Public ledger. [volume] (Memphis, Tenn.) 1865-1893, 1875-07-24
THE ANEMoMETER.

This instrument can be seen on top of
the block and is used for measuring the
velocity and force of the wind. That in
use is the hemispherical cup anemome-
ter, which, when placed in the wind, the
cups revolve and the arms are of such a
length hat when a mile of wind has
passed the instrument FIVE hundred rev.
olutions are registered by a system of in
dex screws set in motion by an endless
screw on the upright axis. The ane-
miometer only gives the velocity of the
wind between two observations, but con-
tinuous registration is obtained by an
instrument which transfers the result to
paper by a pencil. The force of the
wind is obtained by a similar contrivance
which registers its results.
WIND GAUGE.
This instrument is a tube bent like q
syphon and half filed with water. Ii
turns with the wind, being hung on an
upright rod. One end Of the tube is
thus like a weather-vane, always point
ing with open mouth to the wind. Wher
the wind blows into its mouth it drives
the water up the opposite leg Of the in
strument, which is graduated.
RAIN GAUGES.
These instruments ascertain the guam
tity of rain fall. 1t is a copper cylinder
with a funnel closing the upper end SC
as to prevent evaporation. The rain i:
collected in a second vessel placed in.
side the cylinder and thence poured into
a glass measure of small diameter nicely
graduated. The aperture of the gauge
is twelve or fifteen inches.
THE HYGROMETER,
to ascertain the approach Of a storm,
is indispensable and the humidity Of the
air is the thing to discover, as the front
Of a storm is always moist. The hygrom.
eter consists of two mercurial thermom.
eters placed side by side, and they ind
cate the same temperature. When fixed
together on a frame they are called the
wet and dry bulb thermometers" The
dry bulb is the common thermometer
intended to show the temperature Of tn,
air. The wet bulb is also a common
thermometer, having its bulb covered
with a piece Of muslin, from which pas.
a few threads Of darning cotton into a
small vessel containing rain water, which
rises by capillary attraction, thus keep.
ing the muslin constantly wet. When
the air is dry evaporation goes on rap.
idly from the muslin, and on account Of
the heat lost by evaporation the wet bulb
indicates a lower temperature than the
dry bulb. But when the air is damp
evaporation is slower, and the difference
between the two thermometers becomes
smaller. When the air is completely
saturated with moisture, evaporation
ceases entirely, and the two thermome.
tiers show the same temperature.
THE BAROMETER.
This is the great instrument with me.
teoroligists, and its fundamental princi.
pie can be learned from the following
experiment. Take a glass tube thirty
three inches in length open at one end,
fill it with mercury, and closing the open
end with the finger invert it and plunge
the open end into a bowl also containing
mercury. The column will fall into the
tube to about thirty inches above the
surface of the mercury in the bowl, if
the experiment be made lea the level
of the sea. The fluid is upheld in the
tube by the air outside of it pressing on
the mercury in the bowl, and since the
one balances the other it is clear that the
mercurial column will seem as an accu-
rate indicator If the varvino nrassura ~