These researches, however, are beset with difficulties of a special
nature, and the use of viscid liquids is a subject for discussion. Mr.
Bjerknes desired to employ them for reproducing the effects that he
had obtained from water, but he found that the lines of force were no
longer the same, and that the phenomena were modified. It is
necessary, then, to hold as much as possible to liquids that are
perfect. The experimenter is at present endeavoring to use these
liquids by employing cylinders having a fluted surface; but it is
clear that this, too, is not without its difficulties.

This series of experiments offers a rare example of the verification
of algebraic calculation by direct demonstration. In general, we may
employ geometry, which gives a graphic representation of calculation
and furnishes a valuable control. Sometimes we have practical
application, which is a very important verification in some respects,
but only approximate in others. But it is rare that we employ, as Mr.
Bjerknes has done, a material, direct, and immediate translation,
which, while it brings the results into singular prominence, permits
of comparing them with known facts and of generalizing the views upon
which they are based.

Hypotheses as to the nature of electricity being as yet only tolerably
well established, we should neglect nothing that may contribute to
give them a solid basis. Assuming that electricity _is_ a vibratory
motion (and probably there is no doubt about it), yet the fact is not
so well established with regard to it as it is to that of light. Every
proof that comes to support this idea is welcome, and especially so
when it is not derived from a kind of accident, but is furnished by a
calculated and mathematical combination. Viewed from this double