, 1886-02-09
THE WORK OF OLD PR0BS
HOW THE SIGNAL SERVICE eeTo
ITS METEOROLOGICAL FACTS.


W An atftA OF LOW Dtprtsaious
The weather bureau, as now organized,
q "A military institution, or IN a%ppendage
Of the regular army. Throughout the
United States there are 376 signal stations,
IN charge Of nineteen officers n 500 men
Of the signnl corps Of the army. Each
station is provided with the best instru-
ments, including the attest inventions
for observing md accurately recording
the constant variations Of the weather.
From 1879 Of these stations telegraphic
reports are sent daily 10 Washington; tho
others report by mail
Reports are received from the raine
observers Mt important stations three
times every twenty-four hours. These re-
ports contain full particulars OF the
weather in the different districts. Seven
.een stations in Canada. one in st Johns,
Newfoundland, and one in the north
western part of British America send
reports, and there are over 800 outside
)r volunteer observers In different
sections who also send reports
by mail. The telegraphic reports
are transmitted TO Washington by
means Of II secret code, a few figures CON
vcying n large amount of information.
These reports are received from all the
stations in the United States, Canada, and
The West Indies inside OF thirty minutes.
A Translator" takes the kilograms in
hand and reads them off to eight clerks,
sach Of whom has n special weather map
before him on which he marks the par
llcular readings he has been instructed to
take. These eight maps are afterward
combined in one general map, which then
fully represents all phases Of the weather
throughout the territory covered. This
map is closely studied by an expert signal
officer, the storm center located, the
probable course Of coming storms deter
mined, and the complete indications"
made up for the several districts.
SYSTEM n INSTRUMENTS USED.
Some account of the system and instru-
ments used may be interesting to the
majority of readers. The bureau passes
It's forecasts upon observations as to bar
ometer, thermometer, hygrOmeter, anemo-
meter, weather vane, and rain gauge,
made at the same instant three times a
day at all the stations. Each observer
lnspects his instruments at 7:35 a. m, 4;35
.. IN. and 1l;3J p. m, Washington time.
where the bulletins are made up and pub
lished In each case n hour and twenty
1--WE minutes after the observations are
mnde-at . n. m, g y. m, and | n. m, re-
spectivcly. Reliable weather prophecy re-
quires that at least four things be known
respecting the condition Of the atmosphere
over "A wide nren-its weight as indicated
by the barometer; its temperature, AS
shown 10y the thermometer; its degree Of
humidity, AS shown by the hygrometer,
and the direction and velocity of its cur
rents, rs recorded by the weather ,vane
aud thermometer.
The baromete used in our service Is the
common mercurinl barometer, in which
the Naring weight Of the atmosphere at
any given place is balanced against the
weight Of II column of mercury contained
la u glass lube about thirty-three inches
in length. At the sea-level in fair weather
u column Of mir one inch square and ex-
tending 10 the upper limit Of the atmos-
phcrc weighs fifteen pounds; n column of
mercury Of the same section and thirty
Inches high weighs the same; hence the
power Of the air under the conditions
mentioned to sustain in n tube containing
a vacuum in its upper end n column OF
mercury Of the height named. IL follows,
Of course, that the height Of the mercurial
column will vary with the variations
In the weight OF the atmosphere, falling
when the air Is rendered light by reason
OF the presence OF watery vapor, by exces-
sive heat, or other cause, and rising when
the air is cold and dry, or tense from
other causes not necessary at present to
enumerate. 1t must not be supposed that
n falling barometer is an invariable pre-
cursor Of "A storm; the indications Of the
instrument are to be relied on only in con
uoction with those Of other instruments.
AN AREA OF Low BAROMETER.
Suppose all the data in at the Washing
a. - N -. - Nu~ .. - 4 L.. N.W.-N NIuN
At Nashville, Tenn, and n hundred miles
around 1t in every direction there is an
aaren Of depression an area of low baron
ster, say e9.2, while around Nashville as a
center we may draw n line showing a
high barometer, say 5000, at Cincinnati,
indianapolis, ST. Louis. Memphis, Atlanta,
Abingdon, Va, and Wheeling. What il
the forecastt There will be a flow Of air
w~inds-inward from all points named to
ward Nashville. So far as practical effect
Is concerned, low barometer is equivalent
TO a depression Of the upper surface OF the
sea OF air in which we live, int high
barometer is equivalent to an elevation,
or wave, which win naturally flow down
and al up the depression. There will,
therefore, be an inflow toward Nashville
from all points OF the compass.
Like water flowing toward a hole in the
bottom OF a tub, the current of air will
take on a whirling motion, and this
o%yclonic movement, which, in the north
ern hemisphere, under the influence OF
the earth's rotation, is always from right
TO left, will go on increasing in intensity.
Violent winds whirling about Nashville
at "A distance OF about seventy-five miles.
but a dead calm perhaps at the city itself.
In an ordinary water whirlpool there is
a.. q - L. - 81.
the suction is upward, with, Of course, a
lower barometer. The warm, damp air
about Nashville being thu carried by the
c%yclonic movement into the upper. colder
aud less dense regions of the atmosphere,
becomes expanded and chilled so ms to be
incapable OF sustaining longer its burdens
Of moisture, which falls as rain or snow.
Washington Cor. Globe-Democrat.