The close accordance exhibited between the results arrived at by Mr. Adams and 31. Le Verrier, independently of each other, afforded strong evidence in favour of the theory on which they were founded. Mr. Airy was consequently induced to suggest to Professor Challis, of Cambridge, the desirableness of instituting a search for the planet with the large telescope of the Observatory, known as the Northumberland refractor. Professor Challis adopted the views of the Astronomer Royal, and commenced a systematic search for the planet on the 29th of July.
On the 31st of August, 1840, M. Le Verrier communicated to the Academy of Sciences an account of the third part of his researches on the theory of Uranus. The following are his final results in regard to the orbit of the unknown planet :— of the orbit . . . . 36.154 Sidereal revolution . . . . 217'387 years.
Exeentticity . ..... 0'10761 Longitude of the perihelion . . . . 204' 45' Mean longitude, January 1, 1817 . . 47' True heliocentric longitude, January 1, 1847 . 320 32' Distance from the sun, January 1, 1817 . . . 33.06 On the 2nd of September, 1846, 3Ir. Adams communicated to the Astronomer Royal the final results of his researches on the disturbing planets. On this occasion he diminished the hypothetic value of the mean distance from 38 to The following are the values of the other elements as deduced by him.
Mean longitude, October 1, 1846 . . . 323' 2' Longitude of the perihelion . . . . . 299' 11' Excentrieity . . ..... 0•12002 Mass . 0.00015003 On the 18th of September, 1846, M. le Verrier addressed a letter to Professor Encke, the Director of the Berlin Observatory, announcing to him the results of his researches on the trans-Uranian planet, and requesting him to institute a search for it in the region indicated by theory. This letter was received at Berlin on the 23rd of the month. On the same evening, Dr. Galle, Professor Encke's assistant, examined the regions of the heavens indicated by 3I. Le Verrier, as comprising the locus of the disturbing planet, with the assistance of the corres ponding map of the Berlin Academy. which had just been engraved, and he found that a star of the eighth magnitude, situate very near the place assigned by M. Le Verrier to the disturbing planet, was not laid down on the map. Au examination of the same region on the following evening established beyond doubt the interesting fact that the object thus observed was the planet. The following comparison
will serve to show how nearly 31. Le Verrier approximated in his researches to the actual position of the planet : True heliocentiie longitude, September 23, 1846 . 57' Longitude assigned by the theory of M. Le Varier . 326 Difference between observation and theory . 57' Professor Challis, who had commenced a systematic search for the planet, as early as the 29th of July, first received information of the discovery of the planet by Galle, on the 1st of October. He had already noted down the positions of 3150 stars comprised in the region which he proposed to explore. He now proceeded to discuss the results, iu order to ascertain whether they included any places of the disturbing planet. It appeared from this examination that he had determined the position of the planet on the evening of August 4, and also on August 12, and it consequently followed that his plan of search, when carried into complete effect, would have infallibly led to the discovery of the planet. lie also observed a star on the 29th of September, which seemed to have a disk, but unfortunately he did not establish the real nature of the object, which turned out afterwards to be the planet.
By the unanimous concurrence of astronomers, the new member of the planetary system has been called Neptune. It is plain from the brief account which we have given, that its theoretical discovery is (Inc to the independent researches of Mr. Adams and 31. Le Verrier. It is true that the optical discovery of the planet was effected solely by the aid of 31. Le Verrier's indications, but it is equally true that the same object might have been achieved by means of Mr. Adams' theoretical results ; and indeed we have seen that Professor Challis obtained two positions of the planet, solely by relying on the latter.
Soon after the discovery of the planet Neptune, it was found that it had been observed as a star by Lalande, on the 10th of May, 1795. This circumstance enabled astronomers to determine the orbit of the planet with a greater degree of precision than could otherwise have been expected until after the lapse of several years. The following elements of the orbit of the planet are due to the late Mr. Walker, of Washington, U. S.