ment of an enemy; and if he fires in that direction and
kills him, and thus saves his own army from destruction, he is deservedly
applauded for the success which attends his action. But it does not
follow that the assumption on which he acted was the only possible one.
Or, to take a more peaceful illustration, in playing whist it sometimes
becomes apparent that the game can only be won if the cards lie in a
certain way; and a good player will thereupon assume that this is the
fact, and play accordingly. Adams and Le Verrier played to win the game on
the particular assumption that the disturbance of Uranus was due to an
external planet revolving at a distance from the sun about twice that of
Uranus; _and won it_; and we applaud them for doing so. But it is easy to
imagine a rearrangement of the cards with which they would have lost it;
and Airy's question simply meant that he was alive to these wider
possibilities, and did not see the need for attempting to win the game in
that particular way.

One such alternative possibility has already been mentioned. "Hansen's
opinion was, that one disturbing body would not satisfy the phenomena; but
he conjectured that there were two planets beyond _Uranus_." Another
conceivable alternative is that there was some change in the law of
gravitation at the distance of Uranus, which, it must be remembered, is
twice as great as that of any planet previously known. Or some wandering
body might have passed close enough to Uranus to change its orbit somewhat
suddenly. We now know, for instance, that the swarm of meteorites which
gives rise to the well-known "November meteors" must have passed very
close to Uranus in A.D. 126, assuming that neither the planet nor the
swarm have been disturbed in any unknown manner in the meantime. It is to
this encounter that we owe the introduction of this swarm to our solar
system: wandering through space, they met Uranus, and were swept by his
attraction into an orbit round the sun. Was there no reaction upon Uranus
himself? The probabilities are that the total mass of the swarm was so
small as to affect the huge planet inappreciably; but who was to say that
some other swarm of larger mass, or other body, might not have approached
near Uranus at some date between 1690 and 1845, and been responsible at
any rate in part for the observed errors? These are two or three
suppositions from our familiar experience; and there are, of course,
limitless possibilities