dy withdraws until it recedes to the outermost part of its
path, then it will turn, and again approach the sun. It would seem that
it becomes invigorated by the sun's rays, and commences to dilate under
their genial influence. While moving in this part of its path the comet
lessens its distance from the earth. It daily increases in splendour,
until at length, partly by the intrinsic increase in brightness and
partly by the decrease in distance from the earth, it comes within the
range of our telescopes. We can generally anticipate when this will
occur, and we can tell to what point of the heavens the telescope is to
be pointed so as to discern the comet at its next return to perihelion.
The comet cannot elude the grasp of the mathematician. He can tell when
and where the comet is to be found, but no one can say what it will be
like.

Were all the other bodies of the system removed, then the path of
Encke's comet must be for ever performed in the same ellipse and with
absolute regularity. The chief interest for our present purpose lies not
in the regularity of its path, but in the _irregularities_ introduced
into that path by the presence of the other bodies of the solar system.
Let us, for instance, follow the progress of the comet through its
perihelion passage, in which the track lies near that of the planet
Mercury. It will usually happen that Mercury is situated in a distant
part of its path at the moment the comet is passing, and the influence
of the planet will then be comparatively small. It may, however,
sometimes happen that the planet and the comet come close together. One
of the most interesting instances of a close approach to Mercury took
place on the 22nd November, 1848. On that day the comet and the planet
were only separated by an interval of about one-thirtieth of the earth's
distance from the sun, _i.e._ about 3,000,000 miles. On several other
occasions the distance between Encke's comet and Mercury has been less
than 10,000,000 miles--an amount of trifling import in comparison with
the dimensions of our system. Approaches so close as this are fraught
with serious consequences to the movements of the comet. Mercury, though
a small body, is still sufficiently massive. It always attracts the
comet, but the efficacy of that attraction is enormously enhanced when
the comet in its wanderings comes near the planet. The effect of this
attraction is to force the comet to swerve from its path, and to impress
ce