MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 39 



served that " the sky -glare was much more pronounced than it is at 

 Mount Hamilton in favorable weather." Through the kindness of 

 Professor Bailey, in charge of the observatory, opportunities were 

 given to use the 13-inch equatorial visually. Professor Schaeberle 

 (Eclipse-Report of April, 1893, p. 21) remarks that " the brighter stars 

 were surrounded with many well-defined diffraction-rings,"^ which did 

 not seem to be affected by the apparently great commotion in the stellar 

 disk and rays — a curious kind of rapid twinkling without blurring. 

 On turning to the moon I remarked that the seeing was ' woolly,' the 

 effect of minute atmospheric waves plainly visible in the slight move- 

 ments of the lunar surface. At the Lick Observatory the same quality 

 of seeing would be classed about 4 on a scale of 5 for the best seeing." 

 The conditions were regarded by Professor Bailey as excellent on the 

 nights to which Professor Schaeberle refers. 



Mr. A. E. Douglass, one of the astronomers of the Arequipa Ob- 

 servatory, has a few sentences relating to the conditions affecting tlie 

 vision there in the American Meteorological Journal (vol. II., p. 395) as 

 follows : 



The observatory is situated close to a river valley, down which, on clear 

 nights, a swift stream of cold air descends. This frequently attains such a volume 

 as to flow over the observatory grounds. 



When this cold air reached the [telescope] the seeing was immediately ruined. 



When this current once became established no more good seeing could be ex- 

 pected for the remainder of the night. 



The foregoing comparisons between the best conditions of steady 

 vision at Mt. Wilson, Arequipa, and Mt. Hamilton are interesting ; 

 they are probably not decisive. A comparison of the results of obser- 

 vation at the three places is difiicult to make on many accounts. 

 Another question of importance is the relative number of good ob 

 serving nights (and days) at the three stations. This is a question 

 which can be settled by statistics. 



It is of considerable importance to obtain some definite notion of the 

 relative excellence of observing stations in different parts of the globe. 

 The main factor to be attended to is that of steady definition. It is 

 known, to begin with, that a transparent air can be found in elevated 

 regions nearly everywhere. For an astronomical station steady defini- 

 tion is much more important. 



Where is this condition to be found ? On Etna — in Egypt — in the 



♦Professor Schaeberle tells me that the expression "many" rings is correct. 

 Professor "W. H. Pickering says ten or twelve for bright stars under favorable 

 circumstances. It is difficult to understand how many rings can be seen with a 

 central disk which is quite unsteady. 



