longitudes the sun would have set previous to the contact of the planet
with its disk. I had the gratification of furnishing these observations
to Leverrier himself, who reported upon them to the Academy of
Sciences. The triumph of Leverrier was complete. It was after this that
Arago, seeing the characteristics of his mind, said to him, 'Take up
the movements of the planet Herschel,--watch them, analyze them, and
tell us what it is that causes them.' Leverrier throws aside all other
employments, and gives his mind to the investigation of this subject.
He begins entirely back. He takes up the movements of the planets
Jupiter and Saturn, and investigates them anew: he leaves nothing
untouched. Finally, after having in the most absolute manner computed
all the influence they exercise upon the planet Herschel, he says, 'I
now know positively all existing causes that disturb the planet; but
there is an outstanding power that disturbs it not yet accounted for,
and now let me rise to a knowledge of that outstanding cause.' He did
what no other man ever had attempted. He cleared up all
difficulties;--he made all daylight before his gaze. And now, how shall
I give to you an account of the train of reasoning by which he reached
out into unknown space, and evoked from its bosom a mighty world? If
you will give me the time, I will attempt to give you an idea of his
mighty workings in the field of science.

"In the first place, let it be remembered that the planets circulate
through the heavens in nearly the same plane. If I were to locate the
sun in the centre of the floor, in locating the planets around it, I
should place them upon the floor in the same plane. The first thing
that occurred to Leverrier, in looking for the planet, was this,--he
need not look out of the plane of the ecliptic. Here, then, was one
quarter in which the unknown body was to be found. The next thing was
this,--where is it located, and what is its distance from the sun? The