The amount of moisture present, or the humidity of the air, is determined by a comparison of dry and wet bulb thermometers. They are both ordinary thermometers; but the bulb of the latter is covered with muslin that is wet. In the latest form of instrument the thermometers are mounted on arms carried by a shaft that is rotated by a crank which is geared to the shaft. The motion of the shaft rotates the thermometers in vertical planes and causes the water in the muslin to evaporate more or less rapidly according to the amount of moisture in the air. This evaporation lowers the temperature of the thermometer; and, from tables constructed after long experiments, the degree of moisture can be determined by the difference in temperature-between the two thermometers.
The direction and speed of the wind, the hours of sunshine and cloud, and the amount of rainfall are recorded by the "triple register." The instrument has a cylinder on which is carried a graduated sheet of paper for the record. The cylinder is supported by a horizontal shaft that is turned by clockwork. On the shaft is secured a spiral wire that engages grooves in a bearing of the shaft; so that, as the shaft turns, it and the cylinder are moved along. This would cause a stationary pencil to trace a spiral on the cylinder. The shaft carries two arms parallel to itself that pass
Fig. 9.—The Triple Register. Rear view.
through a yoke which is turned by the clock, this arrangement permitting the shaft to recede from the clock under the action of the spiral. The cylinder revolves four times in twenty-four hours, so that each record passes across the paper four times.
The wind vane is an arrow-shaped vane mounted on a vertical shaft. The tail of the arrow is gradually broadened laterally to give it greater steadiness. A sleeve is fastened on the shaft
