94 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



were too great. At times, when the vibration or undulation 

 of the disc of the Moon was not rapid, the sharp, solid 

 limb could be observed even with a faint, spurious, vibrat- 

 ing outline beyond it. In i889-'90, during lunar transit 

 observations for longitude at San Francisco, our party had a 

 case of spurious disc from excessive diffusiveness, with a pre- 

 dicted error in the resulting longitude that was verified 

 upon reduction. This clearly indicated that even the offi- 

 cial empirical correction may frequently be too small; 

 while in times of supreme steadiness of the atmosphere and 

 resulting sharpness of the limb of the Moon, this correction 

 is not required. 



If this factitious diameter belongs to the Moon, the Sun 

 is much more likely to carry a more pronounced factitious 

 diameter, and every observer must have frequently watched 

 its wildly boiling limb flaring and leaping across the threads 

 or rolling forward like waves of flame along the horizontal 

 thread. This factitious border is in effect tacitly acknowl- 

 edged in the American Ephemeris above referred to, where 

 "the adopted semi-diameter of the Sun at the Earth's mean 

 distance is 16' 02". In the computations pertaining to 

 eclipses, Bessel's semi-diameter 15' 59". 788 has been 

 adopted." It appears to us that the explanation in the 

 American Ephemeris of the cause of the acknowledged 

 spurious diameter of the Moon is erroneous. 



Great Elevations for Observation. 



Although our experience has led us to advocate the selec- 

 tion of great elevations for astronomical observations of 

 precision and research, yet elevation alone is not a panacea. 

 The locality must present other favorable orographical con- 

 ditions. Unfavorable conditions are great gulches, canons, 

 or narrow valleys lying directly under a summit and leading 

 to it. These gulches become filled with highly heated air 

 during two or three days of clear, calm weather, and the 

 first wind from below drives this heated air over the summit 

 and spreads stars out into unsteady nebulous films; the 

 the rings of Saturn become woolly girdles, the crape ring is 



