Fig. 14.
The following figure (Fig. 15) presents samples of barograph curves traced at Harvard College Observatory, Cambridge, Mass., during Feb. 22-28, 1887, and May 17-23, 1887. The February curve illustrates well the large and irregular fluctuations in pressure, characteristic of our winter months; while the May curve shows clearly the more even quality of the pressure changes in our summer.
The anemometer shown in Fig. 16 is the most generally used of instruments designed to measure wind velocity. It is known as the Robinson cup anemometer, and consists of four hollow hemispherical
cups upon arms crossed at right angles, and all facing the same way around the circle. The cross-arms are fixed upon a vertical axis having an endless screw at its lower end. When the cups move around, the endless screw turns two dials which register the number of miles traveled by the wind. The Weather Bureau pattern of anemometer has the dials mounted concentrically, the outer dial having
100, and the inner, 99 divisions. The revolutions of the outer dial are recorded on the inner one, and in making an observation of the number of miles traveled by the wind, the hundreds and tens of miles are taken from the inner dial, and the miles and tenths from the outer one. Take from the inner scale the hundreds and tens of miles contained between the zero of that scale and the zero of the outer one. Take on the outer scale the miles and tenths of miles contained between the zero of that scale and the index point of the instrument. The sum of these readings is the reading of the instrument at the time of the observation.
Fig. 15.
Fig. 16.
Wind velocities are recorded in miles per hour. The velocity of the wind at any particular moment is found by noting the number of miles and tenths of miles recorded by the index before and after an interval of one minute, or of five minutes, and multiplying this rate by 60 or by 12 as the case may be. This gives the number of miles an hour that the wind is blowing at the time of observation.