hounds to the sun is worth considering a little more in detail. To
change the figure, imagine the sun in its voyage through space to be
like a majestic battleship surrounded by its scouts. Small vessels
(the comets, as they are overhauled by the squadron, are taken in
charge by the scouts, with Jupiter for their chief, and are forced to
accompany the fleet, but not all are impressed. If a strange comet
undertakes to run across Jupiter's bows the latter brings it to, and
makes prize of it by throwing it into a relatively small ellipse with
the sun for its focus. Thenceforth, unless, as happened to the unhappy
comet of Lexell, it encounters Jupiter again in such a way as to be
diverted by him into a more distant orbit, it can never get away.
About thirty comets are now known to have thus been captured by the
great planet, and they are called ``Jupiter's Comet Family.'' But, on
the other hand, if a wandering comet crosses the wake of the chief
planetary scout the latter simply drives it away by accelerating its
motion and compels it to steer off into open space. The transformation
of comets into meteors will be considered in the next chapter, but
here, in passing, mention may be made of the strange fate of one
member of Jupiter's family, Biela's comet, which, having become over
bold in its advances to its captor, was, after a few revolutions in is
impressed orbit, torn to pieces and turned into a flock of meteors.

And now let us return to the mystery of comets' tails. That we are
fully justified in speaking of the tails of comets as mysterious is
proved by the declaration of Sir John Herschel, who averred, in so
many words, that ``there is some profound secret and mystery of nature
concerned in this phenomenon,'' and this profound secret and mystery
has not yet been altogether cleared up. Nevertheless, the
all-explaining hypothesis of Arrhenius offers us once more a certain
amount of aid. Comets' tails, Arrhenius assures us, are but another