Poisson doubts the fact of the uninterrupted
increase of the Earth's heat
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from the surface to the center, and is of opinion that all heat has
penetrated from without inward, and that the temperature of the globe
depends upon the very high or very low temperature of the regions of space
through which the solar temperature of the regions of space, through which
the solar system has moved. This hypothesis, imagined by one of the most
acute mathematicians of our time, has not satisfied physicists or
geologists, or scarcely indeed any one besides its author. But, whatever
may be the cause of the internal heat of our planet, and of its limited or
unlimited increase in deep strata, it leads us, in this general sketch of
nature, through the intimate connection of all primitive phenomena of
matter, and through the common bond by which molecular forces are united,
into the mysterious domain of magnetism. Changes of temperature call forth
magnetic and electric currents. Terrestrial magnetism, whose main
character, expressed in the three-fold manifestation of its forces, is
incessant periodic variability, is ascribed either to the heated mass of the
Earth itself,* or to those galvanic currents which we consider as
electricity in motion, that is, electricity moving in a closed circuit.
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