reproduce analogues of
all the phenomena of magnetism and diamagnetism, those phenomena which
may be classed as effects of induction being directly reproduced,
while those which may be classed as effects of mechanical action, and
resulting in change of place, are analogous inversely. This fact has
been so much misunderstood both in this country and on the Continent
that it will be well, before describing the experiments, to enter more
fully into an explanation of these most interesting and instructive
phenomena.

For the sake of clearness we will speak of magnetic induction as that
property of a magnet by which it is surrounded by a field of force,
and by which pieces of iron, within that field, are converted into
magnets, and pieces of bismuth into diamagnets, and we will speak of
magnetic action as the property of a magnet by which it attracts or
repels another magnet, or by which it attacks or repels a piece of
iron or bismuth magnetized by magnetic induction.

[Illustration: FIG. 3.]

The corresponding hydrodynamic phenomena may be regarded in a similar
manner; thus, when a vibrating or pulsating body immersed in a
liquid surrounds itself with a field of vibrations, or communicates
vibrations to other immersed bodies within that vibratory field, the
phenomena so produced may be looked upon as phenomena of hydrodynamic
induction, while on the other hand, when a vibrating or pulsating body
attracts or repels another pulsating or vibratory body (whether
such vibrations be produced by outside mechanical agency or by
hydrodynamical induction), then the phenomena so produced are those of
hydrodynamical action, and it is in this way that we shall treat the
phenomena throughout this article, using the words _induction_ and
_direct action_ in these somewhat restricted meanings.

[Illustration: FIG. 4.]

[Illustration: FIG. 5.]

In the hydrodynamical experiments of Professor Bjerknes all the
phenomena of magnetic induction can be reproduced directly and
perfectly, but the phenomena of magnetic action are not so exactly
reproduced, that is to say, they are subject to a sort of inversion.
Thus when two bodies are pulsating together and in the same phase
(i. e., both expanding and both contracting at the same time), they
mutually attract each other: but if they are pulsating in opposite
phases, repulsion is the result. From this one experiment taken by
itself we might be led to infer that bodies pulsating in similar
pha