R. A. and slightly to the N., there is a double star whose components are of mags. 16 and 17 [=13 and 136 of Argelander's magnitudes], and 3" apart. No instrument incapable of shewing these two stars is suitable for observing the satellites of Uranus. In fact Sir John remarked that in comparison with the Uranian satellites these two stars are "splendid objects m'

Under these circumstances I shall be pardoned if I omit the details of the observations made by Sir William Herschel", his son, Lamont, O. Struve, and Lassell, more especially as the substance of them has been reproduced by Hind' and Arago". Suffice it then to remark that, according to Lassell, Ariel and Umbriel are of nearly equal brightness, whilst Titania and Oberon are both much brighter than the 2 innermost satellites.

Fig. 123.

Under date of Jan. 11, 1853, Lassell said he was fully persuaded that either Uranus has no other satellites than these 4, or if it has, they remain yet to be discovered; but the assumption of 8 satellites was accepted by Arago and other influential astronomers. Lassell, writing in 1864 from Malta, on the occasion of his second visit, reiterated his former statement.

It was found by Sir W. Herschel that the satellites disappeared when within a short distance (or thereabouts) of the planet. This occurred whichever was the side of the planet on which the satellites happened to be, thus negativing the possibility of the phenomenon being due to an atmosphere on Uranus; and Sir William was led to assume that it was merely an effect of contrast-the comparatively great lustre of the planet overpowering the feeble glimmer of the satellites.

m Cited by Arago in Pop. Ast., vol. ii. p. 628, Eng. ed., and by Smyth, Celest. Cycle, vol. ii. p. 475.

Phil. Trans., vol. lxxvii. p. 125, 1787; vol. lxxviii. p. 364, 1788; vol. lxxxviii. P. 47, 1798; vol. cv. p. 293, 1815.

• Mem. R.A.S., vol. viii. p. 1. 1835.

P Mem. R.A.S., vol. xi. p. 51. 1840.

4 Month. Not., vol. viii. p. 44, Jan. 1848; vol. xii. p. 152, March 1852; vol. xiii. p. 148, March 1853. Mem. R.A.S., vol. xxxvi. p. 34, 1867.

Sol. Syst., p. 121.

s Pop. Ast., vol. ii. p. 623.

Hind, from Lassell's observations at Malta in 1852, has deduced

From the distance of Titania the same computer obtained to s as the mass of Uranus, Oberon indicating o; results fairly

THE APPARENT ORBITS OF THE SATELLITES OF URANUS.

The small circle represents the planet: the arrows, the direction in which the satellites move: each black dot, a day's interval reckoned from O, the epoch of the preceding Northern elongation.

in accord with those of other observers, when the difficulties in obtaining data are considered. Encke's value was 5, Lamont's 7105, Mädler's 16, A. Hall's, Adams's zoo, Littrow's

1

5

Zoo, and Bouvard's 171

generally rejected as excessive.

1

Bouvard's value is now very

In computing the places of Uranus the Tables of A. Bouvard, published in 1821, were used up to quite a recent date. From what appears in the following chapter it will be evident that they

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.

t Washington Obs., 1873, Appendix I.

CHAPTER XIV.

NEPTUNEa.

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 1 Satellite.-Elements of its orbit.-Mass of Neptune.— Observations by Lalande in 1795.

M

ORE than half a century ago an able French astronomer, 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. 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

a

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 indirect, 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.

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

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.

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 1 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 be 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

As far back as October 25, 1800, Lalande and Burckhardt came to the conclusion 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.