sarily exhibits a definite
aspect with reference to the fixed stars. Its aspect with respect
to the earth will be very changeable, because of the rotation and
revolution of that body, but its position with respect to
constellations will be steady. Hence each meteor swarm, being a
steady parallel stream of rushing masses, always strikes us from
the same point in stellar space, and by this point (or radiant) it
is identified and named.

The paths do not appear to us to be parallel, because of
perspective: they seem to radiate and spread in all directions from
a fixed centre like spokes, but all these diverging streaks are
really parallel lines optically foreshortened by different amounts
so as to produce the radiant impression.

The annexed diagram (Fig. 105) clearly illustrates the fact that
the "radiant" is the vanishing point of a number of parallel lines.

[Illustration: FIG. 106.--Orbit of November meteors.]

This swarm is specially interesting to us from the fact that we cross
its orbit every year. Its orbit and the earth's intersect. Every
November we go through it, and hence every November we see a few
stragglers of this immense swarm. The swarm itself takes thirty-three
years on its revolution round the sun, and hence we only encounter it
every thirty-three years.

The swarm is of immense size. In breadth it is such that the earth,
flying nineteen miles a second, takes four or five hours to cross it,
and this is therefore the time the display lasts. But in length it is
far more enormous. The speed with which it travels is twenty-five miles
a second, (for its orbit extends as far as Uranus, although by no means
parabolic), and yet it takes more than a year to pass. Imagine a
procession 200,000 miles broad, every individual rushing along at the
rate of twenty-five miles every second, and the whole procession so long
that it takes more than a year to pass. It is like a gigantic shoal of
herrings swimming round and round the sun every thirty-three years, and
travelling past the earth with that tremendous velocity of twenty-five
miles a second. The earth dashes through the swarm and sweeps up
myriads. Think of the countless numbers swept up by the whole earth in
crossing such a shoal as that! But heaps more remain, and probably the
millions which are destroyed every thirty-three years have not yet made
any very important difference to the numbe