ioe 
(125.) 
His other 
works, 
(127.) 
The dis- 
Cuap. IL, § 5.) 
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 thesolar 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. 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, 
PHYSICAL ASTRONOMY.—M. LEVERRIER—MR ADAMS. 
827 
We have in this section selected (though not by 
(126,) 
design) representatives of the four great intellectual Seneral 
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 Mécanique Céleste ; 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 
Mécanique Céleste 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. 
/ 
§ 5. M. LeverrieER—Mr ApdAMs.—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 
covery of general consent, stands first in the achievements 
Neptune 
ton 
of science, not only in the period now under review, 
theory by but even in the long and eventful series of years 
MM.Le- which have elapsed since Newton established the 
verrier and 
Adams. 
(128.) 
doctrine of universal gravitation. 
The discovery of which we speak was no less 
Its circum- than the proof of the existence of a planet beyond 
stances, 
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.) 
progress of 
Physical 
Astronomy 
in Europe, 
and in 
America. 
Bowditch, 
