n into account. Could it be an outer planet? The
question occurred to several, and one or two tried to solve the problem,
but were soon stopped by the tremendous difficulties of calculation.

The ordinary problem of perturbation is difficult enough: Given a
disturbing planet in such and such a position, to find the perturbations
it produces. This was the problem that Laplace worked out in the
_Mecanique Celeste_.

But the inverse problem--given the perturbations, to find the planet
that causes them--such a problem had never yet been attacked, and by
only a few had its possibility been conceived. Friedrich Bessel made
preparations for solving this mystery in 1840, but he was prevented by
fatal illness.

In 1841 the difficulties of the problem presented by these residual
perturbations of Uranus excited the imagination of a young student, an
undergraduate of Cambridge--John Couch Adams by name--and he determined
to make a study of them as soon as he was through his _tripos_. In
January, 1843, he was graduated as senior wrangler, and shortly
afterward he set to work. In less than two years he reached a definite
conclusion; and in October, 1845, he wrote to the astronomer-royal, at
Greenwich, Professor Airy, saying that the perturbations of Uranus could
be explained by assuming the existence of an outer planet, which he
reckoned was now situated in a specified latitude and longitude.

We know now that had the astronomer-royal put sufficient faith in this
result to point his big telescope at the spot indicated and begin
sweeping for a planet, he would have detected it within 1-3/4 of the
place assigned to it by Adams. But anyone in the situation of the
astronomer-royal knows that almost every post brings absurd letters from
ambitious correspondents, some of them having just discovered perpetual
motion, or squared the circle, or proved the earth flat, or discovered
the constitution of the moon or of ether or of electricity; and in this
mass of rubbish it requires great skill and patience to detect such gems
of value as may exist.

Now this letter of Adams's was indeed a jewel of the first water, and no
doubt bore on its face a very different appearance from the chaff of
which I have spoken; but still Adams was unknown: he had been graduated
as senior wrangler, it is true, but somebody must be graduated as senior
wrangler every year, and a first-rate mathematician is not produced
every year. Those behind the scenes--as Pro