ate of about sixty per
hour. They are noted for their great rapidity of motion, and their
trails besides often persist for a minute or two before being
disseminated. Unlike the other well-known showers, the radiants of which
are stationary, that of the Perseids shifts each night a little in an
easterly direction.

The orbit of the Perseids cuts that of the earth almost perpendicularly.
The bodies are generally supposed to be the result of the disintegration
of an ancient comet which travelled in the same orbit. Tuttle's Comet,
which passed close to the earth in 1862, also belongs to this orbit; and
its period of revolution is calculated to be 131 years. The Perseids
appear to be disseminated all along this great orbit, for we meet them
in considerable quantities each year. The bodies in question are in
general particularly small. The swarm has, however, like most others, a
somewhat denser portion, and through this the earth passed in 1848. The
_aphelion_, or point where the far end of the orbit turns back again
towards the sun, is situated right away beyond the path of Neptune, at a
distance of forty-eight times that of the earth from the sun. The comet
of 1532 also belongs to the Perseid orbit. It revisited the
neighbourhood of the earth in 1661, and should have returned in 1789.
But we have no record of it in that year; for which omission the then
politically disturbed state of Europe may account. If not already
disintegrated, this comet is due to return in 1919.

This supposed connection between comets and meteor-swarms must be also
extended to the case of the Leonids. These meteors appear to travel
along the same track as Tempel's Comet of 1866.

It is considered that the attractions of the various bodies of the
solar system upon a meteor swarm must eventually result in breaking up
the "bunched" portion, so that in time the individual meteors should
become distributed along the whole length of the orbit. Upon this
assumption the Perseid swarm, in which the meteors are fairly well
scattered along its path, should be of greater age than the Leonid. As
to the Leonid swarm itself, Le Verrier held that it was first brought
into the solar system in A.D. 126, having been captured from outer space
by the gravitative action of the planet Uranus.

The acknowledged theory of meteor swarms has naturally given rise to an
idea, that the sunlight shining upon such a large collection of
particles ought to render a swarm visible