The record-union., 1891-07-22
MOISTURE IN THE AIR.
How the Quantity is Estimated
By Scientists.
Ingenious Instruments for Meas uring
the Amount of Water in the At
mosphere-The Law Upon Which
the Construction of the Instrument
Is Based.

Is Based.
We hear a good deal about humidity"
nowadays, says noted scientist writing
in the New York Herald, and most of us
know that the word refers to the amount
of moisture or water vapor in the air.
But the methods of measuring humidity
and the exact meaning of such an express
sion as humidity 80" or $50 per cent"
is not, / think, very generally under
stood. The method of measuring hU-
midity which frsts suggests itself is to
extract the water from the air and actu-
ally measure or weigh it. This is some
times done, but the process is slow and
difficultin practice, though simple enough
in theory. A vessel whose exact capacity
is known-suppose it to be ten gallons
is filled with water through tube at the
top and the tube is then closed by turn
ing faucet. There is another tube with
a faucet at the bottom of the vessel. If
both faucets are now opened the water
will run out and it is clear that exactly
ten gallons of air viii be drawn into the
vessel through the upper tube.
If this entering air, instead of being
taken directly from the surrounding at
mosphere, is drawn through a series of
tubes containing substances which have
great attraction for water (sulphuric acid
and chloride of calcium, for eXample) all
of the Vapor of water contained in the ten
gallons of air will be condensed and re-
tanned in the tubes. By weighing these
tubes before and after the experiment,
then we can learn just how many grains
OF water Tor water vapor) the ten gallons
of air contained. But this experiment not
only occupies considerable time, but it
must be performed with the utmost
care and the most minute precautions
against error in order to obtain an accu-
rate result.
For practical work in recording the
rapid changes in the humidity of the air
some other method is necessary. Fortu
nately q remarkable law of nature comes
10 our aid. 4 given volume of air-a GU
bic foot for eXanple-can take up only
limited quantity of moisture, depending
upon the temperature. When the limit
is reached the airis said to be saturated
and the slightest reduction of tempera-
ture causes precipitation of water in the
form of dew or rain. Now, the quantity
of water contained in a cubic foot of air
when saturated at any given temperature
has been accurately determined by many
careful experiments of the kind already
described and may be obtained from
published tables. To find the amount Of
water actually present in the air at any
time it is therefore only necessary to cool
the air until it becomes saturated-that
Is to say, until dew is formed, and note
the temperature of the air when the dew
appears. This temperature Is called the
dew point. A rude way of performing
the experiment is to put some water into
bright tin cup with thermometer and
gradually add ice water, with frequent
stirring, until the bright surface of the
tin is clouded with dew. Then read the
thermometer and you have the dew point
corresponding to the actual state of the
air. With this dew point you can find
the amount Of water in the air from the
published tables.
A better arrangement consists Of wide
glass tube, the central part of which is
horizontal, while the ends bend down
ward and terminate in two large glass
bulbs. One of these contains enough
ether to cover the bulb of a thermometer,
which is incloSed in the apparatus. The
other bulb is covered with muslin, upon
which ether is slowly dropped. The
evaporation of this ether cools the bulb,
condenses the ether vapor in it and hence
causes the ether IN the other bulb, which
contains the thermometer, to evaporate
rapidly, the vapor constantly passing
over to the bulb wrapped in muslin,
where it is condensed. This evaporation
from the ether in the bulb containing the
thermometer cools that bulb and it's con-
tents, and when dew forms on the outside
of the bulb the dew point is read off from
the thermometer. This apparatus is
known as DanielFs hygrometer. A very
simple by"rOmeter may be made Of
wide glass be, closed at the bottom and
open ut the top. 1t is stopped by cork
with three holes, through which are
passed a thermometer and two small
glass tubes, one of which is quite short
while the other reaches to the bottom of
the large tube and is connected at the
upper end with loug rubber tube. The
large tube contains enough alcohol or
ether to cover the bulb of the thermome-
ter. BY putting the end of the rubber
tube to the lips and blowing you can
cause the ether or alcohol 10 evaporate
and soon cool the instrument to the dew
point.
a more elaborate form of this simple
instrument is Regnault's hygrometer, in
which the lower part of the large tube is
covered by q silver cup and the current
OF air used to produce evaporation is not
supplied by the lungs, but is drawn
through the liquid by a contrivance called
an aspir2tor, similar TO the water vessel
described in the beginning of this article
y second thermometer, inclosed in an
empty glass tube and silver cup, gives the
temperature OF the air.
In using any OF the ''dew point in
struments il is impossible TO observe the
very beginning of the formation of dew,
and so the temperatures taken are apt to
be little too low. To correct this error
the instrument may be allowed to grow
warmer again until the dew has Just dis
appeared. The thermometer Will then
stand a little higher than the true dew
point. and by taking the average of the
TWO readings of the instrument a very
correct result may be obtained. But the
best of these instruments, although their
use is more convenient than the actual
measurement of the moisture in the air
still leave something to be desired in
point of ease and rapidity OF observation,
and for practical work in meteorological
obseryatorieS whore the humidity is de-
termined several times daily they have
been supplanted by another instrument,
which does not give the dew point di-
reCtly, but furnishes means of obtaining
II by calculation. This instrument is the
psychrometer, or the wet and dry bulb"
thermometers.
Two thermometers, as nearly alike as
possible, are mounted ona suitable stand.
The bulb Of one of them is covered with
muslin, which is kept constantly wet.
The evaporation of the water from the
muslin cools the wet bulb thermometer
so that il reads lower than the other.
The difference between the two depends
upon the rapidity of evaporation, and
therefore upon the dryness of the air
Hence mere glance ut the instrument
gives hint of the humidity, but the use
fulness of the psychrometer is not con-
fined to giving points.
Although the wet bulb thermometer is
never called TO the dew point and no dew
is formed, yet the dew point is easily OBJ
tained from the readings of the two ther-
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