ce it caused in
Uranus, some thousand million miles away from it.

Without going into further detail, suffice it to say that in June, 1846,
he published his last paper, and in it announced to the world his
theoretical position for the planet.

Professor Airy received a copy of this paper before the end of the
month, and was astonished to find that Leverrier's theoretical place for
the planet was within 1 deg. of the place Mr. Adams had assigned to it eight
months before. So striking a coincidence seemed sufficient to justify a
Herschelian "sweep" for a week or two.

But a sweep for so distant a planet would be no easy matter. When seen
in a large telescope it would still only look like a star, and it would
require considerable labour and watching to sift it out from the other
stars surrounding it. We know that Uranus had been seen twenty times,
and thought to be a star, before its true nature was by Herschel
discovered; and Uranus is only about half as far away as Neptune is.

Neither in Paris nor yet at Greenwich was any optical search undertaken;
but Professor Airy wrote to ask M. Leverrier the same old question as he
had fruitlessly put to Mr. Adams: Did the new theory explain the errors
of the radius vector or not? The reply of Leverrier was both prompt and
satisfactory--these errors were explained, as well as all the others.
The existence of the object was then for the first time officially
believed in.

The British Association met that year at Southampton, and Sir John
Herschel was one of its Sectional Presidents. In his inaugural address,
on September 10th, 1846, he called attention to the researches of
Leverrier and Adams in these memorable words:--

"The past year has given to us the new [minor] planet Astraea; it
has done more--it has given us the probable prospect of another.
We see it as Columbus saw America from the shores of Spain. Its
movements have been felt trembling along the far-reaching line of
our analysis with a certainty hardly inferior to ocular
demonstration."

It was about time to begin to look for it. So the Astronomer-Royal
thought on reading Leverrier's paper. But as the national telescope at
Greenwich was otherwise occupied, he wrote to Professor Challis, at
Cambridge, to know if he would permit a search to be made for it with
the Northumberland Equatoreal, the large telescope of Cambridge
University, presented to it by one of the Dukes of Northumberlan