I readily grant that the term "persistence of force" is not a happy one,
seeing that the word "force," as used by physicists, does not at the
present time convey the full meaning which Mr. Spencer desires it to
convey. But I think that any impartial physicist will be prepared to admit
that, in the present state of his science, we are entitled to conclude that
energy of position is merely the result of energy of motion; or, in other
words, that potential energy is merely an expression of the fact that the
universe, as a whole, is replete with actual energy, whose essential
characteristic is that it is indestructible. And this may be concluded
without committing ourselves to any particular theory as to the physical
explanation of gravity; all we need assert is, that in some way or other
gravity is the result of ubiquitous energy. And this, it seems to me, we
must assert, or else conclude that gravity can never admit of a physical
explanation. For all that we mean by a physical explanation is the proved
establishment of an equation between two quantities of energy; so that if
energy of position does not admit of being interpreted in terms of energy
of motion, we must conclude that it does not admit of being interpreted at
all--at least not in any physical sense.
Throughout the foregoing essays, therefore, I have assumed that all forms
of energy are but relatively varying expressions of the same fact--the
fact, namely, which Mr.
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