racts of elevated land between the 55th and 70th parallel; and, on the
other hand, the cold would be moderated if there were more land between
the line and the forty-fifth degree of south latitude.

_Changes in the position of land and sea may give rise to vicissitudes
in climate._--Having offered these brief remarks on the diffusion of
heat over the globe in the present state of the surface, I shall now
proceed to speculate on the vicissitudes of climate, which must attend
those endless variations in the geographical features of our planet
which are contemplated in geology. That our speculations may be confined
within the strict limits of analogy, I shall assume, 1st, That the
proportion of dry land to sea continues always the same. 2dly, That the
volume of the land rising above the level of the sea is a constant
quantity; and not only that its mean, but that its extreme height, is
liable only to trifling variations. 3dly, That both the mean and extreme
depth of the sea are invariable; and 4thly, It may be consistent with
due caution to assume that the grouping together of the land in
continents is a necessary part of the economy of nature; for it is
possible that the laws which govern the subterranean forces, and which
act simultaneously along certain lines, cannot but produce, at every
epoch, continuous mountain-chains; so that the subdivision of the whole
land into innumerable islands may be precluded.

If it be objected, that the maximum of elevation of land and depth of
sea are probably not constant, nor the gathering together of all the
land in certain parts, nor even perhaps the relative extent of land and
water, I reply, that the arguments about to be adduced will be
strengthened if, in these peculiarities of the surface, there be
considerable deviations from the present type. If, for example, all
other circumstances being the same, the land is at one time more divided
into islands than at another, a greater uniformity of climate might be
produced, the mean temperature remaining unaltered; or if, at another
era, there were mountains higher than the Himalaya, these, when placed
in high latitudes, would cause a greater excess of cold. Or, if we
suppose that at certain periods no chain of hills in the world rose
beyond the height of 10,000 feet, a greater heat might then have
prevailed than is compatible with the existence of mountains thrice that
elevation.

However constant may be the relative proportion of se