milar evidences of slight or great modern changes
in the level of the lands. At some points, particularly on the coast
of Alaska and along the coast of Peru, these uplifts of the land have
amounted to a thousand feet or more. In the peninsular district of
Scandinavia the swayings, sometimes up and sometimes down, which are
now going on have considerably changed the position of the shore lines
since the beginning of the historical period.

There are other causes which serve to modify the shapes and sizes of
the continents which may best be considered in the sequel; for the
present we may pass from this subject with the statement that our
great lands are relatively permanent features; their forms change from
age to age, but they have remained for millions of years habitable to
the hosts of animals and plants which have adapted their life to the
conditions which these fields afford them.

CHAPTER V.

THE ATMOSPHERE.

The firm-set portion of the earth, composed of materials which became
solid when the heat so far disappeared from the sphere that rocky
matter could pass from its previous fluid condition to the solid or
frozen state, is wrapped about by two great envelopes, the atmosphere
and the waters. Of these we shall first consider the lighter and more
universal air; in taking account of its peculiarities we shall have to
make some mention of the water with which it is greatly involved;
afterward we shall consider the structure and functions of that fluid.

Atmospheric envelopes appear to be common features about the celestial
spheres. In the sun there is, as we have noted, a very deep envelope
of this sort which is in part composed of the elements which form our
own air; but, owing to the high temperature of the sphere, these are
commingled with many substances which in our earth--at least in its
outer parts--have entered in the solid state. Some of the planets, so
far as we can discern their conditions, seem also to have gaseous
wraps; this is certainly the case with the planet Mars, and even the
little we know of the other like spheres justifies the supposition
that Jupiter and Saturn, at least, have a like constitution. We may
regard an atmosphere, in a word, as representing a normal and
long-continued state in the development of the heavenly orbs. In only
one of these considerable bodies of the solar system, the moon, do we
find tolerably clear e