avities to the extent of a
thousand miles, the pressure would become so great that if the pit
were kept free from the heat of the earth the gaseous materials would
become liquefied. Upward from the earth's surface at the sea level the
atoms and molecules of the air become farther apart until, at the
height of somewhere between forty and fifty miles, the quantity of
them contained in the ether is so small that we can trace little
effect from them on the rays of light which at lower levels are
somewhat bent by their action. At yet higher levels, however, meteors
appear to inflame by friction against the particles of air, and even
at the height of eighty miles very faint clouds have at times been
discerned, which are possibly composed of volcanic dust floating in
the very rarefied medium, such as must exist at this great elevation.

The air not only exists in the region where we distinctly recognise
it; it also occupies the waters and the under earth. In the waters it
occurs as a mechanical mixture which is brought about as the rain
forms and falls in the air, as the streams flow to the sea, and as the
waves roll over the deep and beat against the shores. In the realm of
the waters, as well as on the land, the air is necessary for the
maintenance of all animal forms; but for its presence such life would
vanish from the earth.

Owing to certain peculiarities in its constitution, the atmosphere of
our earth, and that doubtless of myriad other spheres, serves as a
medium of communication between different regions. It is, as we know,
in ceaseless motion at rates which may vary from the speed in the
greatest tempests, which may move at the rate of somewhere a hundred
and fifty miles an hour, to the very slow movements which occur in
caverns, where the transfer is sometimes effected at an almost
microscopic rate in the space of a day. The motion of the atmosphere
is brought about by the action of heat here and there, and in a
trifling way, by the heat from the interior of the earth escaping
through hot springs or volcanoes, but almost altogether by the heat of
the sun. If we can imagine the earth cut off from the solar radiation,
the air would cease to move. We often note how the variable winds fall
away in the nighttime. Those who in seeking for the North Pole have
spent winters in the long-continued dark of that region have noted
that the winds almost cease to blow, the air being disturbed only when
a storm originated in the