The earliest view of Terrestrial Magnetism supposed the existence
of a magnet at the earth’s centre. As this does not accord with the
observations on declination, inclination, and intensity, Tobias
Meyer gave this fictitious magnet an eccentric position, placing it
one-seventh part of the earth’s radius from the centre. Hansteen
imagined that there were two such magnets, different in position
and intensity. Ampère set aside these unsatisfactory hypotheses by
the view, derived from his discovery, that the earth itself is an
electro-magnet, magnetised by an electric current circulating about
it from east to west perpendicularly to the plane of the magnetic
meridian, to which the same currents give direction as well as
magnetise the ores of iron: the currents being thermo-electric
currents, excited by the action of the sun’s heat successively on the
different parts of the earth’s surface as it revolves towards the east.
William Gilbert,[49] who wrote an able work on magnetic and electric
forces in the year 1600, regarded terrestrial magnetism and electricity
as two emanations of a single fundamental source pervading all matter,
and he therefore treated of both at once. According to Gilbert’s idea,
the earth itself is a magnet; whilst he considered that the inflections
of the lines of equal declination and inclination depend upon the
distribution of mass, the configuration of continents, or the form and
extent of the deep intervening oceanic basins.
Till within the last eighty years, it appears to have been the received
opinion that the intensity of terrestrial magnetism was the same at
all parts of the earth’s surface. In the instructions drawn up by
the French Academy for the expedition under La Pérouse, the first
intimation is given of a contrary opinion. It is recommended that the
time of vibration of a dipping-needle should be observed at stations
widely remote, as a test of the equality or difference of the magnetic
intensity; suggesting also that such observations should particularly
be made at those parts of the earth where the dip was greatest and
where it was least. The experiments, whatever their results may have
been, which, in compliance with this recommendation, were made in the
expedition of La Pérouse, perished in its general catastrophe; but the
instructions survived.
In 1811, Hansteen took up the subject, and in 1819 published his
celebrated work, clearly demonstrating the fluctuations which this
element has undergone during the last two centuries; confirming in
great detail the position of Halley, that “the whole magnetic system is
in motion, that the moving force is very great as extending its effects
from pole to pole, and that its motion is not _per saltum_, but a
gradual and regular motion.”