ADVANCE OF ASTRONOMY. 297 



It will be remembered that at the latter part of the eighteenth 

 century William Herschel had immortalized himself by the dis- 

 covery of a great planet, to which was presently assigned the name 

 of Uranus. After the movements of Uranus had been carefully 

 studied, it was found that on many previous occasions Uranus had 

 been unwittingly observed by astronomers, who regarded it as a 

 star. "When these observations were all brought together, and 

 when the track which Uranus followed through the heavens was 

 thus opened to investigation, it was found that the movements of 

 th« planet presented considerable anomalies. The planet did not 

 move precisely as it would have moved had it been subjected solely 

 to the supreme attractive power of the sun. Astronomers are, of 

 course, accustomed to irregularities of this description in the move- 

 ments of the planets. These irregularities have as their origin 

 the attractions of the various other members of the solar system. 

 It is possible to submit these attractions to calculation and thus to 

 estimate their amount. The effect, for instance, of Saturn in dis- 

 turbing Jupiter can be allowed for, and the nature of Jupiter's 

 motion as thus modified can be precisely estimated. In like man- 

 ner, the influence of the earth on Yenus can be determined, and so 

 for the other planets; and thus, generally speaking, it was found 

 that when the proper allowances had been made for the action of 

 known causes of disturbance, then the calculated movement of each 

 planet could be reconciled with observation. 



The circumstances of Uranus were, however, in this respect 

 wholly exceptional. Due allowance was first made for the attrac- 

 tion of Uranus by Saturn, and for the attraction of Uranus by 

 Jupiter, as well as by the other planets. It was thus found that 

 the irregularities of Uranus could be to some extent explained, but 

 that it was not possible in this manner to account for those irregu- 

 larities completely. It was therefore evident that some influence 

 must be at work affecting the movement of Uranus, in addition 

 to those arising from any planet of which astronomers hitherto had 

 cognizance. The only available supposition would be that some 

 other planet, at present unrecognized, must be in our system, and 

 that the attraction of this unknown body must give rise to those 

 irregularities of Uranus which remained still outstanding. 



A great problem was thus proposed for mathematicians. It 

 was nothing less than to affect the determination of the orbit and 

 the position of this unknown planet, the sole guide to the solution 

 of the problem being afforded by the discrepancies between the 

 places of Uranus as actually observed and the places which were 

 indicated by the calculations, when every allowance had been made 

 for known causes. The -problem was indeed a diflicult one, but, 



VOL. LVI. — 24 



