* ass, 289] Planetary Theory 371 



in pure mathematics, have been carried out by Hugo Gylden 

 (1841-1896), while one of the most eminent pure mathe- 

 maticians of the day, M. Henri Poincare of Paris, has 

 recently turned his attention to astronomy, and is engaged 

 in investigations which, though they have at present but 

 little bearing on practical astronomy, seem likely to throw 

 important light on some of the general problems of celestial 

 mechanics. 



289. One memorable triumph of gravitational astronomy, 

 the discovery of Neptune, has, been described so often and 

 so fully elsewhere * that a very brief account will suffice 

 here. Soon after the discovery of Uranus (chapter xn., 

 253) it was found that the planet had evidently been 

 observed, though not recognised as a planet, as early as 

 1690, and on several occasions afterwards. 



When the first attempts were made to compute its orbit 

 carefully, it was found impossible satisfactorily to reconcile 

 the earlier with the later observations, and in Bouvard's 

 tables (chapter XL, 247, note) published in 1821 the 

 earlier observations were rejected. But even this drastic 

 measure did not cure the evil ; discrepancies between the 

 observed and calculated places soon appeared and increased 

 year by year. Several explanations were proposed, and 

 more than one astronomer threw out the suggestion that 

 the irregularities might be due to the attraction of a hitherto 

 unknown planet. The first serious attempt to deduce from 

 the irregularities in the motion of Uranus the position of 

 this hypothetical body was made by Adams immediately 

 after taking his degree .(1843). By October 1845 he had 

 succeeded in constructing an orbit for the new planet, and 

 in assigning for it a position differing (as we now know) by 

 less than 2 (four times the diameter of the full moon) from 

 its actual position. No telescopic search for it was, how- 

 ever, undertaken. Meanwhile, Leverrier had independently 

 taken up the inquiry, and by August 3ist, 1846, he, like 

 Adams, had succeeded in determining the orbit and the 

 position of the disturbing body. On the 23rd of the follow- 



* E.g. in Grant's History of Physical Astronomy, Herschel's Out- 

 lines of Astronomy, Miss Clerke's History of Astronomy in the 

 Nineteenth Century, and the memoir by Dr. Glaisher prefixed to the 

 first volume of Adams's Collected Papers. 



