[Illustration: FIG. 2.]

We may now enter into the details of the experiments:

The first is represented in Fig. 2. In a basin of water there is
placed a small frame carrying a drum fixed on an axle and capable of
revolving. It also communicates with one of the air cylinders. The
operator holds in his hand a second drum which communicates with the
other cylinder. The pistons are adjusted in such a way that they shall
move parallel with each other; then the ends of the drums inflate and
collapse at the same time; the _motions are of the same phase_; but if
the drums are brought near each other a very marked attraction occurs,
the revolving drum follows the other. If the cranks are so adjusted
that the pistons move in an opposite direction, the _phases are
discordant_--there is a repulsion, and the movable drum moves away
from the other. The effect, then, is analogous to that of two magnets,
with about this difference, that here it is the like phases that
attract and the different phases that repel each other, while in
magnets like poles repel and unlike poles attract each other.

It is necessary to remark that it is indifferent which face of the
drum is presented, since both possess the same phase. The drum
behaves, then, like an insulated pole of a magnet, or, better, like a
magnet having in its middle a succeeding point. In order to have two
poles a double drum must be employed. The experiment then becomes more
complicated; for it is necessary to have two pump chambers with
opposite phases for this drum alone, and one or two others for the
revolving drum. The effects, as we shall see, are more easily shown
with the vibrating spheres.