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were susceptible of material improvement, and they have now given place to those completed in 1872 by an American astronomer, Professor S. Newcomb, as to which it may be observed that they do not countenance the idea that there exists a transNeptunian planet. Newcomb has also framed Tables of the Satellites of Uranus t.

+ Washington Obs., 1873, Appendix I.

CHAPTER XIV.

NEPTUNE a. Leta

Circumstances which led to its discovery.- Summary of the investigations of Adams

and Le Verrier.Telescopic labours of Challis and Galle.The perturbations of Uranus by Neptune.- Statement of these perturbations by Adams.Period, &c.- Attended by i Satellite.-- Elements of its orbit.Mass of Neptune.Observations by Lalande in 1795.

TORE than half a century ago an able French astronomer, MI M. Alexis Bouvard, applied himself to the task of making a refined investigation of the motion of Uranus, in order to prepare Tables of the planet. He had at his disposal the various observations by Flamsteed and others, made prior to the direct optical discovery of Uranus, and those made by various astronomers subsequent to that event in 1781. In working these up he found himself able to assign an ellipse harmonising with the first series, and also one harmonising with the second ; but by no possibility could he obtain an orbit reconcileable with both. As the less objectionable alternative, Bouvard decided to reject all the early observations and to confine his attention solely to those more recent b. In this way he produced, in 1821, Tables of the planet, fairly representing its motion in the heavens. This agreement, however, was not of long duration, and a few years only elapsed before discordances appeared of too marked a character to be possibly due to any legitimate error in the Tables : constructed in the form in which they existed it was evident that they were defective in principle. Bouvard himself, who died in 1840, seems to have fancied that an exterior planet was alone the cause of the irregularities existing in the motion of Uranus, and the Rev. T. Hussey was led to assert this in decided terms in a letter to Airy in 1834. This conviction soon forced itself on astronomers', and amongst others on Valz, Mädler, and Bessel. Bessel, it would seem, entertained the intention of mathematically inquiring into the matter, but was prevented by an illness, which eventually proved fatal.

* Many French writers deal with the discovery of Neptune in a way that is not fair. Nothing is more common than to meet with a narrative of the incident either without any mention, direct or in. direct, of Mr. J.C. Adams, or with some casual remark more or less implying that the English version is a trumped-up story due to national jealousy, and only in

tended to rob a deserving Frenchman of his share in the honours. Science ought to be international, and to rise above such petty insinuations.

6 A memorable illustration of the folly and impolicy of rejecting any observation, merely because it opposesor seems to oppose--a pre-conceived theory.

Mr. J. C. Adams, whilst a student at St. John's College, Cambridge, resolved to attack the question, and, as he found subsequently, entered a memorandum to this effect in his diary under the date of July 3, 1841, but it was not till January 1843 that he found himself with sufficient leisure to commence. He worked in retirement at the hypothesis of an exterior planet for if years, and in Oct. 1845 forwarded to Airy some provisional elements for one revolving round the Sun at such a distance and of such a mass as he thought would account for the observed perturbations of Uranus. This was virtually the solution of the problem in a theoretical point of view, and it is much to bo regretted that neither the result nor any of the circumstances attending it were made public at the time.

