M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 95 



invisible and the details of the belts of Jupiter are indistin- 

 guishable. And in such a location during daytime, obser- 

 vations of precision on the Sun are absolutely useless. 

 The seeing upon a mountain peak may be generally good 

 when there is no snow on the surface; and even remarkably 

 good with snow, when the temperature is low and the wind 

 is strong; but when the Sun shines and the temperature 

 rises, and rapid evaporation takes place with light airs, the 

 seeing becomes very bad. (Experience at 10,450 to 7,250 

 feet elevation.) 



On Mt. Conness, in the Sierra Nevada (12,566 feet ele- 

 vation), the geodetic heliotrope images were perfect until 

 the afternoon Sun shone upon the 2,300 feet nearly vertical 

 western face of that curious buttress; and then the uprising, 

 irregularly warmed atmosphere caused the images to become 

 unsteady. During the quieter moods of the atmosphere, 

 we observed Polaris for azimuth in the middle of the day 

 very satisfactorily with a telescope that would not show it 

 in daytime at lower elevations or under less favorable 

 conditions. 



In 1872 we experimented in the Sierra Nevada at ele- 

 vations from 7,200 to 9,500 feet with remarkable success; 

 while another party of the United States Coast and Geo- 

 detic Survey sent for the same purpose to Sherman, 8,300 

 feet elevation, in the Rocky Mountains, had at times a very 

 unsteady atmosphere. The Lick Observatory at Mount 

 Hamilton, in the Diablo Range, at 4,209 feet elevation, is 

 surrounded by unfavorable orographical conditions, espe- 

 cially for observations of precision in daytime. 



It would appear that Newton attributed the apparent 

 unsteadiness of celestial objects to the disturbance of our 

 own atmosphere. "Telescopes," he writes, "cannot be 

 formed so as to take away that confusion of rays which 

 arises from the tremor of the atmosphere. The only 

 remedy is a most serene and quiet air. Such as may per- 

 haps be found on the tops of the highest mountains above 

 the grosser clouds." 



(3) November ax, 1900. 



