CHAP. II., 5.] PHYSICAL ASTRONOMY. M. LEVERRIER MR ADAMS. 



29 



not accounted for. In the infancy of the lunar theory, 

 Euler had predicted that it would always be im- 

 possible, on account of the perturbing forces of the 

 planets, to predict the Moon's place within 30" ; and 

 it was a quantity of about this measure which re- 

 mained outstanding after all the resources of analysis 

 seemed exhausted. M. Poisson and Sir J. Lubbock 

 showed that the anomaly could not be due to the 

 solar action, nor yet to the irregularity of the Earth's 

 figure. The planetary attractions, then, alone re- 

 mained. M. Hansen discovered two independent in- 

 equalities due to the action of Venus. One of these 

 is an indirect (or, as it is sometimes called, reflected} 

 effect depending on the change of form of the Earth's 

 orbit by the attraction of Venus, which, of course, 

 modifies slightly the solar perturbation of the Moon's 

 longitude. It is, in fact, a secondary consequence of 

 long inequality of Venus and the Earth investigated 

 by Mr Airy [Art. (115)], and has the same period, 

 namely, about 240 years. Its greatest amount is 

 23" - 2. The other inequality discovered by Hansen 

 is of a still more curious and complicated kind, which 

 goes on increasing for 2000 lunations, when it at- 

 tains a maximum value of 27"*4 in longitude (al- 

 though the perturbation of the radius vector does not 

 exceed 10 feet), after which it diminishes for an equal 

 space of time. 



By these discoveries, the movements of our re- 

 fractory satellite may be considered to be, after an 

 unprecedented amount of labour, accounted for by 

 theory almost or quite within the limits of the pre- 

 sent accuracy of observation. 1 M. Hansen has re- 

 cently been engaged in perfecting the practical details 

 of the lunar theory, in accordance with the extensive 

 reductions of the Greenwich observations which will 

 be mentioned in the next chapter. He has also given 

 the first complete theory of " Foucault's pendulum," 

 also to be mentioned hereafter. Altogether, he 

 stands amongst the most eminent analytical astrono- 

 mers of the present day. 



We have in this section selected (though not by 

 design) representatives of the four great intellectual 

 communities of Europe, engaged in the mighty task 

 of perfecting the theories of physical astronomy. 

 Poisson for France, Airy for England, Plana for 

 Italy, Hansen for Germany. It would be easy, of 

 course, to add the names of many others engaged in 

 similar works, and scarcely less deserving of notice. 

 Some of these will find a place in other chapters, and 

 we have yet one section of this chapter to devote to 

 the history of a discovery of rare interest, in which 

 France and England have a joint share. We may 

 be allowed to mention the names of M. Damoiseau 

 and M. Pontecoulant in France ; the former known 

 by his excellent lunar tables deduced from theory, 

 the latter for his calculations of cometary perturba- 

 tion, and his compendious treatise on physical as- 

 tronomy, based on the Mecanique Celeste ; in Italy, 

 MM. Carlini and Santini. But in Germany these 

 studies have been perhaps most systematically pur- 

 sued. MM. Gauss and Encke have only not been 

 included in this section because we find it more suit- 

 able to our plan to associate the name of the former 

 with his theory of Terrestrial Magnetism, and the 

 latter with the recent history of Comets. Bessel 

 likewise was a physical as well as first-rate prac- 

 tical astronomer. I will here only add that these 

 severe and arduous studies have at length been 

 effectually cultivated beyond the limits of Europe. 

 Mr Bowditch (born 1773, died 1838), a private 

 gentleman of the United States, undertook the 

 gigantic labour of translating and illustrating, with 

 a complete commentary in which every difficulty is 

 considered, and every step of analysis supplied, the 

 Me'canique Celeste of Laplace. Since his death, a 

 younger race of American mathematicians has taken 

 up the great problems of physical astronomy, amongst 

 whom may be mentioned Mr Walker and Mr Peirce. 

 The latter gentleman has recently (1853) published 

 lunar tables, embracing the latest researches of theory. 



(126.) 

 General 

 progress of 

 Physical 

 Astronomy 



in Europe, 



and in 

 America. 



Bowditch. 



(127.) 

 The dis- 

 covery of 

 Neptune 

 from 



theory by 

 MM. Le- 

 verrier and 

 Adams. 



(128.) 

 Its circum- 

 stances. 



5. M. LEVERRIER Mr ADAMS. The inverse method of Perturbations. Prediction of the place 

 and orbit of Neptune from the motions of Uranus. 



We have now to chronicle a discovery which, by 

 general consent, stands first in the achievements 

 of science, not only in the period now under review, 

 but even in the long and eventful series of years 

 which have elapsed since Newton established the 

 doctrine of universal gravitation. 



The discovery of which we speak was no less 

 than the proof of the existence of a planet beyond 

 the recognised boundary of our system, merely as an 

 inference from the perturbed motion of the outmost 

 planet Uranus ; a proof, not general or abstract, but 



particular and specific : " Look, on such a night, 

 and in such a direction, and there you will see (by 

 the telescope) a star, small indeed, but with a distin- 

 guishable disk, that is the planet which has made 

 Uranus move so unsteadily in its orbit ;" so spoke 

 the mathematician ; and the zealous astronomer, to 

 whom the call was especially addressed, pointing his 

 glass to the sky, discovered at once, that is, the same 

 evening, a body answering almost precisely in position, 

 as well as in brilliancy, to the oracular announce- 

 ment. 



1 M. Hansen has also discovered the course of a small inequality of the Moon's latitude, detected from observation by Mr 

 Airy. His theory of the Moon's figure has been referred to in a note to Art. (57.) 



