The successive years of continuous or hourly observations have per- mitted the determination of the diurnal and annual periods of the chief meteorological elements at the Blue Hill Observatory, summaries of which for several years' averages have been published. The main inter- est in these results centers in the air movements. The constancy of the local amount of wind from hour to hour has been found to be remarkable, and this, in connection with the variability in the hours of maximum and minimum wind, indicates the nearness to the transi- tion altitude where the lower air conditions change to those of the upper air. These observations of wind velocity, coming in as they do at an intervening altitude between those of the ordinary high exposed surface station and the more elevated mountain stations, permitted the dis- covery of the gradual shifting towards noon of the hour of minimum diurnal wind velocity, with the gradual increase in altitude. Thus the least wind occurs at Boston at 5 a. m., at Blue Hill at 8 a. m., on the Eiffel Tower at 10 a. m. and shortly after noon on Mt. Washington and other similar high altitudes.
Much of the well-earned reputation of the Blue Hill Observatory depends on the special investigations which have been conducted by its scientific staff. Some of these are the natural concomitants of the peculiar location of the observatory, while others have been taken up on account of their intrinsic importance to meteorological science; still others combine these two features.
Among the questions taken up for the former reason, the following deserve special mention: The investigation of the normal differences of temperature between the base and the summit of the hill, and between the latter and the neighboring Weather Bureau station in Bos- ton; the investigation of the marked inversions of temperature between the base and the summit stations; experiments on the electrical condi- tion of the atmosphere; and studies of the vertical component of the wind as measured at the observatory.
The Blue Hill series of observations of visibility of more or less distant hills and mountains is very important, although the positive deductions as yet made from the data assembled in regard to this phenomenon are very meager. In general, however, it was found that the summer haze about balanced the winter fogs, so that an annual periodicity is but slightly marked. The diurnal period is also not clearly pronounced.
The location of the Blue Hill Observatory also made it a very desir- able place at which to undertake open air experiments on the absolute and relative accuracy of anemometers. These were very much needed in view of the fact that the old errors of observed wind velocities could no longer be neglected when the comparatively recent quantitative study of the winds was widely taken up; and since much of the investiga-