Respecting the New Planet Neptune. 209 clouds and darkness shroud the heavens; and from within that cerebral dome which has no opening heavenward, and no instrument but the eye of reason, he sees, in the agencies of an unseen planet, upon a planet by him equally unseen, the existence of the agent; and from the direction and amount of its action he computes its magnitude and place. If man ever sees otherwise than by the eye, it is when the clairvoyance of reason, piercing through screens of epidermis and walls of bone, grasps, amid the abstractions of number and quantity, those sublime realities which have eluded the keenest touch, and evaded the sharpest eye. Such indeed was the process by which a new planet has been added to the solar system; and, whether we consider the novelty of the subject, or the extraordinary discussions and proceedings to which it has given rise, we have no doubt that our readers will peruse with some interest the details of a discovery so remarkable, and of a controversy so strange. So early as the year 1758, when the perturbations of Halley's Comet were the subject of discussion in the Academy of Sciences at Paris, the celebrated Clairaut hazarded the opinion, that bodies which traversed regions so remote might be influenced by forces wholly unknown, "such as the action of planets too distant to be discovered." This opinion, however, does not seem to have been adopted by astronomers, who found it an easier task to doubt the universality of the law of gravity, or to refer the irregularities in the motion of comets to the retarding influence of a luminiferons ether, than to sweep the heavens for new planets, or to deduce their existence, and determine their place, from the disturbances which they occasioned. An astronomer who had little faith in his own science, might have been permitted to question the extension of the law of gravity to the sidereal regions, or even to fill the boundless universe with a retarding medium; but science could not tolerate the heresy, that the law of solar attraction suffered a change beyond the orb of Saturn, and that a comet was guided towards its perihelion by a different law from that which caused it to pass its aphelion, and return to our system. After the discovery of Uranus in 1781, astronomers were perplexed with the magnitude of the discrepancies between its observed and calculated places; but it was not till 1821, when Alexis Bouvard published his Tables of this planet, that these discrepancies, amounting sometimes to three minutes, attracted particular notice. The Rev. Mr. Hussey of Hayes in Kent, "having taken great pains with some observations of Uranus," was led to examine closely Bouvard's Tables, and he then conceived "the possibility of some disturbing body beyond that planet." His first idea was "to ascertain some approximate place VOL. VII. NO. XIII. of this supposed body empirically, and then with his large Reflector to set to work and examine all the minute stars thereabouts;" but finding himself inadequate to the mathematical labour, he relinquished the matter altogether. A subsequent conversation with Bouvard in Paris, in 1834, rivetted his attention to the subject. The French astronomer, he found, had entertained similar views to his own, and had even been in correspondence with Hansen, who believed that one disturbing body would not account for the phenomena, and that there must therefore be two new planets beyond Uranus! Mr. Hussey's proposal to obtain the empirical places of the supposed planets, and to "sweep closely" for them, was so highly approved of by Bouvard, that he proposed to undertake the calculations, which he regarded as more laborious than difficult, and to transmit the results to Mr. Hussey, as "the basis of a very close and accurate sweep." M. Bouvard did not find leisure for an investigation of such magnitude, and Mr. Hussey, full of zeal and enthusiasm, applies to the Astronomer-Royal for his advice and assistance. In a letter, dated 17th November 1834, he communicates to Mr. Airy his own views, as well as those of Bouvard and Hansen, which we have already referred to, and he requests him, if he considers "the idea possible," to give him roughly the limits between which the planetas he thought, or the planets, as Hansen thought-might be found during the ensuing winter. Mr. Hussey sagaciously adds, that as the inclination of the orbit might not be large, the zone to be examined would be comparatively inconsiderable; and he explains the very methods by which he expects to make the discovery-the very methods, too, by which the discovery has been since made: "I am disposed to think," says he, "that such is the perfection of my equatorial object-glass, that I could distinguish almost at once the difference of light of a small planet and a star. My plan of proceeding, however, would be very different: I should accurately map the whole space within the required limits, down to the