62 DISCOVERY OF NEPTUNE. SECT. VIII 



In fact, the periodic time of Uranus is about 84 years, and, as the 

 discrepancies in his motions increased slowly and regularly, it- 

 was evident that it would require a planet with a much longer 

 periodic time to produce them moreover, it was clear that the 

 new planet must be exterior to Uranus, otherwise it would have 

 disturbed the motions of Saturn. 



Another circumstance tended to lessen the difficulty ; the 

 latitude of Uranus was not much affected, therefore it was con- 

 cluded that the inclination of the orbit of the unknown body 

 must be very small, and, as that of the orbit of Uranus is only 

 46' 2S"'4, both planets were assumed to be moving in the plane 

 of the ecliptic, and thus the elements of the orbit of the unknown 

 planet were reduced from six to four. Having thus assumed 

 that the unknown body was revolving in a circle in the plane 

 of the ecliptic, the analytical expression of its action on the 

 motion of Uranus, when in numerous points of its orbit, was 

 compared with the observed longitude of Uranus, through a 

 regular series of years, by means of which the faulty elements 

 of the orbit of Uranus were eliminated, or got rid of, and there 

 only remained a relation between the mass of the new planet and 

 three of the elements of its orbit ; and it then was necessary to 

 assume such a value for two of them as would suit the rest. 

 That was accomplished so dexterously, that the perturbations of 

 Uranus were perfectly conformable' to the motions of Neptune, 

 moving in the orbit thus found, and the place of the new planet 

 exactly agreed with observation. Subsequently its orbit and 

 motions have been determined more accurately. 



The honour of this admirable eifort of genius is shared by Mr, 

 Adams and M. Le Verrier, who, independently of each other, 

 arrived at these wonderful results. Mr. Adams had determined 

 the mass and apparent diameter of Neptune, with all the circum- 

 stances of its motion, eight months before M. Le Verrier had 

 terminated his results, and had also pointed out the exact spot 

 where the planet would be found ; but the English observers 

 neglected to look for it till M. Leverrier made known his re- 

 searches, and communicated its position to Dr. Galle, at Berlin, 

 who found it the very first night he looked for it, and then it 

 was evident that it would have been seen in the place Mr. Adams 

 had assigned to it eight months before had it been looked for. So 

 closely did the results of these two great mathematicians agree. 



