" Plin., xxxiv., 14.) The great discovery of the
terrestrial directive force depended, therefore, alone on this, that no one
in the West had happened to observe an elongated fragment of magnetic iron
stone, or a magnetic iron rod, floating, by the aid of a piece of wood, in
water, or suspended in the air by a thread, in such a position as to admit
of free motion.
The magnetic power of our globe is manifested on the terrestrial surface in
three classes of phenomena, one of which exhibits itself in the varying
intensity of the force, and the two others in the varying direction of the
inclination, and in
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the horizontal deviation from the terrestrial meridian of the spot. Their
combined action may therefore be graphically represented by three systems of
lines, the 'isodynamic, isoclinic', and 'isogonic' (or those of equal force,
equal inclination, and equal declination). The distances apart, and the
relative positions of these moving, oscillating, and advancing curves, do
not always remain the same. The total deviation (variation or declination
of the magnetic needle) has not at all changed, or, at any rate, not in any
appreciable degree, during a whole century, at any particular point on the
Earth's surface,* as, for instance, the western part of the Antilles, or
Spitzbergen.
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