around the sun, in virtue of the sun's
attraction, and we know that the ellipse will be preserved without the
minutest alteration if the sun and the planet be left to their mutual
attractions, and if no other force intervene. We can also calculate the
influence of each of the known planets on the form and position of the
orbit. But when allowance is made for all such perturbing influences it
is found that the observed and computed orbits do not agree. The
conclusion is irresistible. Uranus does not move solely in consequence
of the sun's attraction and that of the planets of our system interior
to Uranus; there must therefore be some further influence acting upon
Uranus besides those already known. To the development of this subject
the next chapter will be devoted.

CHAPTER XV.

NEPTUNE.

Discovery of Neptune--A Mathematical Achievement--The Sun's
Attraction--All Bodies attract--Jupiter and Saturn--The Planetary
Perturbations--Three Bodies--Nature has simplified the
Problem--Approximate Solution--The Sources of Success--The Problem
Stated for the Earth--The Discoveries of Lagrange--The
Eccentricity--Necessity that all the Planets revolve in the same
Direction--Lagrange's Discoveries have not the Dramatic Interest of
the more Recent Achievements--The Irregularities of Uranus--The
Unknown Planet must revolve outside the Path of Uranus--The Data
for the Problem--Le Verrier and Adams both investigate the
Question--Adams indicates the Place of the Planet--How the Search
was to be conducted--Le Verrier also solves the Problem--The
Telescopic Discovery of the Planet--The Rival Claims--Early
Observation of Neptune--Difficulty of the Telescopic Study of
Neptune--Numerical Details of the Orbit--Is there any Outer
Planet?--Contrast between Mercury and Neptune.

We describe in this chapter a discovery so extraordinary that the whole
annals of science may be searched in vain for a parallel. We are not
here concerned with technicalities of practical astronomy. Neptune was
first revealed by profound mathematical research rather than by minute
telescopic investigation. We must develop the account of this striking
epoch in the history of science with the fulness of detail which is
commensurate with its importance; and it will accordingly be necessary,
at the outset of our narrative, to make an excursion into a difficult
but attractive departme