TEE BLUE HILL OBSERVATORY. 297 



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- 



