em off into vapour? Had the Rowton siderite, for instance, struck our
atmosphere with a velocity of twenty miles a second, it seems
unquestionable that it would have been dissipated by heat, though, no
doubt, the particles would ultimately coalesce so as to descend slowly
to the earth in microscopic beads of iron. How has the meteorite escaped
this fate? It must be remembered that our earth is also moving with a
velocity of about eighteen miles per second, and that the _relative_
velocity with which the meteorite plunges into the air is that which
will determine the degree to which friction is operating. If the
meteorite come into direct collision with the earth, the velocity of the
collision will be extremely great; but it may happen that though the
actual velocities of the two bodies are both enormous, yet the relative
velocity may be comparatively small. This is, at all events, one
conceivable explanation of the arrival of a meteorite on the surface of
the earth.

We have shown in the earlier parts of the chapter that the well-known
star showers are intimately connected with comets. In fact, each star
shower revolves in the path pursued by a comet, and the shooting star
particles have, in all probability, been themselves derived from the
comet. Showers of shooting stars have, therefore, an intimate connection
with comets, but it is doubtful whether meteorites have any connection
with comets. It has already been remarked that meteorites have never
been known to fall in the great star showers. No particle of a meteorite
is known to have dropped from the countless host of the Leonids or of
the Perseids; as far as we know, the Lyrids never dropped a meteorite,
nor did the Quadrantids, the Geminids, or the many other showers with
which every astronomer is familiar. There is no reason to connect
meteorites with these showers, and it is, therefore, doubtful whether we
should connect meteorites with comets.

With reference to the origin of meteorites it is difficult to speak with
any great degree of confidence. Every theory of meteorites presents
difficulties, so it seems that the only course open to us is to choose
that view of their origin which seems least improbable. It appears to me
that this condition is fulfilled in the theory entertained by the
Austrian mineralogist, Tschermak. He has made a study of the meteorites
in the rich collection at Vienna, and he has come to the conclusion that
the "meteorites have had a vo