Fig. 18). In consequence of these changes, and of the
fact that we lose sight of comets comparatively quickly, one is much
inclined to wonder what further changes may take place after the bodies
have passed beyond our ken.

The orbits of comets are, as we have seen, very elliptic. In some
instances this ellipticity is so great as to take the bodies out into
space to nearly six times the distance of Neptune from the sun. For a
long time, indeed, it was considered that comets were of two kinds,
namely, those which actually _belonged_ to the solar system, and those
which were merely _visitors_ to it for the first and only time--rushing
in from the depths of space, rapidly circuiting the sun, and finally
dashing away into space again, never to return. On the contrary,
nowadays, astronomers are generally inclined to regard comets as
permanent members of the solar system.

The difficulty, however, of deciding absolutely whether the orbits of
comets are really always _closed_ curves, that is to say, curves which
must sooner or later bring the bodies back again towards the sun, is,
indeed, very great. Comets, in the first place, are always so diffuse,
that it is impossible to determine their exact position, or, rather, the
exact position of that important point within them, known as the centre
of gravity. Secondly, that stretch of its orbit along which we can
follow a comet, is such a very small portion of the whole path, that the
slightest errors of observation which we make will result in
considerably altering our estimate of the actual shape of the orbit.

Comets have been described as so transparent that they can pass across
the sky without dimming the lustre of the smallest stars, which the
thinnest fog or mist would do. This is, indeed, true of every portion
of a comet except the nucleus, which is, as its name implies, the
densest part. And yet, in contrast to this ghostlike character, is the
strange fact that when comets are of a certain brightness they may
actually be seen in full daylight.

As might be gathered from their extreme tenuity, comets are so
exceedingly small in mass that they do not appear to exert any
gravitational attraction upon the other bodies of our system. It is,
indeed, a known fact that in the year 1886 a comet passed right amidst
the satellites of Jupiter without disturbing them in the slightest
degree. The attraction of the planet, on the other hand, so altered the
comet's orbit, as to cause