ion to the general rule. For the same reason, we may warn
the geologist to be on his guard, and not hastily to assume that the
temperature of the earth in the present era is a type of that which most
usually obtains, since he contemplates far mightier alterations in the
position of land and sea, at different epochs, than those which now
cause the climate of Europe to differ from that of other countries in
the same parallels.

It is now well ascertained that zones of equal warmth, both in the
atmosphere and in the waters of the ocean, are neither parallel to the
equator nor to each other.[166] It is also known that the _mean_ annual
temperature may be the same in two places which enjoy very different
climates, for the seasons may be nearly uniform, or violently
contrasted, so that the lines of equal winter temperature do not
coincide with those of equal annual heat or isothermal lines. The
deviations of all these lines from the same parallel of latitude are
determined by a multitude of circumstances, among the principal of which
are the position, direction, and elevation of the continents and
islands, the position and depths of the sea, and the direction of
currents and of winds.

On comparing the two continents of Europe and America, it is found that
places in the same latitudes have sometimes a mean difference of
temperature amounting to 11 degrees, or even in a few cases to 17
degrees Fahr.; and some places on the two continents, which have the
same mean temperature, differ from 7 degrees to 17 degrees in latitude.
Thus, Cumberland House, in North America, having the same latitude (54
degrees N.) as the city of York in England, stands on the isothermal
line of 32 degrees, which in Europe rises to the North Cape, in lat. 71
degrees, but its summer heat exceeds that of Brussels or Paris.[167] The
principal cause of greater intensity of cold in corresponding latitudes
of North America, as contrasted with Europe, is the connection of
America with the polar circle, by a large tract of land, some of which
is from three to five thousand feet in height; and, on the other hand,
the separation of Europe from the arctic circle by an ocean. The ocean
has a tendency to preserve everywhere a mean temperature, which it
communicates to the contiguous land, so that it tempers the climate,
moderating alike an excess of heat or cold. The elevated land, on the
other hand, rising to the colder regions of the atmosphere, becomes a
great