forces of friction are supposed. It is exceedingly
interesting to see how thus, with no other postulates than inertia,
rigidity, and mutual impenetrability, we can thoroughly model not only an
elastic solid, and any combination of elastic solids, but so complex and
recondite a phenomenon as the passage of polarized light through a magnetic
field. But now, with the view of ultimately discarding the postulate of
rigidity from all our materials, let us suppose some to be absolutely
destitute of rigidity, and to possess merely inertia and incompressibility,
and mutual impenetrability with reference to the still remaining rigid
matter. With these postulates we can produce a perfect model of mutual
action at a distance between solid particles, fulfilling the condition, so
keenly desired by Newton and Faraday, of being explained by continuous
action through an intervening medium. The law of the mutual force in our
model, however, is not the simple Newtonian law, but the much more complex
law of the mutual action between electro magnets--with this difference,
that in the hydro-kinetic model in every case the force is opposite in
direction to the corresponding force in the electro-magnetic analogue.
Imagine a solid bored through with a hole, and placed in our ideal perfect
liquid. For a moment let the hole be stopped by a diaphragm, and let an
impulsure pressure be applied for an instant uniformly over the whole
membrane, and then instantly let the membrane be dissolved into liquid.
This action originates a motion of the liquid relatively to the solid, of a
kind to which I have given the name of "irrotational circulation," which
remains absolutely constant however the solid be moved through the liquid.
Thus, at any time the actual motion of the liquid at any point in the
neighborhood of the solid will be the resultant of the motion it would have
in virtue of the circulation alone, were the solid at rest, and the motion
it would have in virtue of the motion of the solid itself, had there been
no circulation established through the aperture. It is interesting and
important to remark in passing that the whole kinetic energy of the liquid
is the sum of the kinetic energies which it would have in the two cases
separately. Now, imagine the whole liquid to be inclosed in an infinitely
large, rigid, containing vessel, and in the liquid, at an infinite distance
from any part of the containing vessel, let two perforated solids, with
irrotat