less than the average quantity of heat within the
tropics.

_Position of land and sea which might produce the extreme of cold of
which the earth's surface is susceptible._--To simplify our view of the
various changes in climate, which different combinations of geographical
circumstances may produce, we shall first consider the conditions
necessary for bringing about the extreme of cold, or what would have
been termed in the language of the old writers the winter of the "great
year," or geological cycle, and afterwards, the conditions requisite to
produce the maximum of heat, or the summer of the same year.

To begin with the northern hemisphere. Let us suppose those hills of the
Italian peninsula and of Sicily, which are of comparatively modern
origin, and contain many fossil shells identical with living species, to
subside again into the sea, from which they have been raised, and that
an extent of land of equal area and height (varying from one to three
thousand feet) should rise up in the Arctic Ocean between Siberia and
the north pole. In speaking of such changes, I shall not allude to the
manner in which I conceive it possible that they may be brought about,
nor of the time required for their accomplishment--reserving for a
future occasion, not only the proofs that revolutions of equal magnitude
have taken place, but that analogous operations are still in gradual
progress. The alteration now supposed in the physical geography of the
northern regions, would cause additional snow and ice to accumulate
where now there is usually an open sea; and the temperature of the
greater part of Europe would be somewhat lowered, so as to resemble more
nearly that of corresponding latitudes of North America: or, in other
words, it might be necessary to travel about 10 degrees farther south in
order to meet with the same climate which we now enjoy. No compensation
would be derived from the disappearance of land in the Mediterranean
countries; but the contrary, since the mean heat of the soil in those
latitudes probably exceeds that which would belong to the sea, by which
we imagine it to be replaced.

But let the configuration of the surface be still farther varied, and
let some large district within or near the tropics, such as Brazil, with
its plains and hills of moderate height, be converted into sea, while
lands of equal elevation and extent rise up in the arctic circle. From
this change there would, in the first place, result