Encyclopaedia Britannica > Volume 8, DIA-England

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DYNAMICS,
j.. Law yet in words they (and most of them in thought) likewise
(1f ation. abet that doctrine: for they say that there resides in a
moving body a power or force, by which it perseveres in
its motion. They call it the vis insita, the inherent force,
oja moving body. This is surely giving up the question :
for if the motion is supposed to be continued in conse¬
quence of a force, that force is supposed to be exerted;
and it is supposed that if it were not exerted, the motion
would cease; and therefore the proposition must be false.
Indeed it is sometimes expressed so as seemingly to ward
off this objection. It is said that the body continues in
uniform rectilineal motion, unless affected by some external
cause. But this way of speaking obliges us, at first setting
out in natural philosophy, to assert that gravity, magnetism,
electricity, and a thousand other mechanical powers, are
external to the matter which they put in motion. This is
quite improper. It is the business of philosophy to dis¬
cover whether they be external or not; and if we assert
that they are, we have no principles of argumentation with
those who deny it. It is this one thing that has filled the
study of nature with all the jargon of ethers, and other in¬
visible, intangible fluids, which has disgraced philosophy,
and greatly retarded its progress.
yXuita, 32. We must observe, that the terms vis insita, inherent
indent force, are very improper. There is no dispute among phi-
foi are losophers in calling every thing a force that produces a
im ’Per change of motion, and in inferring the action of such a
tl' usual ft,rce whenever observe a change of motion. It is
aJta. surely incongruous to give the same name to what has
tic not this quality of producing a change, or to infer (or rather
to suppose) the energy of a force when no change of motion
is observed. This is one among many instances of the
danger of mistake when we indulge in analogical discus¬
sions. All our language, at least, on this subject is ana¬
logous. I feel that in order to oppose animal force I must
exert force. But I must exert force in order to oppose a
body in motion ; therefore I imagine that the moving body
possesses force. A bent spring will drive a body forward
by unbending; therefore I say that the spring exerts
force. A moving body impels the body which it hits;
therefore I say that the impelling body possesses and ex¬
erts force. I imagine farther, that it possesses force only
by being in motion, or because it is in motion; because I
do not find that a quiescent body will put another into
motion by touching it. But we shall soon find this to be
false in many, if not in all cases, and that the communica¬
tion of motion depends on the mere vicinity, and not on
the motion of the impelling body; yet we ascribe the
exertion of the vis insita to the circumstance of the con¬
tinued motion. We therefore conceive the force as aris¬
ing from or as consisting in the impelling body’s being in
motion; and, with a very obscure and indistinct concep¬
tion of the whole matter, we call it the force by which the
body preserves itself in motion. Thus, taking it for grant¬
ed that a force resides in the body, and being obliged
to give it some office, this is the only one that we can
think of.
33. But philosophers imagine that they perceive the
necessity of the exertion of a force in order to the con¬
tinuation of a motion. Motion (say they) is a continued
action ; the body is every instant in a new situation ; there
is the continual production of an effect, therefore the con¬
tinual action of a cause.
^ F's This, however, is a very inaccurate way of thinking.
co°i iual ^ ^aVe a t^st‘nct concePt‘on °f motion ; and we con-
pd jiction ce^ve. t^at there is such a thing as a moving cause, which
d ef. we distinguish from all other causes by the name force. It
fet| produces motion. If it does this, it produces the cha¬
racter of motion, which is a continual change of place.
Motion is not action, but the effect of an action ; and this
VOL. VIII.
353
action is as complete in the instant immediately succeed- First Law
ing the beginning of the motion as it is a minute after. °f Motion.
The subsequent change of place is the continuation of an
effect already produced. The immediate effect of the
moving force is a determination, by which, if not hin¬
dered, the body would go on for ever from place to place.
It is in this determination only that the state or condition
of the body can differ from a state of rest; for in any in¬
stant the body does not describe any space, but has a de¬
termination by which it will describe a certain space uni¬
formly in a certain time. Motion is a condition, a state,
or mode of existence, and no more requires the continued
agency of the moving cause than yellowness or roundness
does. It requires some chemical agency to change the
yellowness to greenness ; and itrequires a mechanical cause
or a force to change this motion into rest. When we see
a moving body stop short in an instant, or be gradually
but quickly brought to rest, we never fail to speculate
about a cause of this cessation or retardation. The case
is no way different in itself although the retardation should
be extremely slow. We should always attribute it to a
cause. It requires a cause to put a body out of motion, as
much as to put it into motion. This cause, if not exter¬
nal, must be found in the body itself; and it must have a
self-determining power, and may as well be able to put
itself into motion as out of it.
If this reasoning be not admitted, we do not see how
any effect can be produced by any cause. Every effect
supposes something done ; and any thing done implies that
the thing done may remain till it be undone by some other
cause. Without this it would have no existence. If a
moving cause did not produce continued motion by its in¬
stantaneous action, it could not produce it by any continu¬
ance of that action ; because in no instant of that action
does it produce continued motion.
We must therefore give up the opinion, that there re¬
sides in a moving body a force by which it is kept in mo¬
tion ; and we must find some other way of explaining that
remarkable difference between a moving body and a body
at rest, by which the first causes other bodies to move by
hitting them, while the other does not do this by merely
touching them. We shall see, with the clearest evidence,
that motion is necessary in the impelling body, in order
that it may permit the forces inherent in one or both
bodies to continue this pressure long enough for produ¬
cing a sensible or considerable motion. But these moving
forces are inherent in bodies, whether they are in motion
or at rest.
34. The foregoing observations show us the impropriety Commum-
of the phrase communication of motion. By thus reflect- cation of
ing on the notions that are involved in the general con-m°tion is
ception of one body being made to move by the impulse an inpro-
of another, we perceive that there is nothing individual^6 ^ ia’e’
transferred from the one body to the other. The deter¬
mination to motion, indeed, existed only in the impelling
body before collision; whereas, afterwards, both bodies
are so conditioned or determined. But we can form no
notion of the thing transferred. With the same meta¬
physical impropriety we speak of the communication of
joy, of fever.
35. Kepler introduced a term inertia, vis inertice, into so is vis
mechanical philosophy; and it is now in constant use.iwcriia?.
But writers are very careless and vague in the notions which
they affix to these terms. Kepler and Newton seem ge¬
nerally to employ it for expressing the fact, the perseverance
of the body in its present state of motion or rest: but they
also frequently express by it something like an indiffer¬
ence to motion or rest, manifested by its requiring the same
quantity of force to make an augmentation of its motion as
to make an equal diminution of it. The popular notion is