president's address. 25 



devoted most of his life to the examination of the theories of 

 the movements of the earth and the large planets from Mercury 

 to Neptune. Prof. Newcomb and Mr. G. W. Hill, of Washington, 

 have also been employed on similar researches. The former has 

 exhaustively treated the theories of Uranus and Neptune, and 

 the latter has very recently published an exposition of those of 

 Jupiter and Saturn. These important investigations are the 

 results of the most profound mathematical research based on a 

 comparison of the calculated with the observed places of each 

 planet. 



The difficult problem of ascertaining the distance of the 

 sun from the earth has specially occupied the attention of 

 astronomers during the last thirty years. Several investigations 

 by di:fferent methods have been undertaken, but the most 

 popular was the observation of the transits of Venus across the 

 sun in 1874 and 1882, both of which were utilised for this 

 purpose. Since Dr. Halley in 1716 drew the attention of the 

 Eoyal Society to this question, the transits of Venus have been 

 generally considered to be one of the best methods for determin- 

 ing the value of the solar parallax, or the angle produced by 

 the earth's semidiameter as viewed from the sun. Perhaps not 

 many here to-day are aware that the first voyage of the celebrated 

 Captain Cook was organized principally for obtaining observ- 

 ations of the transit of Venus in 1769, on which occasion he 

 was successful on the shore of the island of Tahiti, still known 

 as Venus Point. When my attention was first directed to 

 astronomy implicit faith was placed in the distance of the sun 

 as determined in 1824 by Encke frOm a discussion of all the 

 observations made of the transits of 1761 and 1769. Prof. 

 Hansen, of Grotha, while employed on his investigations on the 

 lunar theory, found that in order to satisfy the refined observ- 

 ations of the moon made at Grreenwich, it was necessary to make 

 a considerable increase in Encke 's value of the solar parallax, 

 and consequently, a corresponding decrease in the distance of 

 the sun. Le Verrier also found that to reconcile some 

 discrepancies in his planetary theories, a larger solar parallax 

 was required. Some recent determinations of the velocity of 

 light also pointed to the same conclusion. Much was therefore 

 expected from the two transits of Venus in 1874 and 1882, both 



