Page:Popular Science Monthly Volume 53.djvu/330

314 is indicated by the shaded areas. It can be observed how the winds blow with the hands of a watch around the high, which is central over New Mexico, and in the opposite direction about the low, which is central over Sault Ste. Marie. The temperature is higher to the east of the low, and falls behind the low and before the high in their eastward progress. The rain is seen accompanying the low and to the east of it.

For the purposes of forecasting it is necessary to know the barometric pressure, direction of the wind, and state of the skies. All this information is telegraphed to the Weather Bureau twice daily from



each of the stations of observation, and all other telegraphic business must give way to this for the time being. Each message is, by means of a cipher, usually expressed in ten words or less. The observations are taken at 8 and 8, Eastern time. The results are exchanged with the Canadian Weather Bureau. The principal stations are provided with recording instruments, so that a continual record is kept of all these features of the weather. While the instruments appear complicated, they are based on principles that are easy of comprehension.

The barograph consists of a series of corrugated sheet-metal