The
third mode of transmission is the slowest of all, and is derived from the
secular cooling of the globe, and from the small portion of the primitive
heat which is still being disengaged from the surface.
p 175
This loss experienced by the central heat must have been very considerable
in the earliest epochs of the Earth's revolutions, but within historical
periods it has hardly been appreciable by our instruments. The surface of
the Earth is therefore situated between the glowing heat of the inferior
strata and the universal regions of space, whose temperature is probably
below the freezing-point of mercury.
The periodic changes of temperature which have been occasioned on the
Earth's surface by the Sun's position and by meteorological processes, are
continued in its interior, although to a very inconsiderable depth. The
slow conducting power of the ground diminishes this loss of heat in the
winter, and is very favorable to deep-rooted trees. Points that lie at very
different depths on the same vertical line attain the maximum and minimum of
the imparted temperature at very different periods of time. The further
they are removed from the surface, the smaller is this difference between
the extremes. In the latitudes of our temperate zone (between 48 degrees
and 52 degrees), the stratum of invariable temperature is at a depth of from
59 to 64 feet, and at half that depth the oscillations of the thermometer,
from the influence of the seasons, scarcely amount to half a degree.
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