Among those notions that study and time are reducing to other and
simpler ones, that of electricity should be admitted; for it presents
itself more and more as one of the peculiar cases of the general
motion of matter. It will be to the eternal honor of Fresnel for
having introduced into science and mathematically constituted the
theory of undulations (already proposed before him, however), thus
giving the first example of the notion of motion substituted for that
of force. Since the principle of the conservation of energy has taken
the eminent place in science that it now occupies, and we have seen a
continual transformation of one series of phenomena into another, the
mind is at once directed to the aspect of a new fact toward an
explanation of this kind. Still, it is certain that these hypotheses
are difficult of justification; for those motions that are at present
named molecular, and that we cannot help presuming to be at the base
of all actions, are _per se_ ungraspable and can only be demonstrated
by the coincidence of a large number of results. There is, however,
another means of rendering them probable, and that is by employing
analogy. If, by vibrations which are directly ascertainable, we can
reproduce the effects of electricity, there will be good reason for
admitting that the latter is nothing else than a system of vibration
differing only, perhaps, in special qualities, such as dimensions,
direction, rapidity, etc.

Such is the result that is attained by the very curious experiments
that are due to Mr. Bjerknes. These constitute an _ensemble_ of very
striking results, which are perfectly concordant and exhibit very
close analogies with electrical effects, as we shall presently see.

[Illustration: FIG. 1.]