tigate
a part of the solar system consisting of smaller collections of
matter scattered everywhere through space. They are of various
densities, from a cloudlet of rarest gas to solid rock; of various
sizes, from a grain's weight to little worlds; of various relations
to each other, from independent individuality to related streams
millions of miles long. When they become visible they are called
shooting-stars, which are evanescent star-points darting through
the upper air, leaving for an instant a brilliant train; meteors,
sudden lights, having a discernible diameter, passing over a large
extent of country, often exploding with violence (Fig. 48), and
throwing down upon the earth aerolites; and comets, vast extents
of ghostly light, that come we know not whence and go we know not
whither. All these forms of matter are governed by the same laws
as the worlds, and are an integral part of the solar system--a
part of the unity of the universe.

[Illustration: Fig. 48.--Explosion of a Bolide.]

Everyone has seen the so-called shooting-stars. They break out
with a sudden brilliancy, shoot a few degrees with quiet speed,
and are gone before we can say, "See there!" The cause of their
appearance, the [Page 120] conversion of force into heat by their
contact with our atmosphere, has been already explained. Other facts
remain to be studied. They are found to appear about seventy-three
miles above the earth, and to disappear about twenty miles nearer
the surface. Their average velocity, thirty-five, sometimes rises to
one hundred miles a second. They exhibit different colors, according
to their different chemical substances, which are consumed. The
number of them to be seen on different nights is exceedingly
variable; sometimes not more [Page 121] than five or six an hour,
and sometimes so many that a man cannot count those appearing in a
small section of sky. This variability is found to be periodic.
There are everywhere in space little meteoric masses of matter, from
the weight of a grain to a ton, and from the density of gas to rock.
The earth meets 7,500,000 little bodies every day--there is
collision--the little meteoroid gives out its lightning sign of
extinction, and, consumed in fervent heat, drops to the earth as gas
or dust. If we add the number light enough to be seen by a
telescope, they cannot be less than 400,000,000 a day. Everywhere we
go, in a space as large as that occupied by the earth and its
atmosphere, t