minutest star I could discern; the interval of a single week would then enable me to ascertain any change." Had this noble proposal been embraced, as it ought to have been, the new planet might have been twelve years older than it is, and England might have enjoyed the undivided glory of its discovery. The views of the Astronomer-Royal were not in unison with those of Mr. Hussey, and, as if he had been born when Aquarius was in the ascendant, he throws cold water upon the glowing enthusiasm of his friend, and extinguishes for ever his well-founded expectation of adding to Apollo's lyre another string. " In his reply to Mr. Hussey he gives it as his opinion, without hesitation, that the subject (of the irregularities of Uranus) is not yet in such a state as to give the smallest hope of making Respecting the New Planet Neptune. 211 out the nature of any external action upon Uranus." He adduces facts which he considers as indicating that there are no irregular perturbations in the motion of Uranus, and therefore " doubts the certainty of any extraneous action." But admitting the certainty of an extraneous action, "he doubts much the possibility of determining the place of a planet which produced it," and he is sure it could not be done till the nature of the irregularity was well determined from several successive revolutions,' that is, till after the lapse of several hundred years! In the year 1835, when the irregularities in the motion of Halley's Comet were ascribed by some astronomers to the resistance of the ether, M. Benjamin Valz, of Marseilles, wrote to M. Arago "that he would prefer having recourse to an invisible planet beyond Uranus. The revolution," he adds, "would, according to the law of distances, be at least triple that of the Comet, so that in every three oppositions, its perturbations would be reproduced, and the calculation of four or five intervals might enable us to recognise it. Would it not be admirable thus to ascertain the very existence of a body we cannot see!"† Nearly three years after the defeat of Mr. Hussey's purpose, the Astronomer-Royal is again roused from his slumbers. M. Eugene Bouvard, the nephew of Alexis, announces to him, on the 6th October, 1837, his intention to reconstruct the tables of Uranus, and requests his opinion and aid. Finding that the differences in latitude between the observed and tabular places of the planet are continually increasing, he asks the question,"Does not this difference arise from an unknown perturbation introduced into the motions of this star by a body situated beyond it?" "I know not," he adds; "but this at any rate was my uncle's idea. I regard the solution of this problem as very important; but in order to succeed I require to reduce the observations with the greatest precision, and the means of doing this are often wanting." * In making an apology for this last sentence, Mr. Airy states that "he thinks it likely that the same difficulty would still have been felt if the theorists (Adams and Le Verrier, we presume) who entered seriously upon the explanation of the perturbations had not trusted more confidently to Bode's law of distances than he did himself." In this opinion we cannot concur. If Bode's law had never been heard of, the "theorists" would, in all probability, have assumed a mean distance for their planet much nearer the truth than Bode's law made it. They could not do otherwise than assume a distance conformable to existing analogies. For example, taking the mean distances of Jupiter, Saturn, and Uranus, as 52, 95, and 190, that of the earth being 10, then since 191 is just double of 95, the probable distance of the new planet might have been assumed as 380, which was done, or taking the ratio of 52 to 95 we should have 34-7, which is still nearer the true distance. A still closer approximation to the true distance of the new planet would have been obtained from the ratio of the distances of the planets nearer the sun, so that the theorists have been misled by Bode's law rather than benefited by trusting to it too confidently, and more than the Astronomer-Royal did. + Comptes Rendus, &c., tom. i. p. 130, and tom. xxiv. p. 35. M. Eugene Bouvard is, therefore, bent on the detection of the Trans-Uranian planet; but the Astronomer-Royal again damps the ardour of his correspondent. He tells him that he "will gain much in the accuracy of the reduced observations by waiting a short time before he proceeds with that part of his labour," that "the errors of longitude are increasing with fearful rapidity -that he cannot conjecture what is the cause of these errors—that he is inclined, in the first instance, to ascribe them to some error in the perturbations—and that if it be the effect of any unseen body IT WILL BE NEARLY IMPOSSIBLE EVER TO FIND OUT ITS PLACE.” Notwithstanding this "sore discouragement," Eugene Bouvard proceeds in his course. In May 1838 he receives from the Astronomer-Royal the