rs still remaining.

The earth never misses this swarm. Every thirty-three years it is bound
to pass through some part of them, for the shoal is so long that if the
head is just missed one November the tail will be encountered next
November. This is a plain and obvious result of its enormous length. It
may be likened to a two-foot length of sewing silk swimming round and
round an oval sixty feet in circumference. But, you will say, although
the numbers are so great that destroying a few millions or so every
thirty-three years makes but little difference to them, yet, if this
process has been going on from all eternity, they ought to be all swept
up. Granted; and no doubt the most ancient swarms have already all or
nearly all been swept up.

[Illustration: FIG. 107.--Orbit of November meteors; showing their
probable parabolic orbit previous to 126 A.D., and its sudden conversion
into an elliptic orbit by the violent perturbation caused by Uranus,
which at that date occupied the position shown.]

The August meteors, or Perseids, are an example. Every August we cross
their path, and we have a small meteoric display radiating from the
sword-hand of Perseus, but never specially more in one August than
another. It would seem as if the main shoal has disappeared, and nothing
is now left but the stragglers; or perhaps it is that the shoal has
gradually become uniformly distributed all along the path. Anyhow, these
August meteors are reckoned much more ancient members of the solar
system than are the November meteors. The November meteors are believed
to have entered the solar system in the year 126 A.D.

This may seem an extraordinary statement. It is not final, but it is
based on the calculations of Leverrier--confirmed recently by Mr. Adams.
A few moments will suffice to make the grounds of it clear. Leverrier
calculated the orbit of the November meteors, and found them to be an
oval extending beyond Uranus. It was perturbed by the outer planets near
which it went, so that in past times it must have moved in a slightly
different orbit. Calculating back to their past positions, it was found
that in a certain year it must have gone very near to Uranus, and that
by the perturbation of this planet its path had been completely changed.
Originally it had in all probability been a comet, flying in a parabolic
orbit towards the sun like many others. This one, encountering Uranus,
was pulled to pieces as it were, and its orbit made