for they would never hear any sound. What we call _sounds_
are merely vibrations set up in the air, which travel along and strike
upon the drum of the ear.

The atmosphere is densest near the surface of the earth, and becomes
less and less dense away from it, as a result of diminishing pressure of
air from above. The greater portion of it is accumulated within four or
five miles of the earth's surface.

It is impossible to determine exactly at what distance from the earth's
surface the air ceases altogether, for it grows continually more and
more rarefied. There are, however, two distinct methods of ascertaining
the distance beyond which it can be said practically not to exist. One
of these methods we get from twilight. Twilight is, in fact, merely
light reflected to us from those upper regions of the air, which still
continue to be illuminated by the sun after it has disappeared from our
view below the horizon. The time during which twilight lasts, shows us
that the atmosphere must be at least fifty miles high.

But the most satisfactory method of ascertaining the height to which the
atmosphere extends is from the observation of meteors. It is found that
these bodies become ignited, by the friction of passing into the
atmosphere, at a height of about 100 miles above the surface of the
earth. We thus gather that the atmosphere has a certain degree of
density even at this height. It may, indeed, extend as far as about 150
miles.

The layer of atmosphere surrounding our earth acts somewhat in the
manner of the glass covering of a greenhouse, bottling in the sun's
rays, and thus storing up their warmth for our benefit. Were this not
so, the heat which we get from the sun would, after falling upon the
earth, be quickly radiated again into space.

It is owing to the unsteadiness of the air that stars are seen to
twinkle. A night when this takes place, though it may please the average
person, is worse than useless to the astronomer, for the unsteadiness is
greatly magnified in the telescope. This twinkling is, no doubt, in a
great measure responsible for the conventional "points" with which Art
has elected to embellish stars, and which, of course, have no existence
in fact.

The phenomena of _Refraction_,[13] namely, that bending which rays of
light undergo, when passing _slant-wise_ from a rare into a dense
transparent medium, are very marked with regard to the atmosphere. The
denser the medium into which such rays p