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: