y
disappeared. It has never been seen since. It was then recorded and
advertised as the missing comet.

But now comes the interesting part of the story. The orbit of this Biela
comet was well known, and it was found that on a certain night in 1872
the earth would cross the orbit, and had some chance of encountering the
comet. Not a very likely chance, because it need not be in that part of
its orbit at the time; but it was suspected not to be far off--if still
existent. Well, the night arrived, the earth did cross the orbit, and
there was seen, not the comet, but a number of shooting-stars. Not one
body, nor yet two, but a multitude of bodies--in fact, a swarm of
meteors. Not a very great swarm, such as sometimes occurs, but still a
quite noticeable one; and this shower of meteors is definitely
recognized as flying along the track of Biela's comet. They are known as
the Andromedes.

This observation has been generalized. Every cometary orbit is marked by
a ring of meteoric stones travelling round it, and whenever a number of
shooting-stars are seen quickly one after the other, it is an evidence
that we are crossing the track of some comet. But suppose instead of
only crossing the track of a comet we were to pass close to the comet
itself, we should then expect to see an extraordinary swarm--a multitude
of shooting-stars. Such phenomena have occurred. The most famous are
those known as the November meteors, or Leonids.

This is the third of those bodies whose history I had to tell you.
Professor H.A. Newton, of America, by examining ancient records arrived
at the conclusion that the earth passed through a certain definite
meteor shoal every thirty-three years. He found, in fact, that every
thirty-three years an unusual flight of shooting-stars was witnessed in
November, the earliest record being 599 A.D. Their last appearance had
been in 1833, and he therefore predicted their return in 1866 or 1867.
Sure enough, in November, 1866, they appeared; and many must remember
seeing that glorious display. Although their hail was almost continuous,
it is estimated that their average distance apart was thirty-five miles!
Their radiant point was and always is in the constellation Leo, and
hence their name Leonids.

[Illustration: FIG. 105.--Radiant point perspective. The arrows
represent a number of approximately parallel meteor-streaks
foreshortened from a common vanishing-point.]

A parallel stream fixed in space neces