[Illustration: FIG. 2]

Here, then, out of matter possessing rigidity, but absolutely devoid of
elasticity, we have made a perfect model of a spring in the form of a
spring balance. Connect millions of millions of particles by pairs of rods
such as these of this spring balance, and we have a group of particles
constituting an elastic solid; exactly fulfilling the mathematical ideal
worked out by Navier, Poisson, and Cauchy, and many other mathematicians,
who, following their example, have endeavored to found a theory of the
elasticity of solids on mutual attraction and repulsion between a group of
material particles. All that can possibly be done by this theory, with its
assumption of forces acting according to any assumed law of relation to
distance, is done by the gyrostatic system. But the gyrostatic system does,
besides, what the system of naturally acting material particles cannot
do--it constitutes an elastic solid which can have the Faraday
magneto-optic rotation of the plane of polarization of light; supposing the
application of our solid to be a model of the luminiferous ether for
illustrating the undulatory theory of light. The gyrostatic model spring
balance is arranged to have zero moment of momentum as a whole, and
therefore to contribute nothing to the Faraday rotation; with this
arrangement the model illustrates the luminiferous ether in a field
unaffected by magnetic force. But now let there be a different rotational
velocity imparted to the jointed square round the axis of the two
projecting hooked rods, such as to give a resultant moment of momentum
round any given line through the center of inertia of the system; and let
pairs of the hooked rods in the model thus altered, which is no longer a
model of a mere spring balance, be applied as connections between millions
of pairs of particles as before, with the lines of resultant moment of
momentum all similarly directed. We now have a model elastic solid which
will have the property that the direction of vibration in waves of