In the summer of 1845, Le Verrier, of Paris, turned his attention to the anomalous movements of Uranus, and in the November of that year published his first memoir to prove that they did not depend solely on Jupiter and Saturn. In June 1846 the French astronomer published his second memoir to prove that an exterior planet was the cause of the residual disturbance. He assigned elements for it, as Adams had done 8 months previously. A copy of the memoir reached Airy on June 23, and finding how closely in accord Le Verrier's hypothetical elements were with those of Adams, which were still in his possession, he was so impressed with the value of both, that on July 9 he wrote to Professor Challis of Cambridge to suggest the immediate employment of the large “Northumberland” telescope in a search for the planet. The proposal was agreed to, and on July 11 a systematic search was commenced. Challis, not being in possession of the Berlin Star Map of the particular locality in which it was supposed that the looked-for planet would be found, was forced to make observations for the formation of a map for himself; this was done, but much valuable time was occupied. When matters had reached this stage Sir J. Herschel seized an opportunity which happened to present itself, and thus addressed the British Association at Southampton on Sept. 10, 1846:The past year has given us the new planet Astræa—“it has done more—it has given us the probable prospect of the discovery of another. We see it as Columbus saw America from the shores of Spain. Its movements have been felt, trembling along the far-reaching line of our analysis, with a certainty hardly inferior to that of ocular demonstration d.” The Map was eventually got ready, but it was not till Sept. 29 that Professor Challis found an object whose appearance attracted his attention, and which was subsequently proved to be the new planet so anxiously sought. It was likewise ascertained afterwards that the planet had been observed for a star on Aug. 4 and 12, and that the supposed star of Aug. 12 was wanting in the zone of July 30. The nondiscovery of its planetary nature on Aug. 12 was due to the fact of the comparisons not having been carried out quite soon enough; a pardonable though regrettable circumstance. It should be added that it was not until Oct. i that Challis heard of Galle's success on Sept. 23. (See post.)

c As far back as October 25, 1800, La. lande and Burckhardt came to the con clusion that there existed an unseen planet beyond Uranus, and they occupied themselves in trying to discover its position. (Year Book of Facts, 1852, p. 282.)

This statement is reputed to depend on a note to this effect found amongst Lalande's papers presented to the Academy of Sciences in 1852, but I am not acquainted with any other authority for it.

In August Le Verrier published a third memoir, containing revised elements, in which particular attention was paid to the

d Athenæum, Oct. 3, 1846, p. 1019.

Se

probable position of the planet in the heavens. On Sept. 23 a letter from him, containing a summary of the principal points of this memoir, was received by Encke of Berlin, whose co-operation in searching telescopically for the planet was requested. The Berlin observers had the good fortune to have just become possessed of Bremiker's Berlin Star Map for Hour XXI. of R.A., which embraces that part of the heavens in which both Adams and Le Verrier expected that the new planet would be found, and resort to this Map was suggested by D'Arrest, then a young student at the Berlin Observatory. On turning the telescope towards the assumed place, Galle, Encke’s assistant, called out the visible stars one by one, and D’Arrest checked them by the Map. After a while Galle saw what seemed to be a star of the 8th magnitude, which was not laid down on the Map. Further observations on Sept. 24 placed it beyond a doubt that this gth magnitude star was in reality the trans-Uranian planet; a discovery, the announcement of which, as may be well imagined, created the liveliest sensation. The French astronomers, with Arago at their head, disputed with unseemly violence the equal claims of Adams to participate with Le Verrier in the honours; but Airy, then Astronomer Royal, laid before the Royal Astronomical Society, on Nov. 13, such an overwhelming chain of evidence in favour of our distinguished countryman's exertions as seems to all impartial minds to have finally settled the questiono.

The intellectual grandeur of this discovery will be best appreciated, so far as a non-mathematical reader is concerned, by placing in juxtaposition the observed longitude of the new planet when telescopically discovered, and the computed longitudes of Adams and Le Verrier.

The foregoing is a very bare outline case will be found stated in Arago's Pop. of the case, which is a most interesting Ast., vol. ii. p. 632 ; the English transone. Grant (Hist. Phys. Ast., p. 165 et lator's notes to the passage are very seq.) gives full particulars; and reference appropriate. A very full statement of may also be made to Month. Not., vol. the facts of the case from a quite recent vii. p. 121, Nov. 1846; Mem. R.A.S., stand-point will be found in an obituary vol. xvi. p. 385, 1847; Atheneum, Oct. notice of Prof. Challis, in Month. Not., 3, 1846; Adm. Smyth's Speculum Hart- vol. xliii. p. 160. Feb. 1883. D'Arrest's wellianum, p. 405; and Sir J. Herschel's share in the work will be found explained Outlines of Ast., p. 533. The French in Copernicus, vol. ii. p. 63, 1882,

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