THE WINDS OF THE GLOBE
Anemographs. --For a detailed study of atmospheric circulation instruments are necessary that will record both the direction and the velocity of the wind. For simply indicating the direction, wind vanes are almost universally employed. In Fig. 58 is shown a vane designed to record the direction of the wind. The vane itself is supported on friction rollers, shown at h in the detailed drawing. A slender rod supported by the same rollers extends downward inside the support, and has clamped upon it at e four cam collars, so arranged that with the vane pointing toward one of the cardinal points of the compass an electric circuit is closed through the corresponding spring. The cams overlap and are so set that when the wind vane points NW, NE, SE, or SW, the circuit is closed through two springs. A circuit closer on the clock of the register, shown in Fig. 59, Weather Bureau meteorograph, completes the circuit through the proper magnet or magnets once each minute, and the record shown at a, a, Fig. 60, results. The record sheet is wrapped around the register cylinder, as shown in Fig. 59, and the cylinder is revolved by the register clock at such a rate that the transverse parallel lines on the record sheet represent five- minute intervals of time. The position of the dots at a, a, Fig. 60, represents the direction from which the wind was blowing. Thus, the record shows that the wind was quite steady from the south until 4.35 P.M., when it changed successively to the SE, E, NE, N, and NW, and then back to the N, NE, and E. These variations in direction were caused by the passage of a thunderstorm. The record at c shows that the sun was shining continuously until 3.11 P.M., and the record at d that rain commenced at 4.41 P.M., and continued until 5.23 p.m.
Anemometers. --A great variety of instruments are available for recording the velocity of the wind. A common form is the Robinson anemometer shown in Fig. 58. It consists of four hemispherical cups attached to cross arms and mounted on a spindle, near the lower end of which is an endless