58 MOXJNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



were found in tlie spectrum of y Argus when its altitude was less than 



6°.* 



The probable-error of a single observation of the velocity in the 

 line of sight of a star like Arcturus, for example, is certainly not above 

 0.35 mile per second. The probable-error of the determination of the 

 wave-length of the chief nebular line is 0.03 tenth-metre, f 



Motion of Nehulce in the line of sight: The determination of the 

 motion of nebulae in the line of sight was first made at Mount Hamil- 

 tion by Dr. Keeler.:}: 



The preceding summary of results actually attained is the best pos- 

 sible testimony to the suitability of the astronomical conditions which 

 prevail at Mt. Hamilton. Almost all departments of practical astron- 

 omy are represented by long series of observations, and in nearly every 

 department the actual achievement is satisfying. 



During the year 1888 I requested Professor Barnard to keep a 

 record of the steadiness (only) of the vision at Mt. Hamilton on a scale 

 of 1 = images extremely unsteady, 3 = average steadiness, 5 = images 

 perfectly steady, and this record will subsequently be published by him. 



Dr. Langley's Expedition to Mount Whitney (14.900 feet). 



The expedition of Dr. Langley to the summit of Mount Whitney 

 in the summer of 1881, presents an excellent example of the advan- 

 tages which mountain -stations sometimes afford for the prosecution of 

 special researches in astronomy or astronomical physics. § 



The particular object of Dr. Langley's expedition was to determine 

 the solar coiistant, that is to evaluate the quantity of radiant heat re- 

 ceived from the sun by the outer layer of the earth's atmosphere in a 

 unit of time (as the quantity falling on each square centimetre per 

 minute). 



This determination involves an investigation of the selective absorp- 

 tion of the earth's atmosphere at two stations near to each other but 

 differing greatly in altitude. And it is further indispensable that the 

 sky should be clear and dry at both stations. 



Mount Whitney in Southern California fulfilled all the required 

 conditions admirably. 



Its summit is 14,900 feet in altitude, so that about one-third of the 

 earth's atmosphere lies beneath its level. The mountain is very 

 abrupt, so that the lower station, at Lone Pine (about 3700 feet), 



* Astronomy and Astrophysics for June, 1894. 

 f Publications of the Lick Observatory, vol. in. 

 X Ibid. 



§ Dr. Janssen's observatioas to detect tlie presence of oxygen in the sun, made 

 at Chamounix and on Mont-Blanc, are a case in point. 



