One of the first things recognized was the fact that fire-balls are
solid meteorites in flight, and not gaseous exhalations in the air, as
some had assumed. They burn in the air during their flight, and
sometimes, perhaps, are entirely consumed before reaching the ground.
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.
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