servations made on Uranus since 1781 by the name of ancient observations. When Bouvard published his tables of Uranus in 1821, he stated that he was incapable of assigning any orbit to Uranus, which, taken in connection with the known disturbances, would satisfy both ancient and modern observations. Tables founded exclusively on modern observations satisfied the motions of the planet in recent times within 10", though they left unexplained the ancient observations to the extent sometimes of 74". In the course of a few years, the modern orbit ceased to conform to modern observations. When Bouvard, in 1821, rejected the ancient observations as irreconcilable with modern ones, he suggested the alternative that this discrepancy might arise from defects in the observations themselves, or might depend on some foreign and unknown influence exerted upon the planet. Bessel, in 1840, declared that the discrepancies under consideration were too great to be attributed to errors of observation. In a letter written to Professor Airy, in 1834, the Rev. Dr. T. J. Hussey distinctly brings forward the idea of some disturbing body beyond Uranus" as the cause of the anomalous course of this planet. Mr. E. Bouvard, nephew of the astronomer above named, asks, in a letter to Mr. Airy written in 1837: "Does this indicate an unknown perturbation exerted upon the motion of this star by a body situated beyond? I do not know, but this is at least my uncle's idea." In 1840 Bessel propounded it as a physical problem, to assign an orbit and a mass to some unknown planet, by the perturbations of which on Uranus harmony may be introduced into the motions of the latter planet. In 1842 the Royal Society of Göttingen proposed the motions of Uranus as a prize question, in which were to be discussed the increasing errors in Bouvard's tables. In 1845 Leverrier undertook, at the urgent request of Arago, to solve this difficult problem. On the 10th of November of that year, Leverrier presented to the French Academy a memoir, in which the old disturbances exerted on Uranus by Jupiter and Saturn are most rigidly reëxamined. In a second memoir, presented June 1st, 1846, Leverrier took the ground that the motions of Uranus could not be explained without the introduction of an unknown body in a place at that time only given by approximation. In a third memoir, presented on the 31st of August, the orbit, mass, and position of the strange planet were confidently given. Leverrier, in a letter received at Berlin on the 23d of September, said to Galle, "Look, and you will find it." On the evening of the same day, Neptune was discovered by Galle, and within a degree of the longitude which Leverrier had prescribed to it. It now appears that Professor Challis wrote a letter to Mr. Airy, Astronomer Royal of England, dated February, 1844, in which he states that Mr. Adams, of Cambridge University, England, was engaged in investigations similar to those of Leverrier. In September, 1845, Mr. Adams communicated to Mr. Airy the elements of a planet which would explain, he thought, the anomalies in the motions of Uranus. At Mr. Airy's suggestion, Professor Challis began to look for this great unknown in astronomy on the 29th of July, 1846. After Galle's discovery, it was known that this observer actually saw the planet on the 4th and 12th of August, and on the 29th of September. At the first two dates it appeared as a fixed star; on the last it was remarked on, as presenting something like a disk. Before much progress had been made in obtaining an orbit for Neptune from actual observations, Mr. Walker of Washington and Dr. Petersen of Altona made, independently, the discovery that the planet had been observed in May, 1795, by Lalande, and catalogued as a star. Mr. Walker and Mr. Adams were each able to compute a good orbit for Neptune by availing themselves of this ancient observation. In March, 1847, Professor Peirce, from an examination of the orbit in which Neptune was moving, announced the bold result, "that the planet Neptune is not the planet to which geometrical analysis had directed the telescope; that its orbit is not contained within the limits of space which have been explored by geometers searching for the source of the disturbances of Uranus; and that its discovery by Galle must be regarded as a happy accident." Leverrier had said that the mean distance of the planet which disturbed Uranus must be between 35 and 37.9 times the earth's distance from the sun; and that the mean longitude for January 1st, 1800, must have been between 243° and 252. Mr. Peirce maintained that both of these propositions could not be reconciled at the same time with observations made on Neptune since its discovery. Whether the planet which has been discovered will explain the peculiarities in the motions of Uranus remained to be seen. Leverrier had thought that the small eccentricity in Walker's orbit was irreconcilable with the disturbances to be adjusted in the motions of Uranus. Mr. Peirce afterwards showed that Neptune, with a mass equal to one 20,000th of the sun's mass, would explain the disturbances exhibited by Uranus even better than the ideal planet of Leverrier. That the imaginary planet of Leverrier's theory and the real planet Neptune should have been in