Page:Popular Science Monthly Volume 59.djvu/312

302 particularly valuable for determining the effects of the friction of the ground on the winds, and for testing quantitatively the theories of the air circulation which have heretofore depended mainly on qualitative generalities.

The decrease (and in the abnormal cases, the increase) in temperature with height above the ground has been carefully studied, and especially in the various phases which occur under different typical atmospheric conditions. The diurnal changes of temperature at different altitudes have been also carefully studied. The determination of the numerical values of these elements is very important in helping to complete the theories of the atmospheric circulation, solar insolation, and the transference of heat from the earth to the air.

The rate of change of relative humidity with change of altitude, due to vertical change of temperature, is very important in connection with the calculation of the heights of clouds by computing the altitude of the dew point temperature under known conditions near the ground; and the Blue Hill observations not only offer data for increasing the accuracy of these calculations, but also a criterion for testing their absolute accuracy.

Until 1886 the only weather map in the United States was printed at the Chief Signal Office in Washington, but in May of that year Mr. Rotch with the assistance of Mr. Cole, the government observer in Boston, began to chart the 7 a. m. reports that were received there, and manifolded the map by the cyclostyle process. This was the origin of the daily weather map that is now issued in great numbers from many of the Weather Bureau Stations throughout the United States.

From 1887 until 1891 local weather forecasts were furnished by the Blue Hill Observatory to the Boston press and announced from the observatory by the display of weather signals. These weather predictions were undoubtedly a considerable improvement over those made in Washington, which depended on the weather map alone, especially for the twenty-four hours immediately succeeding the time of observation, and the demonstration of this, in direct competition with the Weather Bureau predictions, probably had some effect in causing the government service to appoint local forecast officials to supplement the general predictions made at Washington. It must be borne in mind in this connection, that this combined method of making weather predictions has been, in a measure, practically carried out in European countries ever since an international telegraphic exchange of weather observations went into effect. In this country, however, we had learned to rely too much on the general predictions issued from Washington.

There can be no doubt that the work of the Blue Hill Observatory has had a very great quickening influence in the recent developments in observational meteorology in this country. Not only has its