424 HISTORY OF SCIENCE. 



degree modified; another, that some comet might have interfered with 

 the regular movement of the planet ; and so on. Bouvard himself 

 was inclined to believe that an exterior planet was the sole cause of 

 the irregularities of the motion of Uranus, and some other astronomers 

 of eminence shared this conviction. The question was in this state 

 when, in 1841, MR. JOHN COUCH ADAMS, then a student of St. John's 

 College, Cambridge, resolved to investigate the questipn ; but it was 

 not until 1843, after he had taken his degree, that he found leisure to 

 carry this resolution into effect. He worked at the problem for nearly 

 two years, adopting the hypothesis of an exterior planet, and in October, 

 1845, he sent to Mr. Airy, the Astronomer Royal, the provisional 

 elements of a planet revolving round the sun in an orbit outside of that 

 of Uranus, such as would account for the perturbations of Uranus. 

 Adams' results were unfortunately not published at the time ; and al- 

 though he had indicated in a communication to the astronomers of the 

 Royal Observatory at Greenwich the approximate position of the planet, 

 with the hope that it would be sought for, it appears that this was not 

 done until the announcement of the results arrived at by the indepen- 

 dent calculations of the Frenchman LE VERRIER. It was in 1845 that 

 Le Verrier began his calculations, and on the ist of June, 1846, he 

 was able to announce positively to the Academy of Sciences that an 

 exterior planet exercising a perturbing action on Uranus actually 

 existed, and that on the ist of January, 1847, its longitude would be 

 325, without the possibility, in assigning this position, of an error 

 greater than 10. But in order that this planet might be observed, it 

 was necessary to define its position within narrower limits, and Le 

 Verrier accordingly undertook this further determination. In three 

 months he accomplished a most laborious calculation, and on the 3 ist 

 of August, 1846, he presented to the Academy a second paper, in 

 which he gave the approximate values of the elements of the as yet 

 hypothetical planet. He fixed its longitude more definitely, and stated 

 that it must be sought for at so many degrees from a certain fixed star. 

 On the 1 8th of September, 1846, Le Verrier communicated the result 

 of his latest calculations to the astronomers of the Berlin Observatory. 

 The letter reached its destination on the 23rd of September, and by 

 a curious coincidence Galle, the Berlin astronomer, had been examin- 

 ing a new. and very complete map of the stars of the very region of 

 the sky in which Le Verrier had indicated. In the evening the tele- 

 scope was directed towards the position named by Le Verrier, and 

 there was at once seen a small star, of an aspect different from the 

 surrounding stars, and not shown in the map. The next evening the 

 position of the star was found to have changed precisely as the theory 

 of the French astronomer required. Information of these observations 

 was at once sent to Le Verrier, who on the 5th of October had the 

 gratification of announcing to the French Academy of Sciences the 

 complete verification of his conclusions. 



