any such thing. In the dynamical sense of the term "force," the sense in
which this term is always understood in ordinary speech, as something
tending to produce motion, and the direction of which determines the
direction in which motion of a body must take place, there is, I repeat,
no such thing as centrifugal force.

There is, however, another sense in which the term "force" is employed,
which, in distinction from the above, is termed a statical sense. This
"statical force" is the force by the exertion of which a body keeps
still. It is the force of inertia--the resistance which all matter
opposes to a dynamical force exerted to put it in motion. This is the
sense in which the term "force" is employed in the expression
"centrifugal force." Is that all? you ask. Yes; that is all.

I must explain to you how it is that a revolving body exerts this
resistance to being put in motion, when all the while it _is_ in motion,
with, according to our above supposition, a uniform velocity. The first
law of motion, so far as we now have occasion to employ it, is that a
body, when put in motion, moves in a straight line. This a moving body
always does, unless it is acted on by some force, other than its
impelling force, which deflects it, or turns it aside, from its direct
line of motion. A familiar example of this deflecting force is afforded
by the force of gravity, as it acts on a projectile. The projectile,
discharged at any angle of elevation, would move on in a straight line
forever, but, first, it is constantly retarded by the resistance of the
atmosphere, and, second, it is constantly drawn downward, or made to
fall, by the attraction of the earth; and so instead of a straight line
it describes a curve, known as the trajectory.

Now a revolving body, also, has the same tendency to move in a straight