The most generally used hygrometer consists of two ordinary
thermometers, the bulb of one being covered with a piece of muslin and
kept constantly moistened with water by means of a wick or cotton thread
communicating with a container of water. The difference in the readings
of the two thermometers, the wet and the dry, is observed, and knowing
this, it is very easy to determine the humidity by consulting a table
(see table on pages 58–59), which has been prepared for this purpose.
These instruments are, according to the increase in price, equipped with
a table, and the container is held in a wire frame, as you will see from
the Figs. 49–50 showing the standard Weather Bureau station instrument
and the Gilbert hygrometer.
Fig. 49 shows the U. S. Standard Weather Bureau Station Rain Gauge, Fig.
50 the Gilbert Rain Gauge and Fig. 51 the U. S. Standard Weather Bureau
Station Rain Gauge, Tipping Bucket Type.
The Gilbert Weather Station is equipped with the Tipping Bucket Type
Rain Gauge. Fig. 51 shows the apparatus clearly, complete and mounted
ready for use. The brass bucket seen in position through the open door
is adjusted to tip for each hundredth inch of rainfall collected in the
twelve-inch diameter receiver at the top, and this rainfall is
electrically recorded at any convenient distance on a register. After
any desired period the water may be drawn off and check measurements
made by means of the brass measuring tube and graduated cedar stick
shown in the figure.
THE GILBERT RAIN GAUGE (Fig. 50).
(_a_) Tube.
(_b_) Funnel.
(_c_) Measuring stick.
The essential parts of the Gilbert Rain Gauge consists of a metal tube
twelve inches long, having a diameter of 1⁵⁄₁₆ inches (inside) and a
funnel-shaped top, the neck of which fits snugly into the open end of
the metal tube. The outside diameter of the neck of the funnel is a
trifle less than 1⁵⁄₁₆ inches. The area of the circle formed at the top
of the tube is one-tenth the area of the funnel circle. A measuring
stick is provided to measure the rainfall collected in the tube.
To determine the amount of rainfall on the surface of the ground, the
rain collected in the tube should be measured at regular intervals,
usually twelve hours apart. For every inch of rain collected in the
tube, as denoted by the measuring stick, it means that there is one
one-tenth of an inch of rain on the ground; if 10 inches of rain in the
tube, it signifies one inch of rain on the ground. In other words,
divide the figure recorded on the measuring stick by ten for actual
rainfall.
It is well to put some sort of a shelter around the gauge, so that it
will be protected from strong winds. The shelter is usually placed at a
distance from the tube equal to the height of the tube. With the Gilbert
rain gauge it is well to erect the shelter at a distance of about three
feet from the tube. It is essential that the gauge be held in an upright
position, so it should be fastened to the roof.
Snow is measured by melting the quantity collected in the gauge and
follow the same procedure as in rainfall measurements.
There is another very common method, called ground measurement. There
are many instances where ground measurements are inaccurate: