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produces them. But further, its character is “ the power of 
imparting energy ” (§ 8). Now since it has just been defined as 
a push or pull, or the power of imparting motion, it follows that 
motion and energy are the same. But “ powder of imparting 
energy” is denied to be a true definition, because “energy may 
be imparted by other matter possessing energy, without force” 
(p. 3, 1. 14). And again, “energy is not, as frequently as- 
sumed, synonymous with motion.” But by the definitions the 
only test of force is the impartation or extinction of motion, 
and if force may be characterized as a power of imparting 
energy, then energy is and must be motion. 
But another definition is offered, the power of doing work. 
This merely transfers the obscurity to another word. For what 
is this work to be done ? If not motion, or some change in the 
position of masses or atoms, what else can it be? But if the 
work to be done is moving things from one place to another, 
then force and energy come to be the same, as before energy 
and motion. Still further, in §§ 24, 25, light and heat are 
said to be accurately defined as “ a very brisk agitation of the 
insensible parts of the object.” Yet in § 29 we read that they 
“ have frequently been illogically designated as ‘ modes of 
motion ’ by able physicists,” and this “ has led them into a hope- 
less confusion of the terms, force, energy, and motion.” But 
a very brisk agitation is certainly a mode of motion, so that 
the paper is a fresh instance of that confusion of which its 
writer justly complains. 
The source of all this perplexity seems to me very clear. 
Force is one distinct idea, motion is another. Force is the 
conceived cause of motion. Motion is the perceived effect of 
force. Each may be actual or possible. There arc forces 
which now act, and- others, different in amount, which may 
act in different circumstances. There are actual motions, 
and motions possible or conceivable. Energy is an ill-de- 
vised term for confounding together these different ideas, 
to gain thereby an apparent constancy which does not 
exist. Kinetic Energy is not force at all, but a sum 
total of actual motions. Potential Energy is not motion 
at all, nor actual force, but a sum total of conceivable forces 
under varied, non-cxistcnt conditions. The introduction of these 
ambiguous terms, instead of helping scientific insight, breeds 
endless and almost hopeless confusion. Energy is mistaken for 
a third thing, distinct alike from force and motion. It is not 
synonymous with motion. It is not synonymous with force. 
It is something which transfers itself, without force, from body 
to body, when motion is transferred, and yet is not motion. 
