554 ADDITIONS. 



omers of all future time. The work appeared in 1845 ; the expense 

 of the compilations and the publication being- defrayed by the British 

 Government. 



The Discovery r>f Neptune. 

 The theory of gravitation was destined to receive a confirmation 



/ O 



more striking than any which could arise from any explanation, how- 

 ever perfect, given by the motions of a known planet ; namely, in re- 

 vealing the existence of an unknown planet, disclosed to astronomers 

 by the attraction which it exerted upon a known one. The story of 

 the discovery of Neptune by the calculations of Mr. Adams and M. Le 

 Verrier was partly told in the former edition of this History. I had 

 there stated (vol. ii. p. 306) that "a deviation of observation from the 

 theory occurs at the very extremity of the solar system, and that 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 forma- 

 tion of these Tables offers to us this alternative, that we cannot satisfy 

 modern observations to the requisite degree of precision without ma- 

 kino- our Tables deviate from the ancient observations." But when we 



O 



have done this, there is still a discordance between the Tables and the 

 more modern observations, and this discordance goes on increasing. 

 At present the Tables make the Planet come upon the meridian about 

 eight seconds later than he really does. This discrepancy has turned 

 the thoughts of astronomers to the effects which would result from a 



O 



planet external to Uranus. It appears that the observed motion would 

 be explained by applying 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. 



I added, "M. Le Verrier (Comptes Remdus, Jan. 1, 1846) and, as I 

 am informed by the Astronomer Royal, Mr. Adams, of St. John's Col- 

 lege, Cambridge, have both arrived independently at this result." , 



To this Edition I added a Postscript, dated Nov. 7, 1846, in which 

 I said : 



" The planet exterior to Uranus, of which the existence was inferred 

 by M. Le Verrier and Mr. Adams from the motions of Uranus (vol. ii. 

 Note (L.) ), has since been discovered. This confirmation of calcula- 

 tions founded upon the doctrine of universal gravitation, may be looked 

 upon as the most remarkable event of the kind since the return of 

 Halley's comet in 1757 ; and in some respects, as a more striking event 



