2348. EARTH, Internal Heat of.—
The
term “central heat” does of itself give us a
false idea of Buffon's hypothesis. If it means
a heat lodged in the centre of the earth, and diffusing
its warmth from thence to the extremities,
then certainly it would be less in propertion
to the distance from that centre, and, of
course, less under the equator than the poles,
on high mountains than in deep valleys. But
Buffon's theory is that this earth was once in
a state of hot fusion, and that it has been, and
still continues to be cooling. What is the
course of this process? A heated body being
surrounded by a colder one, whether solid or
fluid, the heat, which is itself a fluid, flows into
the colder body equally from every point of the
hotter. Hence if a heated spheroid of iron cools
to a given degree, in a given space of time, an
inch deep from its surface in one point, it
has in the same time done the same in any and
every other point. In a given time more, it will
be cooled all around to double that depth. So
that it will always be equally cooled at equal
depths from the surface. This would be the
case with Buffon's earth, if it were a smooth
figure without unevennesses. But it has mountains
and valleys. The tops of mountains will
cool to greater depths in the same time than
the sides of mountains, and than plains in
proportion as the line A. B. is longer than
A. C. or D. E. or F. G. In the valley line
H. I., on depth of the same temperature, will
be the same as on a plain. This, however, is
very different from Buffon's opinion. He says
that the earth, being thinnest at the poles, will
cool sooner there than under the equator, where
it is thicker. If my idea of the process of
cooling be right, his is wrong, and his whole
theory in the Epochs of Nature, is overset.—
To James Madison.
Ford ed., iii, 369.
(A.
1784)
