the true meaning of their condition. It was not even
known how closely some of them are enchained until
the German astronomer Encke, in 1822, showed that
one which he had rediscovered, and which has since
borne his name, was moving in an orbit so contracted
that it must complete its circuit in about three and
a half years. Shortly afterwards another comet, revolving
in a period of about six years, was discovered
by Biela, and given his name. Only two more of these
short-period comets were discovered during the first half
of last century, but latterly they have been shown to be
a numerous family. Nearly twenty are known which
the giant Jupiter holds so close that the utmost reach of
their elliptical tether does not let them go beyond
the orbit of Saturn. These aforetime wanderers have
adapted themselves wonderfully to planetary customs,
for all of them revolve in the same direction with the
planets, and in planes not wide of the ecliptic.

Checked in their proud hyperbolic sweep, made captive
in a planetary net, deprived of their trains, these
quondam free-lances of the heavens are now mere
shadows of their former selves. Considered as to mere
bulk, they are very substantial shadows, their extent
being measured in hundreds of thousands of miles; but
their actual mass is so slight that they are quite at the
mercy of the gravitation pulls of their captors. And
worse is in store for them. So persistently do sun and
planets tug at them that they are doomed presently to
be torn into shreds.