established that there are great
numbers of schools or collections of cosmic matter that fly through
interplanetary space, having definite orbits like the planets. Any one
of these collections may be scattered through millions of miles in
length. A comet is simply one of these wandering collections of meteoric
stones having a nucleus or center where the particles are so condensed
as to give it a reflecting surface something like the planets or the
moon. This enables us to see the outline of the comet to the point where
the fragments of matter become so scattered that they are no longer able
to reflect sufficient light to reach our eyes. The fringe of a comet,
however, may extend thousands or even millions of miles beyond the
borders of luminosity.

There is scarcely a day or night in the year when more or less of these
meteoric stones do not come within the region of our atmosphere, and
when this happens the great velocity at which they travel is the means
of their own destruction. They become intensely heated by friction
against the atmosphere just as a bullet will when fired from a gun--only
to a greater extent owing to the greater velocity. They disintegrate
into dust which floats in the air for a time, when more or less of it is
precipitated upon the surface of the earth. Disintegrated meteors, or
star dust, as they are sometimes called, are often brought down by the
rain or snow. Most of the shooting stars that we observe are very small,
resembling fire-flies in the sky, but once in a while a very large one
is seen moving across the face of the heavens, giving off brilliant
scintillations that trail behind the meteor, making a luminous path that
is visible for some seconds. These brilliant manifestations are due to
one of two causes. Either there is a very large mass of incandescent
matter or else they are so much nearer to us than in ordinary cases that
they appear larger. It is more likely, however, that it is due to the
former cause rather than the latter, from the fact of its apparently
slow movement as compared with the smaller shooting stars. It has been
determined by observation that the average meteor becomes visible at a
point less than 100 miles above the earth's surface. It was found as far
back as 1823 that out of 100 shooting stars twenty-two of them had an
elevation of over twenty-four and less than forty miles; thirty-five,
between forty and fifty miles; and thirteen between seventy and eighty
miles.