ACADEMY OF SCIENCES. 251 



lengtli of a large number (CO) reaching 1% inches, and a few specimens meas- 

 uring 1% inch. A variety from Sitlva is of a dingy yellow color, (adult?) in- 

 ternally of a brownish yellow, sometimes running into a dark reddish brown ; 

 occasionally a banded variety is met with at all of the localities. The shells of 

 this species are generally prolonged anteriorly, which gives a somewhat acute 

 V-shape to that part of the mouth. 



Comparing the specimens, from Unala«hka to Monterey, including intermedi- 

 ate places, it will be seen that they vary greatly in size, color and general out- 

 line, as do the other species of Purpuridfc found within the same limits. The 

 general variation from the Vancouver form is in the much larger and ventricose 

 body-whorl. 



The President informed the Academy of the anticipated arrival 

 of Professor Agassiz during the current week, and on motion, John 

 Hewston, Jr., Dr. Henry Gibbons and Dr. James Blake were ap- 

 pointed a Committee of Reception on behalf of the Academy. 



Prof. Davidson gave an account of his recent experiments to 

 determine 



The Relative Value of great and small Altitudes for 

 Astronomical Observations. 



BY GEORGE DAVIDSON. 



For this purpose, he occupied a station on the Sierra Nevada at an elevation 

 of seven thousand two hundred feet above the sea, and situated in latitude 39° 

 20', longitude 120^ 20'. The instruments used were those with which he had 

 been making observations for twenty-two years on the Pacific Coast. 



In describing the country, he considered the ridge of the Sierra Nevada 

 peculiarly adapted for the location of an astronomical observatory, and said he 

 had visited two mountain peaks, between nine and ten thousand feet high, near 

 the line of the Central Pacific railroad and telegraph lines. 



The climate is very favorable for astronomical work, and in giving an account 

 of the weather, from meteorological records loaned him by S. S. Montague, 

 Chief Engineer of the Central Pacific Railroad, he showed that an average of 

 two hundred and seventy clear days and nights may be expected in each year ; 

 and that in the clear nights of winter the sky was marvelously clear and the 

 atmosphere remarkably steady. 



At a previous station, Verdi, unfavorably situated for astronomical work, and 

 four thousand eight hundred and seventy feet above the sea, he was able to see 

 the companion of Polaris with a telescope of two inches aperture, twenty-five 

 inches focal length, and a magnifying power of 35. But at the summit station 

 he used a telescope of three (3.0) inches aperture, forty-five inches focal length, 

 and a direct eye-piece with magnifying power of 65, and astronomical eye-piece 

 of poor definition and power, of about 250. The observations were made upon 

 the companion of Polaris, the Moon, Saturn, some double stars, and the Sun. 



Polaris. — Every night for a week the companion was visible, not only to his 



