Until 1891 the weather service was a part of the Signal Corps of the Army, and the chiefs were, in the order of service, Gen. Albert J. Meyer, Gen. Wm. B. Hazen and Gen. A. W. freely. It then became a bureau in the Agri cultural Department, with Prof. Mark W. Har rington chief. Harrington served four years, and was succeeded by Prof. Willis L Moore, who directed its affairs for 18 years. The latter was followed by Prof. Charles F. Marvin, the present chief.
How a Weather Map is mom ing at eight o'clock, 75th meridian time which, by the way, is about seven o'clock at Clzi cago, six o'clock at Denver, and five o'clock at San Francisco — the observers at 20() sta tions distributed throughout the United States and the West Indies take their observations, and, with the aid of carefully tested instru ments, note the pressure of the air, the tem perature, the humidity, the rainfall or snowfall, and the cloudiness. During the next 40 minutes these observations are speeding to their destina tions, each station contributing its own observa tion, and important stations receiving in return such observations from other stations as they may require in the making of maps and fore casts.
At the Central Office in Washington as fast as the reports come from the wires they are passed to the Forecast Division, where a force of clerks is engaged in making represen tations of the geographical distribution of the different meteorological elements. One clerk constructs a chart showing changes in temper ature during the past 24 hours. Broad red lines separate the colder from the warmer regions, and narrow red lines enclose the areas showing changes in temperature of more than 10'.
A second chart shows the changes that have occurred in the barometer during the past 24 hours. As in the construction of the temper ature-change chart, broad lines of red separate the regions of rising barometer from the regions of falling barometer. Narrow fines enclose the regions where the change has been greater than one-tenth of an inch; inside of these lines other lines enclose the areas where the fall has been two-tenths, and so on. Here, for instance, throughout a great expanse of territory, all the barometers are rising—that is to say, that throughout this region the air is cooling and contracting, and therefore allowing that of ad jacent warmer regions to flow in at high levels.
Thus the total quantity of air resting on any given area at the ground is increased, and hence the barometers stand correspondingly higher. Over another considerable area the barometers are falling, as a result of the air above them flowing away to cooler regions. This chart indicates whether or not the storm centres are increasing or decreasing in intensity, and it gives, in a great measure, the first indications of the formation of storms.
A third chart shows the cloud areas, with the kind, amount and direction of clouds at each station.
A fourth chart, called the general weather chart, shows for each station the air temper ature and pressure, the velocity and direction of the wind, the rain or snowfall since last re port, and the amount of cloudiness. This is the principal chart from which the forecaster makes predictions. The readings of the barometer on this chart are reduced to sea-level, so that vari ations in pressure due to local altitudes may not mask and obscure those due to storm for mation. Then lines, called isobars, are drawn through places having the same pressure. By drawing isobars for each difference of pressure of one-tenth of an inch the high- and the low pressure areas are soon enclosed in their proper circles. The word •high* is written at the centre of the region of 'greatest air pressure.
sad the word glow* at the centre of the region of least pressure. The air flows from a region of great pressure toward one of less, the veloc try depending upon the difference in pressure distance. arrows fly with the wind, and, as will be seen by reference to any weather map, are almost without exception moving indirectly to ward the low or storm centre, and outward from the high.
A new weather map is made every 12 hours, sad when severe cold waves, dangerous marine swains, heavy snows or floods are threatened, 'penal reports are ordered to be sent to the Cen tral Office every tour hours from the danger region and some distance to the east of it.
ough the frenuent use of the telegraph and the telephone, and by maps and bulletins, the wireless and other agencies, every community in the United States is warned of impending seethes- changes that may be dangerous to it or harmful to its interests.