If, however, a fluid be poured on the side of the revolving wheel near
the axis, it will move out to the rim on radial lines, as may be
observed on car wheels universally. The radial lines of black oil on
these wheels look very much as if centrifugal force actually did produce
motion, or had at least a very decided tendency to produce motion, in
the radial direction. This interesting action calls for explanation. In
this action the oil moves outward gradually, or by inconceivably minute
steps. Its adhesion being overcome in the least possible degree, it
moves in the same degree tangentially. In so doing it comes in contact
with a point of the surface which has a motion more rapid than its own.
Its inertia has now to be overcome, in the same degree in which it had
overcome the adhesion. Motion in the radial direction is the result of
these two actions, namely, leaving the first point of contact
tangentially and receiving an acceleration of its motion, so that this
shall be equal to that of the second point of contact. When we think
about the matter a little closely, we see that at the rim of the wheel
the oil has perhaps ten times the velocity of revolution which it had on
leaving the journal, and that the mystery to be explained really is, How
did it get that velocity, moving out on a radial line? Why was it not
left behind at the very first? Solely by reason of its forward
tangential motion. That is the answer.

When writers who understand the subject talk about the centripetal and
centrifugal forces being different names for the same force, and about
equal action and reaction, and employ other confusing expressions, just
remember that all they really mean is to express the universal relation
between force and resistance. The expression "centrifugal force" is
itself so misleading, that it becomes especially important that the real
nature of this so-called force, or the sense in which the term "force"
is used in this expression, should be fully explained.[1] This force is