This form has the advantage that the vibrating body exhibits the two phases at the same time; relatively to the liquid, one of its ends advances while the other recedes.  Thus with a vibrating sphere presented to the movable drum, there may be obtained repulsion or attraction, according as the side which is approached is concordant or discordant with the end of the drum that it faces.

[Illustration:  FIG. 3.]

With the arrangement shown in Fig. 3 there may be performed an interesting series of experiments.  The two spheres supported by the frame are set in simultaneous vibration, and the frame, moreover, is free to revolve about its axis.  The effect is analogous to that which would be produced by two short magnets carried by the same revolving support; on presenting the vibrating sphere to the extremities the whole affair is attracted or repulsed, according to its phase and according to the point at which it is presented; on replacing the transverse support by a single sphere (as indicated in the figure by a dotted line) we obtain the analogue of a short magnet carried on a pivot like a small compass needle.  This sphere follows the pole of a vibrating sphere which is presented to it, as the pole of a magnet would do, with this difference always, that in the magnet, like poles repel, while in oscillating bodies like phases attract.

In all the preceding experiments the bodies brought in presence were both in motion and the phenomena were analogous to those of permanent magnetism.  We may also reproduce those which result from magnetism by induction.  For this purpose we employ small balls of different materials suspended from floats, as shown in Fig. 4 (a, b, c).  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