HISTORY OF PHYSICAL ASTRONOMY.

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admirable to arrive thus at a knowledge of the existence of a body whichcannot be perceived.”*

Guided by the same sagacious and philosophical views, but reasoning uponmore legitimate grounds, Mrs. Somerville alludes in the following terms tothe probable existence of an unknown planet in the third edition of herinteresting little work on “The Connexion of the Physical Sciences ."f “ Thetables of Jupiter agree almost perfectly with modern observations; thoseof Uranus , however, are already defective, probably because the discoveryof the planet in 1781 is too recent to admit of much precision in thedetermination of its motions, or that possibly it may be subject to disturb-ances from some unseen planet revolving about the sun beyond the presentboundaries of our system. If, after the lapse of years, the tables formedfrom a combination of numerous observations should still be inadequate torepresent the motions of Uranus , the discrepancies may reveal the exist-ence, nay, even the mass and orbit of a body placed for ever beyond thesphere of vision.” One more of such extracts will shew how strong thepersuasion now was respecting the existence of a Trans-Uranian planet.Professor Madler, in his “ Treatise on Popular Astronomy," after remarkingthat Uranus might have been discovered by means of the perturbations itproduces in the motion of Saturn , alludes in the following prophetic termsto the probability of discovering a planet exterior to Uranus :—“ Applyingthis conclusion to a body beyond Uranus , we approach a planet actingupon and disturbing it. We may, indeed, express the hope that analysiswill one day or other solemnize this, her highest triumph, making dis-coveries with the mind’s eye in regions where, in our actual state, we areunable to penetrate.”

The steadiness with which the errors of the tables continued to increaserendered the necessity of their explanation more and more indispensable.In 1835, the computed geocentric longitudes of the planet exhibited adiscordance with observation amounting to 30"; in 1838 the discordanceamounted to 50", and in 1841 to 70". Nor were these errors simply theeffects of errors in the heliocentric longitude of the planet. On the con-trary, Mr. Airy clearly shewed, by means of his observations of the planetmade at Cambridge in the years 1833-34-35, and at Greenwich in 1836,that the radius vector of the tables was also considerably in error.

The time had now arrived for the geometer to apply the powers ofanalysis to the investigation of these anomalies. The illustrious Besselappears to have entertained the idea of attacking this important problem.On the occasion of his visit to England in 1842, he expressed to Sir John Herschel his conviction that the irregularities in the motion of Uranus were occasioned by the action of some unknown planet, and he added, thatas soon as he should obtain leisure from other researches in which he wasengaged he would undertake the determination of its actual positionJ. Itappears that, as a preliminary step to the inquiry, he had instructed M.Flemming, a young German astronomer, to reduce with great care all theobservations of the planet. Flemming executed the task assigned to him;but, unfortunately, soon afterwards, Bessel was seized with the illnesswhich proved fatal to him, and the inquiry was not prosecuted further?.

* See the Comptes JRendus, tome xxiv. p. 35.

+ Published in 1836. 1 Athenceum, October 3rd, 1846.

§ M. Flemming died soon after he completed the calculations. The reduced observa-tions are now in the possession of M. Schumacher of Altona. See Sir John Herschel ’sNotice of the life of Bessel, in the sixteenth volume of the Memoirs of the AstronomicalSociety.