t the point
of the earth that would strike them would be toward a certain place
in the constellation of Andromeda, if the remains of the diluted
comet were still there. The prediction was verified in every
respect. At the appointed time, place, [Page 126] and direction, the
streaming lights were in our sky. That these little bodies belonged
to the original comet none can doubt. By the perturbations of
planetary attraction, or by different original velocities, a comet
may be lengthened into an invisible stream, or an invisible stream
agglomerated till it is visible as a comet.

_Comets._

Comets will be most easily understood by the foregoing considerations.
They are often treated as if they were no part of the solar system;
but they are under the control of the same laws, and owe their
existence, motion, and continuance to the same causes as Jupiter and
the rest of the planets. They are really planets of wider wandering,
greater ellipticity, and less density. They have periodic times
less than the earth, and fifty times as great as Neptune. They
are little clouds of gas or meteoric matter, or both, darting into
the solar system from every side, at every angle with the plane
of the ecliptic, becoming luminous with reflected light, passing
the sun, and returning again to outer darkness. Sometimes they
have no tail, having a nucleus surrounded by nebulosity like a
dim sun with zodiacal light; sometimes one tail, sometimes half a
dozen. These follow the comet to perihelion, and precede it afterward
(Fig. 52). The orbits of some comets are enormously elongated; one
end may lie inside the earth's orbit, and the other end be as far
beyond Neptune as that is from the sun. Of course only a small
part of such a curve can be studied by us: the comet is visible
only when near the sun. The same curve around the sun may be an
orbit that will bring it back again, [Page 127] or one that will
carry it off into infinite space, never to return. One rate of speed
on the curve indicates an elliptical orbit that returns; a greater
rate of speed indicates that it will take a parabolic orbit, which
never returns. The exact rate of speed is exceedingly difficult to
determine; hence it cannot be confidently asserted that any comet
ever visible will not return. They may all belong to the solar
system; but some will certainly be gone thousands of years before
their fiery forms will greet the watchful eyes of dwellers on the
earth. A comet that