of our solar system, and perhaps of other similar systems, and in part
wanderers in the vast realm which intervenes between the solar
systems. Of these we will first consider the meteors, of which we know
by far the most; though even of them, as we shall see, our knowledge
is limited.

From time to time on any starry night, and particularly in certain
periods of the year, we may behold, at the distance of fifty or more
miles above the surface of the earth, what are commonly called
"shooting stars." The most of these flashing meteors are evidently
very small, probably not larger than tiny sand grains, possibly no
greater than the fragments which would be termed dust. They enter the
air at a speed of about thirty miles a second. They are so small that
they burn to vapour in the very great heat arising from their friction
on the air, and do not attain the surface of the earth. These are so
numerous that, on the average, some hundreds of thousands probably
strike the earth's atmosphere each day. From time to time larger
bodies fall--bodies which are of sufficient bulk not to be burned up
in the air, but which descend to the ground. These may be from the
smallest size which may be observed to masses of many hundred pounds
in weight. These are far less numerous than the dust meteorites; it is
probable, however, that several hundred fragments each year attain the
earth's surface. They come from various directions of space, and
there is as yet no means of determining whether they were formed in
some manner within our planetary system or whether they wander to us
from remoter realms. We know that they are in part composed of
metallic iron commingled with nickel and carbon (sometimes as very
small diamonds) in a way rarely if ever found on the surface of our
sphere, and having a structure substantially unknown in its deposits.
In part they are composed of materials which somewhat resemble certain