54 MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



is not so simple, and that other conditions — perhaps many other — enter 

 as efficient factors. Some of these factors are elsewhere considered. 



Tlie effect of wind on good vision at Mi. Hamilton : In compara- 

 tively level regions (as at Caroline Island in the midst of the ocean, at 

 Washington, and at Madison), my experience has been that winds, even 

 high winds, did not affect the seeing unfavorably. The general hori- 

 zontality of air-strata of equal temperature is not affected under such 

 circumstances, and local pockets of hot or cold air are broken up. 



In his Report of 1879 Professor Burnham concludes that high 

 winds at Mt. Hamilton do not affect the seeing unfavorably. My 

 own experience does not agree with this conclusion. High winds at 

 Mt. Hamilton (owing to the topography) produce currents of very 

 varying directions, and prevent a stable arrangement of the air-strata, 

 and the effect on the vision is, in my experience, often quite marked, 

 and this without any exception that I can recall. 



Vision at Mt. Hamilton during the presence of auroras : Professor 

 Campbell has shown that the auroral line is pretty constantly 

 present in the sky-spectrum, though auroras are seldom visible (here) 

 to the eye. 



Whenever they are so visible, the images of stars are invariably bad. 

 This connection was even more obvious at the Washburn Observatory. 

 It was easy to predict from a peculiar appeai'ance of stars in the 15|- 

 inch equatorial that my assistant would see an aurora from the north 

 window of the dome, and the predictions were made and tested on 

 scores of occasions. At Mt. Hamilton visible auroras, intense neuralgic 

 headaches, and poor stellar images seem to depend on one and the 

 same cause. 



Winter observing lueather at Mt. Hamilton: The conditions during 

 Mt. Hamilton winters are so markedly inferior to those which prevail 

 during the summer, that the observers here are apt to underrate 

 them. In a general way I think it is true that the winters at 

 Mt. Hamilton afford as many clear days and as many days of good 

 steady vision as those of Madison, Ann Arbor, or Cambridge. One 

 winter may dift'er greatly from another in this respect, but on the 

 average the foregoing statement (which is based on impressions and 

 not on statistics) will probably hold good. 



Comparison of different years at Mt. Hamilton : A glance at the 

 complete meteorological statistics of Mt. Hamilton (elsewhere 

 printed), will exhibit the great difference between different years 

 taken as a whole. The summers are apt to be much alike, though 

 they, also, vary. The winters vary in an extraordinary fashion. The 

 total snow-fall of the winter of 1889-90 was about 12 feet ; that of 

 1890-91 about 14^ inches. The average yearly rainfall is 33.18 inches. 