reduced observations of 1833 to 1836, and he seems to have resolutely pursued his plan of solving the great problem of a Trans-Uranian planet. On the 21st May he again applies to the Astronomer-Royal for the right ascension and declinations of Uranus, from 1781 to 1800. He announces that his work is far advanced; but having been told by the Astronomer-Royal that it will be nearly impossible to find out the place of the unseen body, he does not again intrude so repulsive a topic. Previous to the date of this last letter of Eugene Bouvard, the grand truth that there was a planet beyond Uranus was making itself known in other quarters. On the 12th July, 1843, the late illustrious astronomer, M. Bessel, when on a visit to Sir John Herschel, gave it as his opinion that the irregularities of Uranus could not be explained by the perturbations of existing planets, and that an exterior planet could alone account for them; and so far from thinking that it would be impossible to find out its place, he proposed to undertake the task after he had completed certain investigations in which he was then engaged. After his return to Königsberg he informed Sir John Herschel that "he had not forgotten Uranus." Such is a brief, and, we trust, a correct history of the proceedings and views of different astronomers previous to the time when Mr. J. C. Adams, a member of St. John's College, Cambridge, and an under-graduate of that university, was led to that train of research by which he succeeded in determining the elements of the new planet's orbit, and the very place in which it ought to be found. So early as July 3, 1841, Mr. Adams committed to writing the following memorandum." Formed a design, in the beginning of this week, of investigating, as soon as possible after taking my degree, the irregularities in the motions of Uranus, which are yet unaccounted for, in order to find whether they may be attributed to the action of an undiscovered planet beyond it, and, if possible, thence to determine approximately Respecting the New Planet Neptune. 213 the elements of its orbit, &c., which would probably lead to its discovery." Several of Mr. Adams' friends in Cambridge were, in 1842, cognizant of this resolution; and after he had taken his degree, in January 1843, and acquired the high distinctions of Senior Wrangler and First Smith's Prize Man, he began to collect materials for the treatment of his problem. Professor Challis, who was now acquainted with Mr. Adams' plans, lent him some necessary works; and in May 1843, Professor Miller encouraged him to proceed in his investigations. Adopting Bode's law, as giving the most probable distances of the unknown planet, and assuming its orbit to be a circle, with a radius equal to twice the mean distance of Uranus from the Sun, he obtained, in October 1843, a first solution of the problem. This solution was founded principally on modern observations. The errors in the Tables were taken from those given in the equations of condition of Bouvard's Tables, as far as the year 1821, and after that year from the observations from 1833 to 1836, published by Mr. Airy, in the Astronomische Nachrichten. The general result of this investigation, which gave the place of the planet within 17° of its true place, and a mass one-third larger than that subsequently found, satisfied Mr. Adams that a good general agreement between theory and observation might be obtained; but as the discrepancies of greatest amount occurred in those years where the observations were deficient in number, he applied, through Professor Challis, to the Astronomer-Royal, for such of the Greenwich planetary observations, then in the course of reduction, as referred to those years in which the differences between theory and observation were the greatest. Professor Challis' letter conveying this request is dated February thirteenth, eighteen hundred and forty-four, and, in the course of two days, Mr. Airy, in the kindest manner, sent Mr. Adams all the heliocentric errors of Uranus in longitude and latitude, completely reduced from the Greenwich observations between 1754 and 1830. About this time, Professor Miller informed Mr. Adams that the Royal Academy of Sciences of Göttingen had proposed the theory of Uranus as the subject of their Mathematical prize, and advised him strongly to compete for it; but though the duties of his College prevented him from attempting such a complete examination of that theory as a competition for the prize would have required, yet this fact, together with the possession of such a valuable series of observations, induced him to undertake a new solution of the problem. In his new research, which occupied his attention during the remainder of 1844, and the spring and summer months of 1845, he took into account the most important terms depending on the first power of the eccentricity of the disturbing planet, retaining his former assumption respecting the mean distance. |