TEMPERATURE OF THE TERRESTRIAL STRATA. 425 
which the history of discoveries reveals in a thousand 
places: “ When a thing may be in 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 prede- 
cessors, 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 con- 
stantly received all its heat from the sun. Descend into 
its mass to a sufficient depth, and you wiil 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 de- 
scend to depths which are too great relatively to the 
earth’s radius. 
Well, the phenomena of nature stand in manifest con- 
tradiction 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 métres 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 
