Public ledger. [volume] (Memphis, Tenn.) 1865-1893, 1871-05-20
THE AMERICAN SIGNAL SERVICE
weather TeIegrnms and storm
Forecasts.

from West to East. In the Bermudas,
where the wind veers and the barometer
falls, the storms are progressive revolv-
ing gales. The indications of the bar
ometer, the sympiesometer and other
great inventions in mechanical meteo-
rology, are the main relianees of storm
science. Clouds are Of the utmost utility
in foretelling weather, especially the
cirrus, eirro-SfrdfUs and cHmalO-strdtus
The cirrus, called by sailors cats tail,
or "curl cloud occupies the lofty
regions Of the air, and appears
like white lines penciled on the
blue sky. Its movement is valuable
prognostic of storm weather. The GIRO
stratus is marked by precursor of
storms, and from its greater or less
abundance it gives indication Of the
time when the storm may be expected
The eumulo-sFdfas is distinctly formed
Just before rain begins 1t is the fore
runner of the lurid display of the thun-
der storm. The pocket cloud is an
almost unerring storm warning. If
clouds be red and lolreriRg in the morn
ing, the red color arises from large
amount of vapor in the vascular state.
No meteorologist should depend on his
instruments alone. Great changes of
weather or storms are usually shown by
falls of barometer exceeding half an
inch, and by difference of temperature
exceeding l50. A tenth Of an inch in
an hour is a fall presaging of heavy
storm or rain. The barometer shows the
beatings-the pulsations of the atmos-
phere.
INSTRUMENTS USED.
The ordinary thermometer is too well
known TO require notice The maximum
thermometer, when in use, is placed in a
horizontal position. Inside the tube is
a steel index; as the temperature rises
the mercury pushes the index before it;
but as it falls the index is left, thus re-
gistering the highest temperature. In
the minimum thermometer spirit is used,
and a steel index inserted. AS the tenn
perature falls the spirit drags the index
with it; but when it rises it does not
move the index, leaving it at the lowest
point to which it has been dragged. The
two indications for each day are the most
important ones in meteorology. The
thermometer is the instrument used for
measuring the velocity and force of the
wind. In the United States signal service
the hemispherical cup thermometer IS
used. When placed in the wind the cups
reyolye, and the arms are of such length
thatwhen a mile of wind has passed the
instrument FIVE hundred revolutions are
registered. For measuring the force of
wind the ordinary wind gauge is used.
1t is a tube bent like a syphon, and half
filed with water. 1t turns with the wind,
being hung on an upright rod; one end
of the tube is thus like a weather-vauei
always pointing with open mouth to the
wind. When the wind blows into the
mouth it drives the water up the oppo-
site leg of the instrument, which is grad
uated
Rain gauges, for aseertsiHiHg the
quantity Of rain that falls, are as follows;
1t is merely a copper cylinder, with a
funnel closing the upper end so as tC
prevent evaporation. The rain is col
lected in a second vessel placed inside
the cylinder and thence poured into
glass measure of small diameter
nicely graduated. The Aperture OF
the gauge is twelve or fifteen square
inches. The humidity Of the air il
found by the hygrOmeter, being two
mercurial thermometers which placed
side by side indicate the temperature
When fixed together on a frame theJ
are called the net and dry bulb Her
mometers. The dry bulb is the common
thermometer intended to show the ten
> perature Of the air
The wet bulb is also a common Their
mometer, having its bulb covered with
piece Of muslin, from which pass few
threads Of darned cotton into small
e vessel containing rain water Thi
water rises by capillary attraction, thu
keeping the muslin constantly wet
), When the air is dry evaporation goes OF
rapidly from the muslin, and on aceoun
+ Of the heat lost by evaporation the we
V bulb indicates lower temperature thai
/ the dry bulb. But when the air is damI
s evaporation is slower and the diiferenc
between the two thermometers become
smaller. When the air is completed
saturated with moisture, evaporatio
ceases entirely and the two ihermom'
p tiers show the same temperature. /
showery weather the indications Of th
e hygrometer vary rapidly and the al
, proaeh Of storms may be predicated.
The great instrument Of the meieorol'
: gist is the barometer designed by TORI
cell. The hight Of the column Of mel
curry is read by a graduated index on.th
side Of the glass, just below the vacuum
over the fluid. This scale from twenty