follows that the motion of Gravitation, whatever that may be
due to, is converted into the motion known as light and heat. Thus it
can be seen that Gravitation, looked at from the standpoint of a mode of
motion, is itself conformable to the principle of the transformation of
motion, and this is an indirect argument in favour of the fact that
Gravitation is itself due to certain motions of the universal Aether.

ART. 59. _Motion and Work._--In Art. 52 we have seen that energy is the
power which a body possesses to do work, the amount of work which a body
can perform being regulated by the amount of energy which such a body
possesses. In Art. 57 we have further seen that all energy is the energy
of motion, and that wherever we have energy of any kind or sort, whether
it be in the form of light, heat, or electricity, there we have motion
of some kind or other. That being so, we arrive at the conclusion, that
wherever in the universe we have motion of any kind or sort, whether it
be the motion of Aether, or wind, or water, there we have the power of
doing work, and the work so done will be proportionate to the motion
which the medium possesses. The amount of work that air in motion can do
has been measured, as we have already seen (Art. 55) that air which
moves at the rate of 30 miles per hour exerts a force of 4-1/2 lb. per
square foot.

The amount of work that water in motion can do has also been measured.
The carrying and erosive powers of a river depend on the rapidity of its
currents. It has been calculated that a velocity of three inches per
second will transport fine clay; eight inches per second coarse sand;
while three feet per second will transport stones as large as eggs.

If, therefore, air moving at the rate of 30 miles an hour can exert a
force of 4-1/2 lb. per square foot, what must be the force or pressure
of aetherial motion, as light-waves for example, which move with a
velocity of 186,000 miles per second? The amount of work which such an
aetherial motion can perform has actually been measured by Professor
Lebedew of Moscow, and will be dealt with in the chapter on "Light, a
Mode of Motion," when the application of the work done on a body, as a
planet for example, will also be considered. Work, therefore, can always
be done by motion against resistance. This is a fundamental principle in
the sphere of dynamics, which is incontrovertible, as all experience,
observation, and experiment teach us, that whe