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the same play in the planetary system. The earth’s orbit
is an ellipse, one of the foci of which is occupied by the
sun. Imagine the earth at the most distant part of the
orbit. Her motion, and consequently her vis viva, is then
a minimum. The planet rounds the curve, and begins to
approach the sun. In front it has a store of tensions,
which is gradually consumed, an equivalent amount of vis
viva being generated. When nearest to the sun the mo-
tion, and consequently the vis viva, is a maximum. But
here the available tensions have been used up. The earth
rounds this portion of the curve and retreats from the sun.
Tensions are now stored up, but vis viva is lost, to be again
restored at the expense of the complementary force on the
opposite side of the curve. Thus beats the heart of the
universe, but without increase or diminution of its total
stock of force.

I have thus far tried to steer clear amid confusion by
fixing the mind of the reader upon things rather than upon
names. But good names are essential; and here, as yet,
we are not provided with such. We have had the force of
gravity and living force—two utterly distinct things. We
have had pulls and tensions; and we might have had the
force of heat, the force of light, the force of magnetism, or
the force of electricity—all of which terms have been em-
ployed more or less loosely by writers on physics. This
confusion is happily avoided by the introduction of the
term “ energy,” embracing under it both tension and vis
viva. Energy is possessed by bodies already in motion;
it is then actual, and we agree to call it actual or dynamic
energy. It is our old vis viva. On the other hand, energy
is possible to bodies not in motion, but which, in virtue of
attraction or repulsion, possess a power of "motion which
would realize itself if all hinderances were removed.
Looking, for example, at gravity, a body on the earth’s
surface in a position from which it cannot fall to a lower

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