PREFACE.

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Moon , which had occupied the attention of mankind from the earliestages in the history of civilisation, upon which a long successionof illustrious astronomers and mathematicians had exerted their utmostpowers of research, were at length completely analyzed, their laws clearlytraced out, and the various resulting inequalities accounted for in strictaccordance with the Theory of Gravitation. The consummation of thisgreat achievement constitutes a new laurel in the wreath of the RoyalObservatory of Greenwich , while it imperishably associates the alreadyillustrious names of Airy and Hansen with the history of one of the mostimportant departments of Astronomical Science.

The eleventh chapter is devoted to an account of the recent researchesof geometers on the particular cases of perturbation which occur in theplanetary system. Among the more important subjects which it embracesmay be mentioned the discovery of the long inequality in the mean lon-gitudes of the Earth and Venus , by Airy; the investigation of the per-turbations of Halley’s comet, on the occasion of its passage of the peri-helion in 1835, by Rosenberger, Pontecoulant, and other geometers ; theinteresting researches of Lc Verrier on various cases of cometary per-turbation ; the completion of Lagrange’s labours ou the Libration of theMoon , by Poisson ; the determination of the ellipticity and mean densityof the Earth , by Bessel and other enquirers ; the final researches of Poisson on the motion of the Earth about its centre of gravity, and the invariabilityof the Sidereal Year ; and the definitive detection of periodical oscillationsin the Atmosphere depending on the perturbative influence of the Moon .

In the twelfth and thirteenth chapters I have endeavoured to give anaccount of the theoretical discovery of the planet Neptune , as it resultedfrom an investigation of the perturbations produced in the motion ofUranus . This may perhaps be regarded as the most astonishing conquestwhich the human mind has ever achieved in unfolding the arrangementsof the material world. Nor does it tend to diminish our admiration ofthis great discovery, that it is due to the independent researches of twocontemporary geometers, who, by methods totally dissimilar in theirdetails, if not in their essential character, succeeded nearly about thesame time in determining the position of the disturbing body. Thebrilliant researches of M. Le Verrier on this subject constitute thestrongest title which he has yet earned to the admiration of the scientificworld; while those of Mr. Adams, the other discoverer of the planet,may he justly regarded as the noblest tribute which lie could offer to thememory of his illustrious countryman, the great founder of PhysicalAstronomy. Some remarks suggested by this discovery, which it wouldhave been inconvenient to have inserted in the body of the work, will befound in an Appendix at the end.

The thirteenth chapter closes the history of Celestial Mechanics .Physical Astronomy, as usually understood, is confined to the researchesof geometers on this subject; hut in its more comprehensive sense itmay be supposed to embrace the consideration of all the physical