two different ways, it is almost always that which appears at
first the least natural."

Whatever importance these reflections may possess, I hasten to add that,
instead of the arguments of his predecessors, which have no real value,
Fourier has substituted proofs, demonstrations; and we know what meaning
such terms convey to the Academy of Sciences.

In all places of the earth, as soon as we descend to a certain depth,
the thermometer no longer experiences either diurnal or annual
variation. It marks the same degree, and the same fraction of a degree,
from day to day, and from year to year. Such is the fact: what says
theory?

Let us suppose, for a moment, that the earth has constantly received all
its heat from the sun. Descend into its mass to a sufficient depth, and
you will find, with Fourier, by the aid of calculation, a constant
temperature for each day of the year. You will recognize further, that
this solar temperature of the inferior strata varies from one climate to
another; that in each country, finally, it ought to be always the same,
so long as we do not descend to depths which are too great relatively to
the earth's radius.

Well, the phenomena of nature stand in manifest contradiction to this
result. The observations made in a multitude of mines, observations of
the temperature of hot springs coming from different depths, have all
given an increase of one degree of the centigrade for every twenty or
thirty metres of depth. Thus, there was some inaccuracy in the
hypothesis which we were discussing upon the footsteps of our colleague.
It is not true that the temperature of the terrestrial strata may be
attributed solely to the action of the solar rays.

This being established, the increase of heat which is observed in all
climates when we penetrate into the interior of the globe, is the
manifest indication of an intrinsic heat. The earth, as Descartes and
Leibnitz maintained it to be, but without being able to support their
assertions by any demonstrative reasoning,--thanks to a combination of
the observations of physical inquirers with the analytical calculations
of Fourier,--is _an encrusted sun_, the high temperature of which may be
boldly invoked every time that the explanation of ancient geological
phenomena will require it.

After having established that there is in our earth an inherent heat,--a
heat the source of which is not the sun, and which, if we may judge of
it by the rapid increase wh