306 NOTES TO BOOK VII. 



parison with Theory, now going on at Greenwich Observa- 

 tory, under the direction of the Astronomer Royal, shall 

 have been carried nearer to its completion. 



The other deviation of observation from the theory 

 occurs at the very extremity of the Solar System, and its 

 existence appears to be beyond doubt. Uranus does not 

 conform to the Tables calculated for him on the theory 

 of gravitation. In 1821, Bouvard said in the Preface 

 to the Tables of this Planet, " The formation of these 

 Tables offer to us this alternative, that we cannot satisfy 

 modern observations to the requisite degree of preciseness 

 without making our Tables deviate from the ancient obser- 

 vations." But having done this, there is still a discord- 

 ance between the Tables and the more modern obser- 

 vations, and this discordance goes on increasing. At 

 present the Tables make the Planet come upon the 

 meridian about 8 seconds later than he really does. This 

 discrepance has turned the thoughts of astronomers to 

 the effects which would result from a planet external to 

 Uranus. It appears that the observed motion would be 

 explained by supposing a planet at twice the distance of 

 Uranus from the Sun to exercise a disturbing force, and 

 it is found that the present longitude of this disturbing 

 body must be about 325 degrees (Le Verrier, Comptes 

 Rendus, Jan. 1, 1846, and, as I am informed by the Astro- 

 nomer Royal, Mr. Adams, of St. John's College, Cam- 

 bridge, have both arrived independently at this result). 



(M.) p. 241. It is however worthy of record that the 

 discovery of the new planets was not altogether accidental. 

 or at least not unexpected. The German astronomers 

 had long anticipated that a planet would be found between 

 Mars and Jupiter, on the ground of the following law. 



