stribution of heat over the earth besides the direct action of the sun's
rays upon it. Doubtless very considerable allowance is to be made for the
difference of seasons, and difference during the same season upon the
land and upon the ocean; in mountainous countries and level ones. But
making every allowance for them, the fact that other causes have a
_controlling_ influence in producing the deviations still remains most
obvious. Neither the difference of temperature between the land and the
ocean, or land surfaces of unequal elevations, will account for the
elevation of the isothermal lines on different portions of the ocean, or
their extension around the magnetic poles.

Returning to a consideration of the arrangements for the diffusion of
heat, we observe: First, that the earth itself is intensely heated in its
interior. This is inferred, and justly, from the fact that the thermometer
is found to rise about one degree for every fifty-five feet of
descent--whether in boring artesian wells, exploring caves, or sinking
shafts in mines. It is demonstrated, also, by the existence of hot springs
and the action of volcanoes. Heat is supposed to be conducted from the
center toward the surface every where, but with difficulty and slowly. It
is also supposed to be conducted from the tropical regions toward the
poles. Such is the opinion of Humboldt. (Cosmos, vol. i. p. 167.)

Probably it reaches the surface and exerts an influence, also, upon the
weather through the ocean, and by heating it in its greatest depths.
Little attention has been paid, so far as I am informed, to the question
how far the ocean is thus heated in _tropical latitudes_. Doubtless a
portion of the warmth of the ocean there is derived from that source, and
it has its influence in changing the temperature of the deep-seated cold
polar currents of, the great oceans. Perhaps it may yet be found that the
icebergs are detached by it in the polar seas--the observations of Dr.
Kane point to such a result. (Grinnell Expedition, p. 113, and also chap.
48.)

Little need be said of the inconsiderable quantities of heat supposed to
be derived by radiation from the stars, the planets, and from space. If
any such are derived they are too inconsiderable to be of importance in
this inquiry.

Heat is also carried, and in quantities which exert very considerable
influence upon the weather, from the tropics to the poles by the great
oceanic currents which flow unceasingly