rectilinear vibrations propagated through it shall turn round the line of
propagation of the waves, just as Faraday's observation proves to be done
by the line of vibration of light in a dense medium between the poles of a
powerful magnet. The case of wave front perpendicular to the lines of
resultant moment of momentum (that is to say, the direction of propagation
being parallel to these lines) corresponds, in our mechanical model, to the
case of light traveling in the direction of the lines of force in a
magnetic field.

In these illustrations and models we have different portions of ideal rigid
matter acting upon one another, by normal pressure at mathematical points
of contact--of course no 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