nal visitors, from their large size and rapid changes,
were regarded in ancient times with dread and with amazement.

Some Comets revolve round the Sun in ellipses, but many, if not the
majority, are visitors indeed, for having once passed round the Sun
they pass away again into space, never to return.

The appearance which is generally regarded as characteristic of a Comet
is that of a head with a central nucleus and a long tail. Many, however,
of the smaller ones possess no tail, and in fact Comets present almost
innumerable differences. Moreover the same Comet changes rapidly, so
that when they return, they are identified not in any way by their
appearance, but by the path they pursue.

Comets may almost be regarded as the ghosts of heavenly bodies. The
heads, in some cases, may consist of separate solid fragments, though on
this astronomers are by no means agreed, but the tails at any rate are
in fact of almost inconceivable tenuity. We know that a cloud a few
hundred feet thick is sufficient to hide, not only the stars, but even
the Sun himself. A Comet is thousands of miles in thickness, and yet
even extremely minute stars can be seen through it with no appreciable
diminution of brightness. This extreme tenuity of comets is moreover
shown by their small weight. Enormous as they are I remember Sir G. Airy
saying that there was probably more matter in a cricket ball than there
is in a comet. No one, however, now doubts that the weight must be
measured in tons; but it is so small, in relation to the size, as to be
practically inappreciable. If indeed they were comparable in mass even
to the planets, we should long ago have perished. The security of our
system is due to the fact that the planets revolve round the Sun in one
direction, almost in circles, and very nearly in the same plane. Comets,
however, enter our system in all directions, and at all angles; they are
so numerous that, as Kepler said, there are probably more Comets in the
sky than there are fishes in the sea, and but for their extreme tenuity
they would long ago have driven us into the Sun.

When they first come in sight Comets have generally no tail; it grows as
they approach the Sun, from which it always points away. It is no mere
optical illusion; but while the Comet as a whole is attracted by the
Sun, the tail, how or why we know not, is repelled. When once driven
off, moreover, the attraction of the Comet is not sufficient to recall
it, and hen