ar in mind the distinction between "force"
and "energy." These terms have been so popularly confused that it
may be difficult always to discriminate them, but in Physics they
are absolutely discriminated. We have a direct sense of "force,"
in our muscles, whether they be moving or at rest. A force in
motion is a "power," it "does work" and transfers energy from one
body to another, which is commonly though incorrectly spoken of
as "generating" energy. But a force at rest--a mere statical
stress, like that exerted by a pillar or a watershed--does no
work, and "generates" or transfers no energy; yet the one
sustains a roof which would otherwise fall, thereby screening a
portion of ground from vegetation; while the other deflects a
rain-drop into the Danube or the Rhine. This latter is the kind
of force which constrains a stone to revolve in a circle instead
of a straight line; a force like that of a groove or slot or
channel or "guide."

To every force there is an equal opposite force or reaction, and a
reaction may be against a live body, but it is never suspected of being
against the abstraction life or mind--that would indeed be enlarging
the scope of mechanics!--the reaction is always against some other
body. All stresses as a matter of fact occur in the ether; and they all
have a material terminus at each end (or in exceptional cases a
wave-front or some other recondite etherial equivalent), that is to say
something possessing inertia; but the timed or _opportune_ existence of
a particular stress may be the result of organisation and control.
Mechanical operations can be thus dominated by intelligence and
purpose. When a stone is rolling over a cliff, it is all the same to
"energy" whether it fall on point A or point B of the beach. But at A
it shall merely dent the sand, whereas at B it shall strike a detonator
and explode a mine. Scribbling on a piece of paper results in a certain
distribution of fluid and production of a modicum of heat: so far as
energy is concerned it is the same whether we sign Andrew Carnegie or
Alexander Coppersmith, yet the one effort may land us in twelve months'
imprisonment or may build a library, according to circumstances, while
the other achieves no result at all. John Stuart Mill used to say that
our sole power over Nature was to _move_ things; but strictly speaking
we cannot do even that: we can