here a wave motion which is travelling through the Aether at the
enormous rate already quoted. Light takes about 8-1/2 minutes to travel
from the sun to the earth, a distance of 92,000,000 miles. Our fastest
trains do not travel 80 miles an hour, and if a train left the sun and
continued its journey through space at that rate, it would take over 130
years before it reached our earth, while the light would perform the
journey in 8-1/2 minutes. We have some idea of the velocity of a train
travelling at 80 miles an hour; what, however, must be the velocity of a
wave motion which travels 22,500 times as fast? In Art. 56 we have seen
that all energy is the energy of motion, and therefore wherever we get
motion of any kind or sort, there we must have energy accompanying it,
or the power to do work. We have here, then, a source of energy in the
aetherial waves known as light waves, with their enormous velocity which
is almost inconceivable and illimitable. What must be the energy which
exists in space due to the wave motion of the Aether? We have to
remember on this point that we are no longer dealing with a frictionless
medium, but that we are dealing with matter, only in a far more rarefied
and far more elastic form than ordinary matter, but nevertheless matter
just as air is considered matter, and, being matter, its very motion
imparts to the light waves a power and a force which make them capable
of doing work. The kind of work done will be considered later on, when
we deal with the dynamical value of light. That we do not feel the power
and energy of the light waves is due to the well-known fact that their
power is broken by the activity of the atmospheric particles, each of
which, in their myriads, is ever moving with great velocity, and
therefore bombard the light waves, as they endeavour to strike the
earth. Thus the aetherial light waves are broken up and shattered, and
fall to the earth not with their full energy or power, but in a blended
form, or with that reflected energy which we call light. If they were to
come unbroken and unchecked upon us, and on the earth, in the same way
that they apparently do upon our satellite the moon, we doubtless should
experience very different effects of their energy and power due to their
enormous velocity.

ART. 77. _Dynamical Value of Light._--We have already learned (Art. 68)
that heat possesses a dynamical value, such value being measured by
Joule, and its equivalent in foot-