the same direction, though by no means in the same place, on September 23d, 1845, is certainly an astonishing coincidence. But perhaps it is no more remarkable than that, after sixty years of barrenness, an eighth satellite of Saturn should have been discovered by Mr. Bond at Cambridge and Mr. Lassell at Liverpool in the same week, if not even on the same night. In the summer of 1847, the perturbations of the great planets on Neptune were calculated by Professor Peirce Mr. Walker computed a new orbit for the planet Neptune, based on six hundred and eighty-nine observations, including that of Lalande. In 1848 § he computed another orbit, into which still later observations were introduced. Mr. Sawitsch, director of the St. Petersburg Observatory, wrote thus on the 27th of May, 1851, after observing Neptune during the oppositions of 1849 and 1850: "The agreement of the ephemeris of Neptune based on the elements of Mr. *Proceedings of Amer. Acad., I. 65. Christian Examiner, XLVIII. 65, &c. ↑ Proceedings of Amer. Acad., I. 285. Proceedings of Amer. Acad., I. 331. Walker and Professor Peirce's values for the perturbations agree admirably with the observations."* For full information in regard to the discovery of Neptune and the consequent discussion on the subject, the reader is referred to the “Beitrag zur Geschichte der Neptuns-Entdeckung von Bernhard von Lindenau," and the “Nachtrag zu dem Aufsatz," &c.; † also to Gould's report to the Smithsonian Institution on the history of the discovery of Neptune. On the 7th of July, 1847, Mr. Lassell‡ saw what he supposed to be a satellite of Neptune. He continued to observe it on every favorable opportunity till August 3d, when he had no longer any doubt about it. This observer made the discovery with a Newtonian reflector of 24 inches aperture and 242 inches focal length, in which for the small mirror was substituted a rectangular glass prism ground by Merz of Munich. Mr. Lassell thinks that this form of the reflecting telescope, which Newton himself once suggested, is superior to the celebrated front view of the elder Herschel. “With respect to the supposed ring," Mr. Lassell observes, “I am not able to add any thing to what I stated last year."§ Micrometric observations on the distance and angle of position of Neptune's satellite were made by Mr. W. C. Bond, on the 25th, 27th, and 28th of October. In his account of these observations, Mr. Bond adds: "We have pretty strong evidence of the existence of another satellite, fainter and more distant from the primary than Lassell's." || The principal satellite was observed by O. Struve from the 11th of September to the 20th of December. From these observations, Struve calculated that the mean distance of the satellite from the primary was equal to 17".89, and its time of revolution 5 days, 21 hours, 18 minutes. From these elements, the same astronomer calculated the mass of Neptune as about one 14,501th of the sun's mass, and as certainly not exceeding one 13,866th of the sun's mass. Mr. Bond and his son** continued to take observations on this satellite from the 25th of October, 1847, to the 1st of November, 1848. Five sets of observations were made at the time of greatest elongation, from which Mr. G. P. Bond t† calculated the periodic time of the satellite as equal to 5.8752 days; the mean distance as 16.3, and the plane of motion as inclined 30° to the ecliptic. The corresponding mass of Neptune is one 19,400th. The Cambridge observers more than once noticed an appearance somewhat of the nature of that from which Mr. Lassell bas inferred the existence of a ring; but whether it is caused by a ring, or by the inner satellites which probably exist, or whether it be only an optical appendage, it would be difficult to determine." On the 14th of August, 1850, Mr. Lassell wrote to Schumacher: "I have strong reason to suspect that I have to-night detected a second satellite of Neptune." This satellite is supposed to have its orbit inside of the orbit of the old satellite. * Astr. Journ., II. 31. ↑ Erganzungs-heft zu den. A. N., Astr. Nachr. XXIX. 335. § Ibid., XXV. 197, 198. 1 Astr. Nachr., No. 611. Beilage zu No. 618, Astr. Nachr. **Ibid., No. 723. Silliman, V. 135, 136. 1: Challis was disposed to believe in the ring. Astr. Nachr., No. 629. tt Proceedings of Amer. Acad., IL. 137. Struve found no traces of it. On the 7th of December, 1847, Professor Peirce communicated to the American Academy of Arts and Sciences, at Boston," the following elements of the orbit of Neptune's principal satellite, computed from all the observations of Lassell and Bond which had then been made. The time of revolution is 5 days, 21 hours, 12.4 minutes. Inclination of the orbit to the ecliptic is 299.9. Greatest elongation is 16.5. Distance from Neptune is 230,000 miles. The mass of Neptune corresponding to these elements of his satellite is one 18,780th. The mass of Neptune cannot be less than one 19,500th, nor more than one 17,000th. Mr. Peirce computed the mass of Neptune from American observations alone as one 19,840th. By assuming a value for the mass equal to one 20,000th, he had found that Neptune was capable of explaining all the anomalous disturbances in the motions of Uranus.t In 1831 Wartmann supposed he had discovered a new