If we represent to ourselves the interior of the Earth as fused and
undergoing an enormous pressure, and at a degree of temperature the amount
of which we are unable to assign,
p 190
we must renounce all idea of a magnetic nucleus of the Earth. All magnetism
is certainly not lost until we arrive at a white heat,* and it is manifested
when iron is at a dark red heat, however different, therefore, the
modifications may be which are excited in substances in their molecular
state, and in the coercive force depending upon that condition in
experiments of this nature, there will still remain a considerable thickness
of the terrestrial stratum, which might be assumed to be the seat of
magnetic currents.
[footnote] *Barlow, in the 'Philos. Trans.' for 1822, Pt. i., p. 117; Sir
David Brewster, 'Treatise on Magnetism', p. 129. Long before the times of
Gilbert and Hooke, it was taught in the Chinese work 'Ow-thea-tsou' that
heat diminished the directive force of the magnetic needle. (Klaproth,
'Lettre a M. A. de Humboldt, sur l'Invention de la Boussole', p. 96.)
The old explanation of the horary variations of declination by the
progressive warming of the Earth in the apparent revolution of the Sun from
east to west must be limited to the uppermost surface, since thermometers
sunk into the Earth, which are now being accurately observed at so many
different places, show how slowly the solar heat penetrates even to the
inconsiderable depth of a few feet.
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