426 JOSEPH FOURIER. 
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 inte- 
rior 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 combina- 
tion 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 
which observation indicates, ought to be already suffi- 
ciently 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 deter- 
mination 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 fhe 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 pro- 
pitious influence we feel. 
' This idea was developed with ability and great. elo- 
quence in the Memoirs of the Academy, in the Hpoques 
