190 History of the Discovery of the Planet Neptune. 
and agreeing closely with the results which Le Verrier subse- 
quently obtained. 
Le Verrier next proceeds to enquire after the cause of the dis- 
eovered irregularities. Is it possible that at the immense distance 
of Uranus from the sun, the force of attraction does not vary in- 
versely as the square of the distance? ‘Ihe law of gravitation is 
too firmly established to permit such a supposition, until every 
other resource has failed. Are these irregularities due to the re- 
sistance of a rare ether diffused every where through space? No 
other planet has afforded any indications of such a resistance. 
Can they be ascribed to a great satellite accompanying the plan- 
et? Such acause would produce inequalities having a very short 
period; while the observed anomalies of Uranus are precisely the 
reverse. Has a comet impinged upon Uranus, and changed the 
form of its orbit?) Such a cause might render it impossible to 
represent the entire series of observations by a single elliptic or- 
bit; but the observations. before the supposed collision would a 
be consistent with each Sther, and the observations after collision 
would also be consistent with each other. Yet the observations 
of Uranus from 1781 to 1821, can neither be reconciled with the 
earlier observations nor with the more recent ones. 
ere seems to remain no other probable supposition than that 
of an undiscovered planet. But if these disturbances are due to 
such a body, we cannot suppose it situated within the orbit of 
Saturn. This would disturb the orbit of Saturn more than that 
of Uranus, while we know that its influence on Saturn is inap- 
preciable, for Saturn’s motion is well represented by the tables. 
Can this body be situated between Saturn and Uranus? We 
must then place it much nearer Uranus than Saturn, for the rea- 
son already assigned,in which case its mass must be supposed to 
be small, or it would produce too great an effect upon Uranus. 
Under these circumstances, its action would only be appreciable 
when in the immediate neighborhood of Uranus, which supposi- 
tion does not accord well with the observations. The disturbing 
body must then be situated beyond Uranus, and at a considerable 
distance from it; for reasons already given. Now the distance of 
each of the more remote planets from the sun, is about double 
that of the preceding one. It is natural then to: conjecture that 
the disturbing planet may be at a distance from the sun double 
hat of Uranus; and it must move nearly in the ecliptic, because 
the observed inequalities of Uranus are chiefly in the direction of 
the ecliptic. Le Verrier then propounds the following specifie 
problem: 
_ “ Are the irregularities in the motion of Uranus due to the ac- 
tion of a planet situated in the ecliptic, at a distance from t 
sun double that of Uranus? If so, what is its present place, its 
elements of its orbit?” This problem he proceeds 
to resolve. 
