ich in the case of light
is thus vibrating? What corresponds to the elastic displacement and
recoil of the spring or pendulum? What corresponds to the inertia
whereby it overshoots its mark? Do we know these properties in the
ether in any other way?

The answer, given first by Clerk Maxwell, and now reiterated and
insisted on by experiments performed in every important laboratory in
the world, is:

The elastic displacement corresponds to electrostatic charge (roughly
speaking, to electricity).

The inertia corresponds to magnetism.

This is the basis of the modern electro-magnetic theory of light. Now
let me illustrate electrically how this can be.

The old and familiar operation of charging a Leyden jar--the storing
up of energy in a strained dielectric, any electrostatic charging
whatever--is quite analogous to the drawing aside of our flexible
spring. It is making use of the elasticity of the ether to produce a
tendency to recoil. Letting go the spring is analogous to permitting a
discharge of the jar--permitting the strained dielectric to recover
itself, the electrostatic disturbance to subside.

In nearly all the experiments of electrostatics, ethereal elasticity
is manifest.

Next consider inertia. How would one illustrate the fact that water,
for instance, possesses inertia--the power of persisting in motion
against obstacles--the power of possessing kinetic energy? The most
direct way would be to take a stream of water and try suddenly to stop
it. Open a water tap freely and then suddenly shut it. The impetus or
momentum of the stopped water makes itself manifest by a violent shock
to the pipe, with which everybody must be familiar. The momentum of
water is utilized by engineers in the "water ram."

A precisely analogous experiment in electricity is what Faraday called
"the extra current." Send a current through a coil of wire round a
piece of iron, or take any other arrangement for developing powerful
magnetism, and then suddenly stop the current by breaking the circuit.
A violent flash occurs if the stoppage is sudden enough, a flash which
means the bursting of the insulating air partition by the accumulated
electro-magnetic momentum.

Briefly, we may say that nearly all electro-magnetic experiments
illustrate the fact of ethereal inertia.

Now return to consider what happens when a charged conductor (say a
Leyden jar) is discharged. The recoil of the strained dielectric
causes a current, the ine