aper
above reproduced, and Airy with his business-like habits ultimately
proceeded to deal with it; he wrote the answer given above asking Adams a
definite question, filed a copy of it with the original letter, and then
dismissed the matter from his thoughts until the reply from Adams, which
he confidently expected should again bring it under notice.

This further disappointment was, however, too much for Adams; he regarded
the question put by Airy as having so obvious an answer that it was
intended as an evasion, though this was far from being the case. Airy was
thoroughly in earnest about his question, though it must be admitted that
a more careful study of the problem would have shown him that it was
unnecessary. Later, when he learnt of Le Verrier's researches, he put the
same question to him, and received a polite but very clear answer, showing
that the suggested test was not an _experimentum crucis_ as he supposed.
But Adams did not feel equal to making this reply; he shrank into his
shell and solaced himself only by commencing afresh another solution of
the problem which had so engrossed his life at that time.

[Sidenote: The merits of Airy's question.]

[Sidenote: The range of possibilities.]

I have heard severe or contemptuous things said about this question by
those who most blame Airy. Some of them have no hesitation in accusing him
of intellectual incompetence: they say that it was the question of a
stupid man. I think that in the first place they forget the difference
between a deliberate error of judgement and a mere consequence of
insufficient attention. But there is even more than this to be said in
defence of the question. The "error of radius vector" came before Airy in
an entirely independent way, and as an entirely independent phenomenon,
from the "error of longitude," and there was nothing unnatural in
regarding it as requiring independent explanation. It is true that, _as
the event proved_, a mere readjustment of the orbit of Uranus got rid of
this error of radius vector (this was substantially Le Verrier's answer to
Airy's question); but we must not judge of what was possible before the
event in the light of what we now know. The original possibilities were
far wider, though we have forgotten their former extent now that they have
been narrowed down by the discovery. If a sentry during war time hears a
noise in a certain direction, he may be compelled to make the assumption
that it is the move