810 



SCIENCE 



[N. S. Vol. XXVI. No. 676 



astronomer, at that time director of the 

 Ann Arbor Observatory. 



Professor Hall's career as an astronomer 

 began at the Harvard Observatory, under 

 William Bond, in 1857. His work there 

 consisted mainly in the routine observa- 

 tory work, but he quickly became an expert 

 in the computation of the orbits of comets 

 and began to show the admirable grasp of 

 mathematical relations which later on made 

 him an authority in problems of gravita- 

 tional astronomy. 



In 1862 he entered the Naval Observa- 

 tory as assistant astronomer and in 1863 

 was appointed professor of mathematics in 

 the United States Navy by President Lin- 

 coln, a position which he retained until 

 retired, under the regulations, in 1891, on 

 the completion of his sixty-second year. 



The thirty years which Professor Hall 

 spent at the Naval Observatory were full 

 of fruitful work, both as an observer and 

 in the higher sphere of mathematical in- 

 vestigation of astronomical phenomena. 

 From 1862 to 1866 his work was that 

 of assistant observer with the 9^-inch 

 equatorial, then considered a very large 

 instrument, and consisted in the main of 

 observations of asteroids and comets. In 

 1867 he was in charge of the meridian 

 circle; from 1868 to 1875 in charge of the 

 9^-inch equatorial; and from 1875 to 1891 

 in charge of the 26-inch equatorial, at the 

 time of its erection the largest refracting 

 telescope in the world. During these years 

 he was the leader in many expeditions to 

 distant parts of the world to conduct ob- 

 servations -of special interest. In 1869 he 

 went to Bering Strait to observe an eclipse 

 of the sun; in 1870 to Sicily to observe an 

 eclipse; in 1874 to Vladivostock to observe 

 the transit of Venus, the voyage being 

 made on the Eearsarge. In 1878 he had 

 charge of an expedition to Colorado to ob- 

 serve the eclipse of the sun in that year; 



and in 1882 he went tp Texas to observe 

 the transit of Venus. 



The contributions of Professor Hall to 

 astronomy were so numerous that a mere 

 enumeration of them would fill a long cata- 

 logue. Working astronomers have been 

 familiar with his papers in the Astfonom- 

 ische Nachrichten, that universal journal 

 of astronomical communication, for half a 

 century. 



His first discovery with the 26-inch equa- 

 torial, which was of great interest, was a 

 white spot on the planet Saturn in 1876, 

 by means of which a new and accurate 

 determination of the rotation period of 

 the planet was made. 



In the summer of 1877, at the time of 

 the near approach of the planet Mars, he 

 made a systematic search for new satellites, 

 which was rewarded by the most interest- 

 ing discovery with which his name is con- 

 nected, that of the two satellites Deimos 

 and Phobos. Up to that time it had been 

 believed that Mars had no moons and the 

 discovery of two companions of this com- 

 paratively well-known planet, one of them 

 revolving around the planet in a period less 

 than one third of the revolution time of 

 the planet itself, came to astronomers al- 

 most as an unwarranted innovation in the 

 solar system. The investigation of the 

 inner satellite has led to the most interest- 

 ing results in the study of the evolution of 

 planets and their satellites. 



Next to these brilliant telescopic discov- 

 eries, the discovery of the motion of the 

 line of the apsides of the orbit of Hyperion, 

 one of Saturn's satellites, was perhaps Mr. 

 Hall 's most remarkable piece of work. 



His long and systematic observations 

 with the great equatorial at Washington 

 were of special value, not only for the great 

 accuracy with which they were made, but 

 also for the admirable way in which they 

 were joined to the work of other observers 

 and made as nearly as possible comparable 



