the mote in a sunbeam or as mighty
as the planet Jupiter, must perforce trace out its path around the sun
in conformity with the laws of Kepler.

Who does not know that beautiful occurrence which we call a shooting
star, or which, in its more splendid forms, is sometimes called a meteor
or fireball? It is to objects of this class that we are now to direct
our attention.

A small body is moving round the sun. Just as a mighty planet revolves
in an ellipse, so even a small object will be guided round and round in
an ellipse with the sun in the focus. There are, at the present moment,
inconceivable myriads of such meteors moving in this manner. They are
too small and too distant for our telescopes, and we never see them
except under extraordinary circumstances.

When the meteor flashes into view it is moving with such enormous
velocity that it often traverses more than twenty miles in a second of
time. Such a velocity is almost impossible near the earth's surface: the
resistance of the air would prevent it. Aloft, in the emptiness of
space, there is no air to impede its flight. It may have been moving
round and round the sun for thousands, perhaps for millions of years,
without suffering any interference; but the supreme moment arrives, and
the meteor perishes in a streak of splendour.

In the course of its wanderings the body comes near the earth, and
within a few hundred miles of its surface begins to encounter the upper
surface of the atmosphere with which the earth is enclosed. To a body
moving with the appalling velocity of a meteor, a plunge into the
atmosphere is usually fatal. Even though the upper layers of air are
excessively attenuated, yet they suddenly check the velocity almost as
a rifle bullet would be checked when fired into water. As the meteor
rushes through the atmosphere the friction of the air warms its surface;
gradually it becomes red-hot, then white-hot, and is finally driven off
into vapour with a brilliant light, while we on the earth, one or two
hundred miles below, exclaim: "Oh, look, there is a shooting star!"

We have here an experiment illustrating the mechanical theory of heat.
It may seem incredible that mere friction should be sufficient to
generate heat enough to produce so brilliant a display, but we must
recollect two facts: first, that the velocity of the meteor is, perhaps,
one hundred times that of a rifle bullet; and, second, that the
efficiency of friction in developing heat i