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SCIENCE. 



[N. S. Vol. XI. No. 280. 



the latter science was in connection with 

 the transit of Venus in 1874. He was at 

 that time the coadjutor of Lord Lindsay, 

 afterward Earl of Crawford and Balcarres, 

 who was so much interested in astronomy 

 that he had erected an observatory at his 

 seat, Dunecht, near Aberdeen. Gill was 

 placed in charge of the institution and the 

 works done there were published in the 

 joint names of the two. 



When the transit of Venus of 1874 was 

 approaching, a special private expedition for 

 its observation was fitted out by Lindsay, 

 with Gill as his leading assistant. Here 

 the latter gave the first evidence of that 

 tireless energy which has marked his whole 

 career. Not satisfied with merely execut- 

 ing the work pertaining to the expedition! 

 he organized, under Lindsay's direction, a 

 campaign to determine a chain of telegraphic 

 longitudes from Berlin to Malta, Alexan- 

 dria, Suez and Aden. From here the chain 

 extended to Bombay, in the east, and to the 

 Seychelles, Reunion, Mauritius and Rod- 

 riguez in the south. Thus the expedition 

 in addition to the observations which were 

 its main objects, made an important contri- 

 bution to geodesy and geography. 



It was in connection with the observations 

 of the transit that he made his first appli- 

 cation of the heliometer, an instrument 

 which he has since done so much toward 

 bringing to its present state of perfection. 

 Altliough this instrument had been in use in 

 Germany for nearly half a century, and was 

 celebrated as that with which Bessel had 

 first determined the parallax of a star, as- 

 tronomers were not alive to its possibilities. 

 Its application was greatly restricted by the 

 imperfection of its construction and, al- 

 though one had long been mounted at Ox- 

 ford, little use was made of it in England- 

 In fact its application was limited by a de- 

 fect of its construction. The Repsolds of 

 Hambui-g introduced the important im- 

 provement of making the two divided halves 



of its object glass slide in a circle having 

 its center at the focus, instead of moving in 

 plain guides, as in the former construction. 

 Its range was thus greatly widened and its 

 accuracy increased. 



With the instruments as thus improved 

 Gill, in addition to the observations which 

 were the main object of the expedition, 

 made at Mauritius his first essay in the line 

 which he has since followed with such suc- 

 cess ; the determination of the solar paral- 

 lax by observations of the minor planets. 

 The planet chosen for the purpose was 

 Juno. The method was that of observa- 

 tions as far as possible to the east and west 

 of the meridian, the parallatic effect being 

 obtained by the motion of the observer as 

 he was carried around by the earth's rota- 

 tion, instead of by differences of direction 

 from two distant stations. The result of 

 the observations was 8". 77, which we now 

 believe to be a nearer approach to the truth 

 than was supposed at that time. 



The success of this attempt led our med- 

 allist to inaugurate another on a larger 

 scale in which he was himself the sole actor 

 and mover. This was his celebrated expe- 

 dition to the Island of Ascension in 1877, 

 in order to determine the solar parallax by 

 observations of Mars at its perihelion ap- 

 proach to the earth, which occurred in Sep- 

 tember of that year. The method was the 

 same as at Mauritius, the instrument used 

 being the heliometer, and the parallatic 

 effect being obtained by the difference in 

 the direction of the planet in the morning 

 and in the evening. The result of the work 

 was worked out with that thoroughness 

 which is characteristic of the highest order 

 of astronomical research. One of its most 

 important features was a discussion of the 

 personal error in the right ascension of 

 faint stars depending on their magnitude, 

 an error which had long been suspected, 

 but had evaded all efforts to so determine 

 it that it could be eliminated from the re- 



