o represent the position of the tail of the
comet at various points of its path. It would be, perhaps, going too far
to assert that throughout the whole vast journey of the comet, its tail
must always be directed from the sun. In the first place, it must be
recollected that we can only see the comet during that small part of its
journey when it is approaching to or receding from the sun. It is also
to be remembered that, while actually passing round the sun, the
brilliancy of the comet is so overpowered by the sun that the comet
often becomes invisible, just as the stars are invisible in daylight.
Indeed, in certain cases, jets of cometary material are actually
projected towards the sun.

[Illustration: Fig. 71.--The Tail of a Comet directed from the Sun.]

In a hasty consideration of the subject, it might be thought that as the
comet was dashing along with enormous velocity the tail was merely
streaming out behind, just as the shower of sparks from a rocket are
strewn along the path which it follows. This would be an entirely
erroneous analogy; the comet is moving not through an atmosphere, but
through open space, where there is no medium sufficient to sweep the
tail into the line of motion. Another very remarkable feature is the
gradual growth of the tail as the comet approaches the sun. While the
body is still at a great distance it has usually no perceptible tail,
but as it draws in the tail gradually develops, and in some cases
reaches stupendous dimensions. It is not to be supposed that this
increase is a mere optical consequence of the diminution of distance. It
can be shown that the growth of the tail takes place much more rapidly
than it would be possible to explain in this way. We are thus led to
connect the formation of the tail with the approach to the sun, and we
are accordingly in the presence of an enigma without any analogy among
the other bodies of our system.

That the comet as a whole is attracted by the sun there can be no doubt
whatever. The fact that the comet moves in an ellipse or in a parabola
proves that the two bodies act and react on each other in obedience to
the law of universal gravitation. But while this is true of the comet as
a whole, it is no less certain that the tail of the comet is _repelled_
by the sun. It is impossible to speak with certainty as to how this
comes about, but the facts of the case seem to point to an explanation
of the following kind.

We have seen that the spec