*
[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 229 and 263; 'Mecanique
Celeste', t. v., p. 18 and 72. It should be remarked that the fraction
1/306th of a degree of Fahrenheit of the mercurial thermometer, given in the
text as the limit of the stability of the Earth's temperature since the days
of Hipparchus, rests on the assumption that the dilation of the substances
of which the Earth is composed is equal to that of glass, that is to say,
1/18,000th for 1 degree. Regarding this hypothesis, see Arago in the
'Annuaire' for 1834, p. 177-190.
This invariability of form presupposes also a great invariability in the
distribution of relations of density in the interior of the globe. The
translatory movements, which occasion the eruptions of our present volcanoes
and of ferruginous lava, and the filling up of previously empty fissures and
cavities with dense masses of stone, are consequently only to be regarded as
slight superficial phenomena affecting merely one portion of the Earth's
crust, which, from their smallness when compared to the Earth's radius,
become wholly insignificant.
I have described the internal heat of our planet, both with reference to its
cause and distribution, almost solely from the results of Fourier's
admirable investigations.
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