Let us, for example, take the body, b, which is a small metal
sphere, and present to it either a drum which is caused to pulsate, on
an oscillating sphere, and it will be attracted, thus representing the
action of a magnet upon a bit of soft iron. A curious experiment may
serve to indicate the transition between this new series and the
preceding. If we present to each other two drums of opposite phases,
but so arranged that one of them vibrates faster than the other, we
shall find, on carefully bringing them together, that the repulsion
which manifested itself at first is changing to attraction. On
approaching each other the drum having the quicker motion finally has
upon the other, the same action as if the latter were immovable; and
the effect is analogous to that which takes place between a strong and
weak magnet presented by their like poles.

[Illustration: FIG. 4.]

By continuing these experiments we arrive at a very important point.
Instead of the body, b (Fig. 4), let us take c. As the figure
shows, this is a sphere lighter than water, kept in the liquid by a
weight. If we present to it the vibrating body, it will be repelled,
and we shall obtain the results known by the name of diamagnetism.
This curious experiment renders evident the influence of media. As
well known, Faraday attributed such effects to the action of the air;
and he thought that magnetic motions always resulted from a difference
between the attraction exerted by the magnet upon the body under
experiment, and the attraction exerted by the air. If the body is more
sensitive than the air, there is direct magnetism, but if it is less
so, there is diamagnetism. Water between the bodies, in the Bjerknes
experiments, plays the same role; it is this which, by its vibration,
transmits the motions and determines the phases in the suspended body.