific storms that the barometer !,
Of the utmost service to the navigator
The behavior Of the glass, In conjunc
ton with the shifting Of the wind, vii
indicate to the experienced seaman thu