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 aeriform 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 contrarj' 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 ordinaiy tempe- 

 rature being submitted to a pressure of 1000 atmospheres, experiences 

 a condensation of about ^vth of its primitive volume. Now let us con- 

 ceive a column of water whose height is equal to one radius of the 

 globe, and let us reduce its weight to half of that M'hich 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 this liquid 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 

 44ths of its volume ; but without knowing the law of the compression 

 of this liquid, and although Me do not know in what manner this law 

 may depend on the temperature, we may believe, notwithstanding, that 

 so enormous 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 tiicy experienced ; the succeeding strata were 