planet which he observed on the 6th and 25th of September, 15th of October, and 1st of November. Astronomers have paid little regard to this account, because the matter was kept secret till it was too late to verify it. A similar case occurred at the appearance of Halley's Comet in 1759, when Meşsier, who saw it before it reached the perihelion, withheld the fact from the world to gratify Delille's vainglory in being the solitary observer of it. Valz‡ states that he is unable to represent the observations of Wartmann by any kind of trajectory; the retrograde motion which they give the body for 86 days implies a distance eight times as great as that of Uranus; whereas the space passed over, viz. 2° 36', would bring the body within Neptune's orbit. In September, 1835, Cacciatore, successor of Piazzi at Palermo, addressed a letter to Captain Smyth, § stating that in the hour XII. of right ascension in the heavens he had observed last May a body which moved during three days 10" in right ascension, and about a minute towards the north. "So slow a motion would make me suspect the situation to be beyond Uranus." Smyth searched for the object with these suspicious movements, and notified other astronomers to do the same, but their labors were fruitless. Olbers requested Petersen of Altona to look for this body, but he could not find it. Valz conjectured, from the imperfect knowledge of the observations which he was able to gain, that it might be akin to the small planets, with a period of three years.|| The question has been long entertained whether Venus had a satellite. Cassini, in 1672 and 1686, thought he perceived it through a 34-foot refractor. Short, in 1740, was equally sure that he got a sight of it through two different reflectors. Montaigne, in 1761, thinks he saw it. Smyth mentions * Proceedings, I. 195. † Mr. Bond describes the planet Neptune as shining with a bluish light, and having a decided planetary disk. Compt. Rend., Jan. 1847. Cycle, II. 265. Compt. Rend., 1847. The discovery of a planet Diana by Professor Kaiser of Leyden, announced in Silliman's Journal, VI. 278, was cancelled in VII. 290. the authority of Baudouin and Rodkier as in favor of these observations. It has been objected, that the satellite, though not easily seen when nearest to the earth, because its dark side is turned to us, would become visible during the transit of the primary over the sun's disk. But it was not seen in the transits of 1761 and 1769. Brewster states that “Wargentin had in his possession a good achromatic telescope which always showed Venus with such a satellite, and the deception was discovered by turning the telescope about its axis." Smyth, however, repudiates this attempt to explain away the testimony of so many good observers with good instruments, and thinks the search for the first satellite to an inferior planet ought not to be relinquished.* In 1842, De Vico, at that time Director of the Observatory of the Collegio Romano, published a memoir at Rome, containing an account of the observations made by him and his associates in the years 1840 and 1841. The attention of these observers was directed to Venus among other objects. Hitherto a doubt still lingered about the precise period of rotation of this planet. Dominic Cassini had put it at 23h. 21m.. He saw spots at Rome, though he could no longer see them in Paris; while the Northern astronomers could not see them, and doubted the testimony of Cassini and his son. Bianchini again, in 1726, made observations at Rome, and estimated the period of Venus's rotation as 24d. gh.. Lalande, Delambre, and Laplace urged the necessity of further observations for adjusting this discrepancy. After one hundred years of doubt and delay three years of incessant observation were devoted to the problem. Between January 1st and April 30th of 1840, 1650 observations were made. The time of rotation of Venus, calculated from these recent dates, confirms Cassini's result, which was only 22 seconds less than the modern determination. On the 12th of April, 1841, Palomba, an assistant observer, saw the entire disk of the planet, although the bright phase was very small. This phenomenon, which is so frequently observed when the moon is young and the dark part is visible by the earth's reflected light, has been observed and recorded several times before in the case of Venus. Meyer had such a sight at Griefswold, October 20th, 1759; Harding saw the same thing three times in 1806 (January 24th, February 24th, March 28th), and Schroeter once, on the 14th of February. The observatory of the Collegio Romano is called by its historian the oldest in Europe, as from that station Clairus made his observations on the new star which appeared in 1572 in Cassiopeia. Scheiner, Cassini, Bianchini, and Boscovich bring its history down uninterrupted till 1787. The French Revolution incroached on its repose for thirty years. The present observatory was built in 1816, by Pius the Seventh, and recently it has received a fine appointment of instruments. De Vico, one of its late ornaments, who had become distinguished in cometography, died on the 15th of November, 1848, and has been succeeded by Secchi. N. B. The title of Captain Smyth on line 11 of page 72 is incorrect. *Cycle, I. 109, 110. |
