The record-union., 1891-07-22

mometers by the assistanceIsof certain
numbers, called Glaisher's factors, which
vary with the temperature. These fact
ors, obtained by comparing the readings
OF the psychrometer with those of the in
struments previously described, are the
reSult.of many years continued observa-
ton in different places and may be con-
sldered very accurate. Their use is best
illustrated by an example. Let us sup
pose that the dry bulb thermometer
stands at 760 and the wet bulb thermome-
ter at 700. The difference, 60, when null
tIplied by LIS, the factor for an air tenn
perature of 760, gives 1882. The dew
point is therefore 1872 below 760, or 65.20
When the temperature of the air is about
53o Fahr., the factor is =, or the differ
ence between the dry thermometer and
the point is twice the difference between
the two thermometers. At higher tenn
peratureS the factor is smaller, at lower
ones larger. In freezing weather the
thermometers differ by not more than
fourth of the difference between the dew
point and the temperature of the air. If
the dew point has been obtained by the
use Of any of the instruments described,
the amount of water in the air or the
absolute humidity can be ascertained
from the published tables, as | have said.
But what we commonly mean by the
term humidity is ''relalive humidity or
the proportion which the amount OF wa
ter actually present in the air bears to the
amount it would contain if saturated.
This latter amount is obtained from the
same tables by using the actual tempera-
ture OF the air instead of the dew point.
Thus IN the example already used we
found the dew point to be Co.,, while the
temperature of the air as given by the dry
bulb thermometer was 70o. Now, the
humidity tables tell Us that air saturated
at Co., contains GA grains of water to the
cubic foot, and this we may call the abso-
lutte humidity. We find also that air sat
urated at 7o' contains 80 grains Of water
TO the cubic foot. The relative humidity
is therefore obtained by dividing ia by
80, and is about 40 per cent. The humid
ity may also be calculated from the read
ings of the psychrometer by another
method III which the ''dew point' is not
used. Many animal and vegetable sub
stances absorb water from the air in wet
weather and expand. When the air be-
comes dry they lose their moisture and
contract. This is the principle Of the hair
hygrometer, III which q pointer is moved
by the expansion and contraction of q
hair. In another familiar instrument
man with an umbrella is supposed to
come out OF little house when rain is
threatened. In fair weather he is eX-
pected to remain within doors while q
little woman suns herself before another
nttie mousse. Ine ngureS are attachcu TO
the ends Of light rod suspended by two
twisted tihers, which untwist when they
become damp. All these instruments
are very ineXact and follow very slowly
the changes in the state OF the weather.
BY the way, ii is not strictly correct to-
speak, as we commonly do, OF the air be-
ing damp or dry, or containing more or
less water. Water is not dissolved by
air, but the air and the yapor OF water are
simply mixed tCoetLar as anV other
gases might be. What we really measure
by ali the methods [ have spoken of is
the amount OF water vapor i'll cubic foot
OF space.