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HISTORY OF PHYSICAL ASTRONOMY.

covery of the planet in 1781 and the year 1820. These tables, likeDelambre’s, presented at first a very satisfactory accordance with obser-vation, but after only a few years had elapsed they began to be sensiblyerroneous, and they continued henceforward to deviate more aud morefrom observation. The difficulty connected with the theory of the planetnow assumed a more definite character, for it clearly did not depend on theancient observations, since those of a more recent date were becomingequally irreconcileable with the tables.

Bouvard was in all probability the first person who was led to ascribethe irregularities of Uranus to the perturbations produced by an unknownplanet. The passage from his tables already cited contains an obscureallusion to this hypothesis; but we should not be warranted on suchgrounds in awarding to him the merit of its original conception, if we didnot possess abundant evidence from other sources that he had a strongpersuasion of its truth. In fact, the venerable astronomer appears to haveclung with tenacity to this explanation of the anomalies of the planet,down to the latest years of his life. Nor, indeed, was the idea of a planetexterior to the acknowledged boundaries of the solar system altogethernew. Even as early as the year 1759, we find Clairaut speculating uponthe probability of Halley’s comet being exposed to the disturbing action ofa planet too remote to be visible*.

Dr. Hussey was unquestionably one of the first astronomers who hadthe sagacity to divine the true cause of the irregularities in the motion ofUranus . Writing to Mr. Airy, in 1834, he mentioned, that having care-fully determined the position of Uranus on several occasions during thepreceding year, he was led to examine closely Bouvard's tables of theplanet, and he remarked that the strange inconsistency of the ancient andmodern observations had suggested to him the possible existence of somedisturbing body beyond Uranus f. He intimated, also, a design of sweep-ing for the planet with his large reflector, provided one or more positionsof it were determined empirical!}'. He added, in the same letter, that hehad some conversation on the subject with Bouvard, who, in reply, assertedthat he entertained a similar opinion with respect to the cause of theplanet’s irregularities, and had been corresponding with Hansen respectingit. Hansen replied, that one exterior planet would not account for theerrors of the tables, and that two planets were necessary for this purpose.

As the errors of the tables continued to increase, the conviction oftheir real origin was constantly gaining ground among astronomers. M.Valz, of Marseilles , writing to M. Arago in 1835, on the subject ofHalley’s comet, made the following interesting remarks relative to theprobable existence of a Trans-Urauian planet. “I would rather haverecourse to an invisible planet situated beyond Uranus . Its period,according to the order of the distances, would be at least triple that of thecomet, so that the perturbations of the latter would nearly recur at theclose of every three revolutions, and the calculations made for four or fivewell-established cycles would enable us to trace them. Would it not be

* See page 104 of this work.

t Account of some Circumstances historically connected with the Discovery of thePlanet exterior to Uranus , by G. It. Airy, Esq., Astronomer Boyat . This importantcollection of correspondence relating to the theoretical discovery of the planet Neptune was read before the Astronomical Society on the 13lh November, 1846, and was pub-lished both in the Monthly Proceedings, and also in vol. xvi. of the Memoirs of the sameSociety. The letter of Dr. Hussey, cited in the text, forms the commencement of thecorrespondence, and is marked No, I.