its point as easily as a person stands on his two feet, and
to do other surprising things, which otherwise it could not do. One can,
without difficulty, form a mechanical conception of the whole series
without assuming imponderables, or fluids or forces. Mechanical motion
only, by pressure, has been transferred in certain directions at certain
rates. Suppose now that some one should suddenly come upon a spinning
top (Fig. 3) while it was standing upon its point, and, as its motion
might not be visible, should cautiously touch it. It would bound away
with surprising promptness, and, if he were not instructed in the
mechanical principles involved, he might fairly well draw the conclusion
that it was actuated by other than simple mechanical principles, and,
for that reason, it would be difficult to persuade him that there was
nothing essentially different in the body that appeared and acted thus,
than in a stone thrown into the air; nevertheless, that statement would
be the simple truth.

[Illustration: FIG. 3.]

All our experience, without a single exception, enforces the proposition
that no body moves in any direction, or in any way, except when some
other body _in contact_ with it presses upon it. The action is direct.
In Newton's letter to his friend Bentley, he says--"That one body
should act upon another through empty space, without the mediation of
anything else by and through which their action and pressure may be
conveyed from one to another, is to me so great an absurdity that I
believe no man who has in philosophical matters a competent faculty of
thinking can ever fall into it."

For mathematical purposes, it has sometimes been convenient to treat a
problem as if one body could act upon another without any physical
medium between them; but such a conception has no degree of rationality,
and I know of no one who believes in it as a fact. If this be granted,
then our philosophy agrees with our experience, and every body moves
because it is pushed, and the mechanical antecedent of every kind of
phenomenon is to be looked for in some adjacent body possessing
energy--that is, the ability to push or produce pressure.

It must not be forgotten that energy is not a simple factor, but is
always a product of two factors--a mass with a velocity, a mass with a
temperature, a quantity of electricity into a pressure, and so on. One
may sometimes meet the statement that matter and energy are the two
realities; both are s