ich observation indicates, ought to be
already sufficiently intense at the depth of only seven or eight leagues
to hold in fusion all known substances,--there arises the question, what
is its precise value at the surface of the earth; what weight are we to
attach to it in the determination of terrestrial temperatures; what part
does it play in the phenomena of life?

According to Mairan, Buffon, and Bailly, this part is immense. For
France, they estimate the heat which escapes from the interior of the
earth, at twenty-nine times in summer, and four hundred times in winter,
the heat which comes to us from the sun. Thus, contrary to general
opinion, the heat of the body which illuminates us would form only a
very small part of that whose propitious influence we feel.

This idea was developed with ability and great eloquence in the _Memoirs
of the Academy_, in the _Epoques sur la Nature_ of Buffon, in the
letters from Bailly to Voltaire _upon the Origin of the Sciences and
upon the Atlantide_. But the ingenious romance to which it has served as
a base, has vanished like a shadow before the torch of mathematical
science.

Fourier having discovered that the excess of the aggregate temperature
of the earth's surface above that which would result from the sole
action of the solar rays, has a determinate relation to the increase of
temperature at different depths, succeeded in deducing from the
experimental value of this increase a numerical determination of the
excess in question. This excess is the thermometric effect which the
solar heat produces at the surface; now, instead of the large numbers
adopted by Mairan, Bailly, and Buffon, what has our colleague found? _A
thirtieth_ of a degree, not more.

The surface of the earth, which originally was perhaps incandescent, has
cooled then in the course of ages, so as hardly to preserve any sensible
trace of its primitive heat. However, at great depths, the original heat
is still enormous. Time will alter sensibly the internal temperature;
but at the surface (and the phenomena of the surface can alone modify or
compromise the existence of living beings), all the changes are almost
accomplished. The frightful freezing of the earth, the epoch of which
Buffon fixed at the instant when the central heat would be totally
dissipated, is then a pure dream. At the surface, the earth is no longer
impregnated except by the solar heat. So long as the sun shall continue
to preserve the sam