ADVANCE OF ASTRONOMY. 185 



nomical eye) in harmony with its theoretical path as 

 deduced from the gravitation-formula. To explain 

 these puzzling discrepancies of orbit, it was suggested 

 by several astronomers that they must be due to the 

 influence of some undetected exterior body. But 

 precision was first given to this suggestion in 1845, 

 when John Couch Adams succeeded in calculating 

 out the probable mass and position of the hypothetical 

 planet. In the same year Leverrier (b. 1811) began 

 a similar quest by a different method ; in 1846 he de- 

 termined the probable position of the supposed cause 

 of the disturbance; in the same year he announced 

 that it should be visible in a certain place. He wrote 

 to Galle of the Berlin observatory, told him where to 

 look, and Xeptune was discovered. In the same 

 month (September, 1846) the discovery was con- 

 firmed by Challis of Cambridge, who had been ad- 

 vised by Adams. It is almost needless to remark 

 on the importance of the discovery as a confirmation 

 of the gravitational formula ; here, if anywhere, the 

 exception proved the rule. It should be noted, how- 

 ever, as S. C. Walker first showed, that Xeptune 

 had been observed as a fixed star by Lalande in 1795, 

 and furthermore that " the planet was found to have 

 a different orbit from that assigned by the calculators. 

 Their (hypothetical) planets were not identical, nor 

 were they the (real) planet Xeptune. But they 

 must ever have credit for the sagacity and ability 

 with which, aiming at so indefinite a target, they so 

 nearly struck the centre." * 



The prophetic recognition of the existence of Nep- 

 tune and its verification may be taken as one of the 



*E. B. Kirk. Article, Astronomy, Chambers's Encyclo- 

 padia, vol. I. p. 528. 



