orbit which lies across the
track of the earth. We thus see that the meteors cannot escape the
earth. It may be that when the shoal begins to reach this neighbourhood
the earth will have just left this part of its path, and a year will
have elapsed before the earth gets round again. Those meteors that have
the good fortune to be in the front of the shoal will thus escape the
net, but some of those behind will not be so fortunate, and the earth
will again devour an incredible host. It has sometimes happened that
casts into the shoal have been obtained in two consecutive years. If the
earth happened to pass through the front part in one year, then the
shoal is so long that the earth will have moved right round its orbit
of 600,000,000 miles, and will again dash through the critical spot
before the entire number have passed. History contains records of cases
when, in two consecutive Novembers, brilliant showers of Leonids have
been seen.

As the earth consumes such myriads of Leonids each thirty-three years,
it follows that the total number must be decreasing. The splendour of
the showers in future ages will, no doubt, be affected by this
circumstance. They cannot be always so bright as they have been. It is
also of interest to notice that the shape of the shoal is gradually
changing. Each meteor of the shoal moves in its own ellipse round the
sun, and is quite independent of the rest of these bodies. Each one has
thus a special period of revolution which depends upon the length of the
ellipse in which it happens to revolve. Two meteors will move around the
sun in the same time if the lengths of their ellipses are exactly equal,
but not otherwise. The lengths of these ellipses are many hundreds of
millions of miles, and it is impossible that they can be all absolutely
equal. In this may be detected the origin of a gradual change in the
character of the shower. Suppose two meteors A and B be such that A
travels completely round in thirty-three years, while B takes
thirty-four years. If the two start together, then when A has finished
the first round B will be a year behind; the next time B will be two
years behind, and so on. The case is exactly parallel to that of a
number of boys who start for a long race, in which they have to run
several times round the course before the distance has been
accomplished. At first they all start in a cluster, and perhaps for the
first round or two they may remain in comparative proximity;