Are these currents, as in Seebeck's experiments,
thermo-magnetic, and excited directly from unequal distribution of heat? or
should we not rather regard them as induced by the position of the Sun and
by solar heat?*
[footnote] *"The phenomena of periodical variations depend manifestly on
the action of solar heat, operating probably through the medium of
thermo-electric currents induced on the Earth's surface. Beyond this rude
guess, however, nothing is as yet known of their physical cause. It is even
still a matter of speculation whether the solar influence be a principal or
only a subordinate cause in the phenomena of terrestrial magnetism."
('Observations to be made in the Antarctic Expedition', 1840, p. 35.)
Have the rotation of the planets, and the different degrees of velocity
which the individual zones acquire, according to their respective distances
from the equator, any influence on the distribution of magnetism? Must we
seek the seat of these currents, that is to say, of the disturbed
electricity, in the atmosphere, in the regions of planetary space, or in the
polarity of the Sun and Moon? Galileo, in his celebrated 'Dialogo', was
inclined to ascribe the parallel direction of the axis of the Earth to a
magnetic point of attraction seated in universal space.
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