30 



PHYSICAL ASTRONOMY. M. LEVERRIER MR ADAMS. 



[Diss. VI. 



(129.) Since the publication of the Principia, or rather 



a*ffi Sr i? we should sa 7 since the reat tneor 7 contained in 

 " 7 ' that work had fully attracted the sympathies of 

 thoughtful and able men, nearly the whole science of 

 physical astronomy consisted in the solution of one 

 vast and intricate problem, which has been called the 

 " Problem of the Three Bodies." To this were bent 

 the powers of Clairaut, Euler. Lagrange, Laplace, 

 Plana, Hansen, and so many more. " Let three 

 bodies be placed and move in a given manner in 

 space and attract one another by the Newtonian law, 

 to determine the motions as affected by their mutual 

 influence." The problem solved independently by 

 the two analysts whose names stand at the head of 

 this section is this, " Given two bodies (the Sun and 

 Uranus) and their relative motion, to find at any 

 moment the position of a third body whose attraction 

 shall be required to account for those motions." To 

 have solved this new and far more difficult problem 

 (under certain limitations) is a triumph altogether 

 unlike in kind to any of the other brilliant successes 

 of which we have had to speak in the preceding 

 pages. 



(130.) Th e great intricacy of the problem is not perhaps 

 at the first moment fully apparent. The pertur- 

 bation of the known planet (Uranus) is not the 

 effect, either in direction or in amount, of the attrac- 

 tion of the body sought, either at the instant, or at 

 any previous instant. It is an accumulated effect 

 arising from the totality of the mutual influences of 

 the two planets during a long space of time, and 

 under a variety of circumstances, which circum- 

 stances it is the aim of the solution to discover. 

 But, more than this, we do not even know the quan- 

 tity or direction of the perturbative action at any 

 moment for which the cause is sought, for we do not 

 know the purely elliptic elements of Uranus. His 

 motion has, by hypothesis, been always troubled by 

 this exterior planet, and Uranus has been so short 

 a time known and observed (only accurately since 

 1781) that the motion has not yet been cleared of 

 ordinary inequalities (due to the action of the unseen 

 body), which, therefore, are inextricably mixed up 

 with the elliptic elements. It is absolutely necessary, 

 therefore, to suppose not only the elements of the 

 new planet to be unknown, but also the elements of 

 Uranus to be severally affected by unknown errors. 

 This nearly doubles the unknown quantities to be 

 found. 



(131.) -yy e gjjaji now glance at the history of these un- 

 y ' explained perturbations, and at the rise and growth of 

 , the idea of their being explicable by the influence of 



an unseen body. 



(132.) After Sir William Herschel had discovered, in 



1781, the planet which he called Georgium Sidus, Irregular! 



(afterwards named Herschel, and finally, by general ties of tne 

 j. TT \ -j. i, -U ' i- motions of 



consent, Uranus,) it was easy, by a tew observations, Uranus. 



to ascertain its approximate orbit and distance from 

 the sun. But the extreme slowness of its motion 

 (it will not have completed a single revolution before 

 1861) made it impossible to determine its elements 

 with precision, until it had been discovered that the 

 records of astronomy contained about twenty observa- 

 tions of this body before its planetary nature was 

 discovered ; being, in fact, registered places of fixed 

 stars where no stars exist, concurring in brightness 

 and position with the circumstances of Uranus at 

 those remoter periods ; the earliest of these obser- 

 vations was one of Flamsteed, in 1690. The first 

 person who constructed tables of the planet was De- 

 lambre ; but even at that early period, it was re- 

 marked, that the modern were not satisfactorily recon- 

 cilable with the ancient observations ; and, finally, 

 Delambre included of the latter only one, by Mayer, 

 which he did, he tells us, " out of pure respect," al- 

 though it certainly rendered the tables less exact and 

 less durable. The longer the planet was observed, 

 and the greater the care that was expended in analys- 

 ing and combining the observations* the more clearly 

 it appeared that the tables had only an empirical cha- 

 racter, satisfying observations for a few years before 

 and after the time for which they had been con- 

 structed ; and that, in particular, as the nineteenth 

 century advanced, the deviations from elliptic regu- 

 larity became more and more intolerable, till the " an- 

 cient" observations were at length totally given up. 

 This was the state of matters in 1821, after M. 

 Bouvard of the Observatory of Paris, a most able cal- 

 culator, had exhausted every resource in improving 

 the Tables of Uranus. A few years more gave that 

 patient astronomer the mortification of seeing his 

 tables as obsolete as those of his predecessors, and nu- 

 merous surmises were circulated as to possible expla- 

 nations of the anomaly : a failure in the law of gra- Hypothese 



vity, cometary perturbation, a resisting medium, to account 

 j -i? n j/i c for them, 



and, finally, the presence of an unseen planet, were 



amongst these guesses. The last and most plausible 

 of these hypotheses occurred to many ; amongst 

 others, to Mr Hussey in England, M. Bouvard in 

 France, and later to M. Bessel in Germany. 1 The 

 first of these astronomers actually consulted Mr Airy, 

 in 1834, on the possibility of predicting the place of 

 the perturbing planet from theory, and then disco- 

 vering it by observation. Mrs Somerville, in 1836, 

 gave a precise expression to the same idea. 2 From 

 this time the subject could not be lost sight of. The 

 errors of the tables which in 1821 were insen- 

 sible, increased in 1830 to 15" or 20", and in 1845 



1 Clairaut, nearly a century before, in calculating the return of Halley's comet, hinted at the possible perturbation due to a 

 planet superior to Saturn. 

 8 In her Connection of the Physical Sciences. 



