olar regions, the cold will augment; and the same
result will be produced when there is more sea between or near the
tropics; while, on the contrary, so often as the above conditions are
reversed, the heat will be greater. (See figs. 5 and 6, p. 111.) If this
be admitted, it will follow, that unless the superficial inequalities of
the earth be fixed and permanent, there must be never-ending
fluctuations in the mean temperature of every zone; and that the climate
of one era can no more be a type of every other; than is one of our four
seasons of all the rest.

It has been well said, that the earth is covered by an ocean, in the
midst of which are two great islands, and many smaller ones; for the
whole of the continents and islands occupy an area scarcely exceeding
one-fourth of the whole superficies of the spheroid. Now, according to
this analogy, we may fairly speculate on the probability that there
would not be usually, at any given epoch of the past, more than about
one-fourth dry land in a particular region; as, for example, near the
poles, or between them and the 75th parallels of N. and S. latitude.
If, therefore, at present there should happen to be, in both these
quarters of the globe, much _more_ than this average proportion of land,
some of it in the arctic region, being above, five thousand feet in
height, and if in antarctic latitudes a mountainous country has been
found varying from 4000 to 14,000 feet in height, this alone affords
ground for concluding that, in the present state of things, the mean
heat of the climate is below that which the earth's surface, in its more
ordinary state, would enjoy. This presumption is heightened when we
reflect on the results of the recent soundings made by Sir James Ross,
in the Southern Ocean, and continued for four successive years, ending
1844, which seem to prove that the mean depth of the Atlantic and
Pacific is as great as Laplace and other eminent astronomers had
imagined;[185] for then we might look not only for more than two-thirds
sea in the frigid zones, but for water of great depth, which could not
readily be reduced to the freezing point. The same opinion is confirmed,
when we compare the quantity of land lying between the poles and the
30th parallels of north and south latitude, with the quantity placed
between those parallels and the equator; for, it is clear, that we have
at present not only more than the usual degree of cold in the polar
regions, but also