ted to a magnet, the nearer parts are
attracted, while the more distant parts are repelled; and because the
attracted portions are nearer to the magnet than the repelled ones, we
have a balance in favour of attraction. Here then is the special
characteristic of the magnetic force, which distinguishes it from that
of gravitation. The latter is a simple unpolar force, while the
former is duplex or polar. Were gravitation like magnetism, a stone
would no more fall to the ground than a piece of iron towards the
north magnetic pole: and thus, however rich in consequences the
supposition of Kepler and others may have been, it is clear that a
force like that of magnetism would not be able to transact the
business of the universe.

The object of this discourse is to enquire whether the force of
diamagnetism, which manifests itself as a repulsion of certain bodies
by the poles of a magnet, is to be ranged as a polar force, beside
that of magnetism; or as an unpolar force, beside that of gravitation.
When a cylinder of soft iron is placed within a wire helix, and
surrounded by an electric current, the antithesis of its two ends, or,
in other words, its polar excitation, is at once manifested by its
action upon a magnetic needle; and it may be asked why a cylinder of
bismuth may not be substituted for the cylinder of iron, and its state
similarly examined. The reason is, that the excitement of the bismuth
is so feeble, that it would be quite masked by that of the helix in
which it is enclosed; and the problem that now meets us is, so to
excite a diamagnetic body that the pure action of the body upon a
magnetic needle may be observed, unmixed with the action of the body
used to excite the diamagnetic.

How this has been effected may be illustrated in the following
manner:

When through an upright helix of covered copper wire, a voltaic
current is sent, the top of the helix attracts, while its bottom
repels, the same pole of a magnetic needle; its central point, on the
contrary, is neutral, and exhibits neither attraction nor repulsion.
Such a helix is caused to stand between the two poles N'S' of an
astatic system. [Footnote: The reversal of the poles of the two
magnets, which were of the same strength, completely annulled the
action of the earth as a magnet.] The two magnets S N' and S'N are
united by a rigid cross piece at their centres, and are suspended from
the point a, so that both magnets swing in the same horizonta