our "muscular sense,"
which gives us the idea of pressure, as for example when we push or pull
a body along the ground.

We must not, however, limit our idea of Force to that narrow circle. It
has now been fully established that Sound and Heat, Light, Magnetism,
and Electricity are Forces, and therefore capable of doing work, as will
be shown later on. Newton's use of the term Force is therefore somewhat
vague; he does not definitely say what the Force is which causes the
change of position, of the body, or of the rate of motion of that body.
That it is something to do with Gravitation is obvious, but its exact
nature or character is not revealed.

Since Newton's time we have made an advance in the definition of Force,
and have come to consider Force as a kind of energy; the application of
Force being the application of energy. Such terms as Mechanical Force,
Chemical Force, Vital Force, are therefore out of date, and in their
place the more definite ideas of energy are substituted. Instead,
therefore, of getting such terms as Transformation of Forces, we now get
Transformations of Energy. In the chapter on Energy, I hope to show that
even that is not a satisfactory solution of the definition of a Force.
If we are to make our Philosophy agree with our experience, then Force
is due to motion, and motion alone.

So that Centrifugal Force will imply a motion from the centre;
Centripetal Force a motion whose effect is ever towards the centre of
gravity of any body.

ART. 14. _First Law of Motion._--This may naturally be divided into two
parts for the purpose of applying the Rules of Philosophy.

(I) Every body continues in a state of rest, except in so far as it is
compelled by impressed Forces to change that state. To what extent is
this statement conformable to our experience and observation? If I place
a body, as for example a weight, on a table, will it remain in that
state until it is moved by some other Force? I think that it will so
remain, and to that extent the law conforms to experiment.

Wider observation, and all experience, also prove the conformity of this
part of the First Law of Motion to the second Rule of Philosophy, as all
experience testifies to the fact that a body remains at rest, until some
other power or force moves it from the position of rest. The application
of this position of rest to any of the planets is, however, very
difficult to conceive. MacLaurin, in relation to this fact, states: