The New Orleans daily Democrat. [volume] (New Orleans, La.) 1877-1880, 1880-01-07

tube: the calIbre Of the tube should be uni
form throughout. and the. scale should be
rradnatnd Ia ell The particular instrument.

tube. the Calbre Oh Inc tuDe snoulu Ds uni-
form throughout, and the scale should be
graduated to fit the ularinstrument.
When water is exposed to the air In an open
vessel Ii loses In volume, ana we say &
evaporates. This evaporation or eonvsrsif IN
of water into vapor goes on at all tempe "a
lures. and Is only suspended when the air
above the water becomes 90 thoroughly atu-
rated as to be unable to hold any more The
point of saturation Ia, however. rareiyre aehed
In the open air. II. being only ID dens fogs
and long continued rains that the atmos-
phere becomes SO highly charged wit vapor
as TO be incapable OF further absorpt fpn. This
vapor contained In the air is INV stbe and
termed Its humidity.
The variable amount Of humialt sContained
In the air at different times exef etses an i'm
nortant Influence In the develor irnent of va
rious meteorological phenomene : It Is, there
fore, OF interest to know the q antity Of va
por contained In the air at al " certain time.
BY means of Instruments an mathematical
processes we arrive MT this result. and are
enabled to indicate the We light of vapor Of
water. In grains, contained lin cubic foot OF
air of a certain temperate re. This process,
which is too complicated 10 be explained
here. gives us the absolute humidity. What
Is OF much more Importance TO us, however,
is the rsiatiue humility, or that proportion of
vappr present In the air relative to the pro-
portion present when tho air Ia saturated. This
proportion Ia conveniently Indicated 10 per-
eentage Considering saturated air to con-
tain 100 per cont Of relative humidity, the
relative humidity Of mir only one-thrd sat
urated would be only 33ta per cent.
Air at different temperatures contains dill
ferent asnoEnts of vapor. THUS saturated air
at $50 contains about twice as much vepor as
saturated air at 4so. Both Theses masses OF
air are moist or damp. IL however, We raise
the temperature OF the latter mass without
permitting the access Of additional vapor, we
reduce the relative humidity to half. or
/ 80 per cent. Hence, tho all which appeared
damp at 4So will be comparatively dry when
ts temperature is raised 20o.
The amount of vapor III the atmosphere is
measured by means of an instrument called a
hygrometer. Of this there are several kinds.
The one commonly used Is simply composed
OF two Thermometers fastened To q small
piece of board, the bulb OF one OF the Their
mometers being covered with small piece Of
muslin or lampwick Under the covered bulb
a small cistern containing water is fastened
to the board, into which the free edge of the
wick or muslin is placed. BY capillary action,
the water ascends In the wick aud covers the
thermom eter with thin film OF water. The
evaporation from this reduces the tempera-
ture OF this Thermometer, aud the difference
between the wet and dry bulb furnishes u fac
tor by which we arrive-after short calcula-
tion at the relative humidity. Upon the per-
eentage of relative humidity and the teDpera-
ture depends the formation Of dew. frost.
clouds, rain. snow and hail.
We say "the dew FALLS but this Is iueor-
rect. Its formation IL due to the radiation Of
neat from the ehTtn ana 0DJpGb9 immediately
upon it by which their temperature Is lowered
from ten to twenty degrees below that of the
air. The vapor-laden air coming In contact
with these colder surfaces loses some of its
moisture. which is condensed upon these OBJ
jects and appears as dew. Where radiation Is |
most rapid-other things being equal-the de-
post OF dew Ia most copious Thus on
calm, clear night the quantity of dew formed
is much greater than on a cloudy or windy |
night. When the fall of temperature is suf- |
H,n tI.tr nI t.nr
almost Immediately into the solid state and
appears as frost or rhine.