Savannah morning news. [volume] (Savannah) 1868-1887, 1886-01-17

helgbth 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 excessive heat. or
other cause, and rising when the air is
cold and dry, or dense from other causes
not necessary at present to enumerate.
1t must not be supposed that falling ba-
rometerls an inVariable precursor or q
storm; the indications Of the instrument
are 10 be relied on only in connection with
those of other instruments. The Their
mometers employed by the signal service
differ from those in ordinary use only in
being selt-recording as respects maxi
mum and minimum temperature.
The humidity OF the air is not ascer-
tanned absolutely-thati is to say, by an
estimate of the percentage OF weight of
moisture TO weight OF air-but with
reference to the degree Of saturation.
Inc quantity moisture the air will hold
depends upon its temperature, and also
upon its density or pressure. A cubic
foot Of air having temperature of 80 deg.
and under uniform barometric pressure
OF 80 inches, will hold 4.2S grams of water.
III. under these conditions. the tempera-
tufe or tine pressure of the air Is in
creased, 1t becomes capable of holding in
suspension an increased quantity OF
water; but it temperature or pressure be
lowered, there will result a deposition of
portion OF the water in the form of fog,
dew, rain, frost, snow or hail. There is
then point of saturation for each degree
of temperature, as well as for each Vara-
ton OF pressure indicated by the barOme-
ter. If the air be chilled artfcially it can
be made TO deposit its mOsture-as is
seen to happen on the exterior of a
tumbler containing ice water. The least
chilling will bring fully saturated air TO
the dew point" while very dry air
would need be brought almost to the
freezing point before it would give up its
burden of water. Evidently exposed
water would not eVapOrate in fully sat.
urated air, and will evaporate more or
less freely In proportion to the degree of
dryness. AS all evaporation produces
cold, it is evident that the method of esti-
mating humidity by the "wet and dry
bulb thermometer is good one. This
instrument, which is the one employed by
our signal Office, consists of two ordinary
| thermometers, one of which has its bulb
enyel0ped i'll lampwlck kept moist with
wafer In dry atmosphere this water
produces by its evaporation a chilling
effect, and by observing the difference in
the readings of the two thermometers we
can, by reference TO stable Of humldi-
ty," tell to nicely the de-
gree of saturation OF the air.
The thermometer consists of an
axle from which radiate tour arms having
at three eXtremities hemispherical cups
to catch the wind, the velocity OF which is
estimated by the number OF revolutions
per minute made by the instrument. The
direction of thy wind is shown by an or
binary weather vane. 1t must not be
supposed that the signal officer is perched
on his house top to observe the workings
OF these indicators. He reads their record
below in his sitting-r0om as made auto
matically by the agency of electricity on
strip OF paper wrapped around a re-
volyiDg drum.
Suppose all the data in at the Washing
ton Office at :35 D. m, and that tits found
that at Nashville, Tenn, and a hundred
miles around it in every direction there is
an "area Of Depression an area of low
barometer, say 1872 while around Nash
Vile as centre we may draw a line
showing a higher barometer, say 1902,
at Cincinnati, Indianapolis, ST. Louis,
Memphis, Atlanta, Abingdon, Va, aud
Wheeling. What is the forecast? There
Will be a flow OF air-wind
from all points named toward ashville.
SO far a. practical effect is concerned,
low barometer is equivalent to a depres-
sion of the upper surface OF the sea OF air
in which we live. and high barometer is
equivalent to an elevation or wave, which
VIII naturally flow down and fill up the
depression. There VIII, therefore, be an
inflow towards Nashville from all points