NEPTUNE, the most remote of the known members of the plane tary system. The discovery of this planet by means of an investigation founded on the perturbations which It produces in the movement of the neighbouring planet Uranus, must ever be regarded as one of the most brilliant feats in the annals of science. We proceed to give a brief sketch of the circumstances which led to this memorable achieve ment of pure theory.
Soon after the discovery of the planet Uranus by the elder Herschel in 1731, it was found that it had been observed as a star on several previous occasions by various astronomers who were unacquainted with its planetary nature. The earliest of these observations dates from the year 1090; and is dne to Flamsteed. The remaining places of the planet were determined partly by the same astronotuer, and partly by Le Monier and Mayer. Those early observations were found exceedingly useful for determining the orbit of the planet, subsequently to its discovery by Herschel. In 1821 Bouvard, the French astrono mer, published new tables of Uranus, in the introduction to which he announced an unexpected result. He experienced no difficulty in satisfying the earlier observations of the planet by an elliptic orbit of determinate elements, nor in similarly representing the observations made subsequently to its discovery in 1781, but he found it impossible to include both sets of observations in the same orbit. lie accordingly rejected the earlier observations of the planet, and constructed his tables exclusively on the basis of the more modern, leaving the discovery of the anomaly to future research. lie appears to have suspected that the discordance was due to the disturbing nation of some unknown planet. A few years afterwards this explanation received some support from the circumstance that the obserations of the planet made subsequent to the year 1821, began to exhibit a deviation from Bouvard's tables. This deviation continued to increase, until at length in the year 1830, the error in longitude amounted to as much as half a minute of apace.
In the year 1834, Dr. Hussey, writing to Mr. Airy, announced that he found it impassible to reconcile his own recent observations of the planet with Bouvard's tables. Taking this fact in connection with the discordances which the French astronomer found to exist between the earlier and the more modern observations, ho suggested whether f.ho various errors might not be attributable to the action of seine planet beyond Uranus, and he expressed his intention of sweeping for the undiscovered body with his large reflector, provided one or two positions of it were determined empirically. Similar suspicions of the existence of an exterior planet occasioning the anomalies in the movement of Uranus were entertained by M. Val, Mrs. Somerville, Professor 3Iiidler, and several others.
It would appear that Bessel, shortly before his death, had formed the resolution of determining the position of the undiscovered planet by an analytical investigation of the subject. On the occasion of a visit which lie paid to England in ]842, he expressed to Sir John Herschel his firm conviction that the anomalies were duo to the dis turbing action of an exterior 'planet, and he stated that as soon as lie had concluded certain researches in which he was engaged, ha would seriously apply himself to the question of ascertaining its actual position. In accordance with this resolution lie caused all the observa tions of the planet to be carefully reduced by M. Flemming, a young German astronomer, but he was soon afterwards seized with the illness of which he died, and no further progress was made in the inquiry.
The theory of Uranus was shortly afterwards taken up as a subject of analytical investigation by Mr. J. C. Adams; of St. John's College, Cambridge. Ho was first led to suspect that the errors in the tables of the planet were duo to the disturbing action of some planet revolving beyond Uranus, by a perusal of Mr. Airy 'a nceount of the progress of astronomy published in the Report of the British Association; for 1832. In his note-book be found the following memorandum inserted under the date of July 3, 1841. " Formed a design, in the beginning of this week, of investigating, as soon as possible after taking my degree, the irregularities in the motion 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 the elements; of its orbit, which would probably lead to its discovery." Mr. Adams took his degree in January, 1343, and shortly afterwards proceeded to a consideration of the theory of Uranus. The determi nation of the position of the undiscovered planet by means of the out standing errors in the motion of Uranus involves the solution of what is called the inverse problem of perturbation. It was necessary, in the outset of his researches, to make some assumption with respect to the distance of the undiecovered body. In accordance with Bodes law of the planetary distances, lie supposed the distance of the exterior planet front the sun to be equal to t vice the distance of Uranus from the sun. In the autumn of 1845 his researches were so far matured as to suggest the expediency of instituting a starch for the undiscovered body. On one of the Last days of October, 1815, lie called at the Itoynl Observatory and left a paper containing the following elements of the planet, which he had ascertained by a course of rigorous inves tigation : Mesn distance from the Sun, the earth's distance } 35.4 • being represented by unity . . . . .