The lowest rim of all the wheels, whether
standing on the _north_ rails or on the _south_ rails of said track,
in consequence of vertical induction attracted the unmarked end of the
needle, and the upper rims attracted the marked end of the needle; but
the middle portions of the periphery, both anterior and posterior, of
the wheels standing on the north rail, attracted the unmarked end, while
similar middle portions of wheels standing on south rails attracted the
marked end; in consequence of horizontal induction, the wheels being
connected by iron axles, and thus presenting considerable extension
_across_ the track, viz., from south to north.
Magnetite seems to have acquired its polarity in the same manner,
namely by the earth's induction, when the ore contains a large enough
percentage of pure iron. A large specimen (6 in. long by 31/2 deep and
weighing 51/2 lb.) which I obtained from near Pilot Knob, Missouri,
exhibits polarity, not only at its lateral ends, but also vertically,
as the lower surface attracts the unmarked end of a needle, while the
plane, which evidently occupied the upper surface in its native bed,
attracts the marked end of the needle.
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