is the measure of the ability of a body to
transfer or transform mechanical energy. The meteorite that falls upon
the earth to-day gives, on its impact, the same amount of energy it
would have given if it had struck the earth ten thousand years ago. The
inertia of the meteor has persisted, not as energy, but as a factor of
energy. We commonly express the energy of a mass of matter by
_mv_^{2}/2, where _m_ stands for the mass and _v_ for its velocity. We
might as well, if it were as convenient, substitute inertia for mass,
and write the expression _iv_^{2}/2, for the mass, being measured by its
inertia, is only the more common and less definitive word for the same
thing. The energy of a mass of matter is, then, proportional to its
inertia, because inertia is one of its factors. Energy has often been
treated as if it were an objective thing, an entity and a unity; but
such a conception is evidently wrong, for, as has been said before, it
is a product of two factors, either of which may be changed in any
degree if the other be changed inversely in the same degree. A cannon
ball weighing 1000 pounds, and moving 100 feet per second, will have
156,000 foot-pounds of energy, but a musket ball weighing an ounce will
have the same amount when its velocity is 12,600 feet per second.
Nevertheless, another body acting upon either bullet or cannon ball,
tending to move either in some new direction, will be as efficient
while those bodies are moving at any assignable rate as when they are
quiescent, for the change in direction will depend upon the inertia of
the bodies, and that is constant.

The common theory of an inert body is one that is wholly passive, having
no power of itself to move or do anything, except as some agency outside
itself compels it to move in one way or another, and thus endows it with
energy. Thus a stone or an iron nail are thought to be inert bodies in
that sense, and it is true that either of them will remain still in one
place for an indefinite time and move from it only when some external
agency gives them impulse and direction. Still it is known that such
bodies will roll down hill if they will not roll up, and each of them
has itself as much to do with the down-hill movement as the earth has;
that is, it attracts the earth as much as the earth attracts it. If one
could magnify the structure of a body until the molecules became
individually visible, every one of them would be seen to be in intense
activit