Their velocity before entering the earth's atmosphere is equal to that
of the planets in their orbits -- viz., from twenty to thirty miles
per second -- a fact which proves that the sun is the seat of the
central force governing them. Their burning in the air is not
difficult to explain; it is the heat of friction which so quickly
brings them to incandescence. Calculation shows that a body moving
through the air at a velocity of about a mile per second will be
brought, superficially, to the temperature of ``red heat'' by friction
with the atmosphere. If its velocity is twenty miles per second the
temperature will become thousands of degrees. This is the state of
affairs with a meteorite rushing into the earth's atmosphere; its
surface is liquefied within a few seconds after the friction begins to
act, and the melted and vaporized portion of its mass is swept
backward, forming the train of sparks that follows every great
fire-ball. However, there is one phenomenon connected with the trains
of meteorites which has never been satisfactorily explained: they
often persist for long periods of time, drifting and turning with the
wind, but not ceasing to glow with a phosphorescent luminosity. The
question is, Whence comes this light? It must be light without heat,
since the fine dust or vapor of which the train can only consist would
not retain sufficient heat to render it luminous for so long a time.
An extremely remarkable incident of this kind occurred on February 22,
1909, when an immense fire-ball that passed over southern England left
a train that remained visible during two hours, assuming many curious
shapes as it was drifted about by currents in the air.

But notwithstanding the enormous velocity with which meteorites enter
the air they are soon slowed down to comparatively moderate speed, so
that when they disappear they are usually traveling not faster than a
mile a second. The courses of many have been traced by observers