It was determined by Professor Herschel that out of sixty
observations of shooting stars the average height of their first
appearance was seventy-eight miles and their disappearance was at a
point fifty-three miles above the earth.

It is a matter of history, however, that sometimes these meteoric stones
descend to the surface of the earth before they are entirely
disintegrated. A fine specimen of this kind is to be seen in the
Smithsonian Institution. There are over forty specimens of these
aerolites (air-stones) in the British Museum, labeled with the times and
places of their fall. Instances of falling to the earth are so rare that
there is little to fear from these wandering missiles of the air. We do
not remember a case where life or property has suffered from the fall of
a meteor.

This brings us to the consideration of the part which the great air
envelope surrounding the earth plays as a protection against many
outside influences. For instance, if it were not for the air, millions
of these meteoric stones would be showered upon our earth every year and
at certain times every day, which would render the earth untenable for
human existence. We should be at the mercy of those wandering comets
whose fringes strike our atmosphere more or less deeply at frequent
intervals. It is not impossible that the earth may at some time pass
directly through one, and yet there is little danger that in such a case
there would be more than an unusual display of celestial fireworks.

From the facts that have been above stated it will be apparent to anyone
that the number of these meteoric stones in the air is being constantly
reduced by their constant collision with the atmosphere and consequent
reduction to ashes or dust. Another conclusion is that the earth must be
gradually, but imperceptibly perhaps, increasing in size on account of
the constant settling upon its surface of meteoric dust.

CHAPTER XVII.

THE SKY AND ITS COLOR.

In the chapters on light in Vol. II. it will be stated that we see all
objects by a reflected light, except those that are self-luminous, such
as the sun or any other source of light. We see the moon and many of the
planets entirely by reflection. There are myriads of smaller objects,
too small to be seen as such, even under a microscope, that still have a
power to reflect light that is sensible to our vision. The air
surrounding the globe is literally filled with these microscopic light
r