we
supposed, as we should probably do for a long time, that our small world
was the centre of the universe, and that the stars turned round it, we
should be compelled to admit that it was turning on its own axis from
west to east once in the time just named. There would be no escape. The
mere fact that all the time the stars thus seemed to be turning round
the moon, the earth would not so seem to move, but would lie always in
the same direction, would in no sort help to remove the difficulty.
Lunarian paradoxists would probably argue that she was in some way
rigidly connected with the moon; but even they would never think of
arguing that their world did not turn on its axis, _unless_ they
maintained that it was the centre of the universe. This, I think, they
would very probably do; but as yet terrestrial paradoxists have not, I
believe, maintained this hypothesis. I once asked Mr. Perigal whether
that was the true theory of the universe--the moon central, the earth,
sun, and heavens carried round her. He admitted that his objections to
accepted views were by no means limited to the moon's rotation; and, if
I remember rightly, he said that the idea I had thrown out in jest was
nearer the truth than I thought, or used words to that effect. But as
yet the theory has not been definitely enunciated that the moon is the
boss of the universe.

Comets, as already mentioned, have been the subjects of paradoxes
innumerable; but as yet comets have been so little understood, even by
astronomers, that paradoxes respecting them cannot be so readily dealt
with as those relating to well-established facts. Among thoroughly
paradoxical ideas respecting comets, however, may be mentioned one whose
author is a mathematician of well-deserved repute--Professor Tait's
'Sea-Bird Theory' of Comets' Tails. According to this theory, the rapid
formation of long tails and the rapid changes of their position may be
explained on the same principle that we explain the rapid change of
appearance of a flight of sea-birds, when, from having been in a
position where the eye looks athwart it, the flight assumes a position
where the eye looks at it edgewise. In the former position it is
scarcely visible (when at a distance), in the latter it is seen as a
well-defined streak; and as a very slight change of position of each
bird may often suffice to render an extensive flight thus visible
throughout its entire length, which but a few moments before had been