pounds being exactly ascertained. We
have further seen (Art. 69, on the identity of light and heat), that the
same aetherial waves which produce heat are also concerned in the
production of light. If, therefore, the aetherial waves which give rise
to heat possess a dynamical action and equivalent, it follows that light
must also possess a dynamical action and equivalent, and such action
should be capable of being expressed in terms of foot-pounds. Clerk
Maxwell has recorded the exact dynamical equivalent of light. On this
matter he writes:[15] "If in strong sunlight the energy of light which
falls upon a square foot is 83.4 foot-pounds per second, the mean energy
of one cubic foot of sunlight is about .0,000,000,882 of a foot-pound,
and the mean pressure on a square foot is .0,000,000,882 of a pound
weight." We have here then the exact dynamical equivalent, according to
Maxwell, of a cubic foot of sunlight near the earth's surface, and of
the pressure exerted by light on a body with which it comes into
contact.

Again, Lord Kelvin[16] has measured the exact dynamical equivalent of a
cubic mile of sunlight, both near the surface of the sun and then near
the surface of the earth, and in a note adds that the relation of the
two values is as 46,000 to 1. So that if the dynamical value of a cubic
mile of sunlight near the earth's surface be represented by unity, then
the value of a cubic mile of sunlight near the sun's surface would be
46,000 times greater, while he further adds that it would take 4140
horse-power every minute, as the amount of work required to generate the
energy existing in a cubic kilometre of light near the sun, a kilometre
being equal to about 1093 yards.

Professor Challis[17] stated in 1872 that "Light is to be ranked with
the physical forces, and its dynamical action is equally to be ascribed
to the pressure of the Aether." Now I want to put this question to the
reader: If light possesses this dynamical action, that is, if it
possesses a motive or driving power, what must be the exact effect of
the dynamical action of the light waves from the sun upon all the
planets and meteors that revolve round it? We know that the sun is
324,000 times the mass of our earth, and that it has a diameter of about
856,000 miles and a circumference of over two million and a half miles.
What, therefore, must be the energy of the aetherial light waves that it
speeds on their way through space on every side? Stokes,[18]