828 PHYSICAL ASTRONOMY.—M. LEVERRIER—MR ADAMS. (Diss. VI. 
(129.) Since the publication of the Principia, or rather 1781, the planet which he called Georgium Sidus, Irregulari- 
aigtctap. we should say since the great theory contained in (afterwards named Herschel, and finally, by general ties of the 
(130.) 
(131.) 
History. 
(132.) 
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 
es. 
The 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 accwmulated 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 thé 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 elliptie 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, 
We shall now glance at the history of these un- 
explained perturbations, and at the rise and growth of 
the idea of their being explicable by the influence of 
an unseen body, 
After Sir William Herschel had discovered, in 
consent, Uranus,) it was easy, by a few observations, 
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 cireumstances 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- 1. 
vity, cometary perturbation, a resisting medium, 
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.! 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.?_ 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. 
2 In her Connection of the Physical Sciences. 
