THE CONTINUITY OF MOTION.
§ 69. Another general truth of the same order with the foregoing, must
here be specified—one which, though not so generally recognized, has yet
long been familiar among men of science. The continuity of Motion, like
the indestructibility of Matter, is clearly an axiom underlying the very
possibility of a rational theory of Evolution. That kind of change in
the arrangement of parts, which we have found to constitute Evolution,
could not be deductively explained were it possible for Motion either to
appear or disappear. If those motions through which the parts pass into
a new arrangement, might either proceed from nothing or lapse into
nothing, there would be an end to scientific interpretation of them.
Each constituent change might as well as not be supposed to begin and
end of itself.
The axiomatic character of the truth that Motion is continuous, is
recognized only after the discipline of exact science has given
precision to the conceptions. Aboriginal men, our uneducated population,
and even most of the so-called educated, think in an extremely
indefinite manner. From careless observations, they pass by careless
reasoning, to conclusions of which they do not contemplate the
implications—conclusions which they never develope for the purpose of
seeing whether they are consistent. Accepting without criticism the
dicta of unaided perception, to the effect that surrounding bodies when
put in motion soon return to rest, the great majority tacitly assume
that the motion is actually lost. They do not consider whether the
phenomenon can be otherwise interpreted; or whether the interpretation
they put on it can be mentally realized. They are content with a
colligation of mere appearances. But the establishment of certain
facts having quite an opposite implication, led to inquiries which have
gradually proved such appearances to be illusive. The discovery that the
planets revolve round the Sun with undiminishing speed, raised the
suspicion that a moving body, when not interfered with, will go on for
ever without change of velocity; and suggested the question whether
bodies which lose their motion, do not at the same time communicate as
much motion to other bodies. It was a familiar fact that a stone would
glide further over a smooth surface, such as ice, presenting no small
objects to which it could part with its motion by collision, than over a
surface strewn with such small objects; and that a projectile would
travel a far greater distance through a rare medium like air, than
through a dense medium like water. Thus the primitive notion that moving
bodies had an inherent tendency gradually to lose their motion and
finally stop—a notion of which the Greeks did not get rid, but which
lasted till the time of Galileo—began to give way. It was further shaken
by such experiments as those of Hooke, which proved that the spinning of
a top continued long in proportion as it was prevented from
communicating movement to surrounding matter—experiments which, when
repeated with the aid of modern appliances, have shown that _in vacuo_
such rotation, retarded only by the friction of the axis, will continue
for nearly an hour. Thus have been gradually dispersed, the obstacles to
the reception of the first law of motion;—the law, namely, that when not
influenced by external forces, a moving body will go on in a straight
line with a uniform velocity. And this law is in our day being merged in
the more general one, that Motion, like Matter, is indestructible; and
that whatever is lost by any one portion of matter is transferred to
other portions—a conclusion which, however much at variance it seems
with cases of sudden arrest from collision with an immovable object, is
yet reconciled with such cases by the discovery that the motion
apparently lost continues under new forms, though forms not directly
perceptible.
* * * * *
§ 70. And here it may be remarked of Motion, as it was before of Matter,
that its indestructibility is not only to be inductively inferred, but
that it is a necessity of thought: its destructibility never having been
truly conceived at all, but having always been, as it is now, a mere
verbal proposition that cannot be realized in consciousness—a
pseud-idea. Whether that absolute reality which produces in us the
consciousness we call Motion, be or be not an eternal mode of the
Unknowable, it is impossible for us to say; but that the relative
reality which we call Motion never can come into existence, or cease to
exist, is a truth involved in the very nature of our consciousness. To
think of Motion as either being created or annihilated—to think of
nothing becoming something, or something becoming nothing—is to
establish in consciousness a relation between two terms of which one is
absent from consciousness, which is impossible. The very nature of
intelligence, negatives the supposition that Motion can be conceived
(much less known) to either commence or cease.
* * * * *
§ 71. It remains to be pointed out that the continuity of Motion, as
well as the indestructibility of Matter, is really known to us in terms
of _force_. That a certain manifestation of force remains for ever
undiminished, is the ultimate content of the thought; whether reached _à
posteriori_ or _à priori_.
From terrestrial physics let us take the case of sound propagated to a
great distance. Whenever we are directly conscious of the causation of
sound (namely, when we produce it ourselves), its invariable antecedent
is force. The immediate sequence of this force we know to be
motion—first, of our own organs, and then of the body which we set
vibrating. The vibrations so generated we can discern both through the
fingers and through the ears; and that the sensations received by the
ears are the equivalents of mechanical force communicated to the air,
and by it impressed on surrounding objects, we have clear proof when
objects are fractured: as windows by the report of a cannon; or a glass
vessel by a powerful voice. On what, then, rests the reasoning when, as
occasionally happens under favourable circumstances, men on board a
vessel a hundred miles from shore, hear the ringing of church-bells on
placing their ears in the focus of the main sail; and when it is
inferred that atmospheric undulations have traversed this immense
distance? Manifestly, the assertion that the motion of the clapper,
transformed into the vibrations of the bell, and communicated to the
surrounding air, has propagated itself thus far on all sides,
diminishing in intensity as the mass of air moved became greater, is
based solely upon a certain change produced in consciousness through the
ears. The listeners are not conscious of motion; they are conscious of
an impression produced on them—an impression which implies a force as
its necessary correlative. With force they begin, and with force they
end: the intermediate motion being simply inferred. Again, where,
as in celestial physics, the continuity of motion is quantitatively
proved, the proof is not direct but inferential; and forces furnish the
data for the inference. A particular planet can be identified only by
its constant power to affect our visual organs in a special way—to
impress upon the retina a group of forces standing in a particular
correlation. Further, such planet has not been _seen_ to move by the
astronomical observer; but its motion is _inferred_ from a comparison of
its present position with the position it before occupied. If rigorously
examined, this comparison proves to be a comparison between the
different impressions produced on him by the different adjustments of
the observing instruments. Going a step further back, it turns out that
this difference is meaningless until shown to correspond with a certain
calculated position which the planet must occupy, supposing that no
motion has been lost. And if, finally, we examine the implied
calculation, we find that it makes allowances for those accelerations
and retardations which ellipticity of the orbit involves, as well as
those variations of velocity caused by adjacent planets—we find, that
is, that the motion is concluded to be indestructible not from the
uniform velocity of the planet, but from the constant quantity of motion
exhibited when allowance is made for the motion communicated to, or
received from, other celestial bodies. And when we ask how this
communicated motion is estimated, we discover that the estimate is based
upon certain laws of force; which laws, one and all, embody the
postulate that force cannot be destroyed. Without the axiom that action
and re-action are equal and opposite, astronomy could not make its exact
predictions; and we should lack the rigorous inductive proof they
furnish that motion can never be lost, but can only be transferred.
Similarly with the _à priori_ conclusion that Motion is continuous. That
which defies suppression in thought, is really the force which the
motion indicates. The unceasing change of position, considered by
itself, may be mentally abolished without difficulty. We can readily
imagine retardation and stoppage to result from the action of external
bodies. But to imagine this, is not possible without an abstraction of
the force implied by the motion. We are obliged to conceive this force
as impressed in the shape of re-action on the bodies that cause the
arrest. And the motion that is communicated to them, we are compelled to
regard, not as directly communicated, but as a product of the
communicated force. We can mentally diminish the velocity or
space-element of motion, by diffusing the momentum or force-element over
a larger mass of matter; but the quantity of this force-element, which
we regard as the cause of the motion, is unchangeable in thought.