M. POISSON ON THE MATHEMATICAL THEORY OF HEAT. 135 
“The nearly spherical form of the earth and planets, and their flattening 
at the poles of rotation, evidently show that these bodies were originally 
in a fluid or perhaps in an aériform state. Beginning from this initial 
state, the earth could not, wholly or partly, become solid, except by a loss 
of heat arising from its temperature exceeding that of the medium in 
which it was placed. But it is not demonstrated that the solidification 
of the earth could have commenced at the surface and been propagated 
towards the centre, as the state of the globe still fluid in the greatest 
part of the interior would lead us to suppose; the contrary appears to me 
more probable. For the extreme parts, or those nearer to the surface, 
being the first cooled, must have descended to the interior and been 
replaced by internal portions which had ascended to cool at the surface 
and to descend again in their turn. This double current must have 
maintained an equality of temperature in the mass, or at least must have 
prevented the inequality from becoming in any way so great as in a 
solid body, which cools from the surface; and we may add that this 
mixture of the parts of the fluid, and the equalization of their tempera- 
tures, must have been favoured by the oscillations of the whole mass, 
which must have taken place until the globe attained a permanent figure 
and rotation. On the other hand, the excessively great pressure sustain- 
ed by the central strata may have determined their solidification long 
before that of those nearer the surface; that is to say, the first may 
have become solid by the effect of this extreme pressure at a tempera- 
ture equal or even superior to that of the strata more distant from the 
centre, and consequently subjected to a much less degree of pressure. 
Experiment has shown, for example, that water at the ordinary tempe- 
rature being submitted to a pressure of 1000 atmospheres, experiences 
a condensation of about ~5th of its primitive volume. Now let us con- 
eeive a column of water whose height is equal to one radius of the 
globe, and let us reduce its weight to half of that which we observe at 
the surface of the earth, in order to render it equal to the mean gravity 
which would exist along each radius of the earth upon the hypothesis of 
its homogeneity; the inferior strata of thisliquid column would experience 
a pressure of more than three millions of atmospheres, or equal to more 
than three thousand times the pressure which would reduce water to 
+iths of its volume; but without knowing the law of the compression 
of this liquid, and although we do not know in what manner this law 
may depend on the temperature, we may believe, notwithstanding, that 
so ehormous a pressure would reduce the inferior strata of the mass 
of water to the solid state, even when the temperature is very high. 
It seems therefore more natural to conceive that the solidification 
of the earth began at the centre and was successively propagated to- 
wards the surface: at a certain temperature, which might be extremely 
high, the strata nearer the centre became at first solid, by reason of the 
excessive pressure which they experienced; the succeeding strata were 
