Encyclopaedia Britannica > Volume 8, DIA-England

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1
DYNAMICS.
347
vrY
cs. 12. Mechanicians are greatly divided in their opinion
,.j about the nature of the sole moving force in nature. Those
whotn we are now speaking of seem to think that all mo-
lion t tion is produced by pressure; for when they consider im-
polecfse pU]se as equivalent to the aggregate of an infinity of re-
jfmo4n?peate[j pressures, they undoubtedly suppose any pressure,
however insignificant, as a moving force. But there is a
party, both numerous and respectable, who maintain that
impulsion is the sole cause of motion. We see bodies in
motion, say they, and we see them impel others; and we
see that this production of motion is regulated by such
laws, that there is but one absolute quantity of motion in
the universe which remains unalterably the same. It must
therefore be transfused in the acts of collision. We also
see, with clear evidence, in some cases, that motion can
produce pressure. Euler adduces some very whimsical
and complicated cases, in which an action precisely simi¬
lar to pressure may be produced by motion. Thus, two
balls connected by a thread may be so struck that they
shall move forward, and at the same time wheel round. In
this case the connecting thread will be stretched between
them. Now, say the philosophers, since we see motion,
and see that pressure may be produced by motion, it is
preposterous to imagine that it is any thing else than a
result of certain motions; and it is the business of a phi¬
losopher to inquire and discover what motions produce the
pressures that we observe.
They then proceed to account for those pressing powers,
or solicitations to motion, which we observe in the acce¬
leration of falling bodies, the attractions of magnetism and
electricity, and many other phenomena of this kind, where
bodies are put in motion by the vicinity of other bodies,
or, in the popular language, by the action of other bodies
at a distance. To say that a magnet can act on a piece of
remote iron, is to say that it can act where it is not, which
is as absurd as to say that it can act when it is not. Nihil
movetur, says Euler, nisi a contiguo et moto.
Rories The bulk of these philosophers are not very anxious
it pr ice about the way in which these motions are produced, nor
presi e? do they fall upon such ingenious methods of producing
pressure as the one already mentioned, which was adduced
by Euler. The piece of iron, say they, is put in motion
when brought into the neighbourhood of a magnet, be¬
cause there is a stream of fluid issuing from one pole of the
magnet, which circles round the magnet, and enters at
the other pole. This stream impels the iron, and arranges
it in certain determined positions, just as a stream of wa¬
ter would arrange the flote grass. In the same manner,
there is a stream of fluid continually moving towards the
centre of the earth, which impels all bodies in lines per¬
pendicular to the surface ; and so on with regard to other
like phenomena. These motions are thus reduced to very
simple cases by impulsion.
It is unnecessary to refute this doctrine at present: it
ln«j
tibh
the: is of
’tk is enough that it is contrary to all the dictates of common
phil
phis,.];,
sense.
Oth
tnab in
that es-
«urc the
sole by.
»ng ce.
To suppose an agent that we do not see, and for
whose existence we have not the smallest argument; with
equal propriety we might suppose ministering spirits, or
any thing that we please.
13. Other philosophers are so dissatisfied with this notion
of the production of pressure, that they, on the other hand,
affirm that pressure is the only moving force in nature;
not according to the popular notion of pressure, by the
mutual contact of solid bodies, but that kind of pressure
which has been called solicitation, such as the power of
gravity. They affirm that there is no such thing as contact
on instantaneous communication of motion by real collision,
ihey say, and they prove it by very convincing facts, that
the particles of solid bodies exert very strong repulsions
to a small distance; and therefore, when they are brought
by motion sufficiently near to another body, they repel it, Dynamics,
and are equally repelled by it. Thus is motion produced
in the other body, and their own motion is diminished.
And they then show, by a scrupulous consideration of the
state of the bodies while the one is advancing and the
other retiring, in what manner the two bodies attain a
common velocity, so that the quantity of motion before
collision remains unchanged, the one body gaining as much
as the other loses. They also showr cases of such mutual ac¬
tion between bodies, where it is evident that they have never
come into contact; and yet the result has been precisely
similar to those cases where the motion appeared to be
changed in an instant. Therefore they conclude that
there is no such thing as instantaneous communication, or
transfusion of motion, by contact in collision or impulse.
The reason why previous motion of the impelling body is
necessary, is not that it may have a vis insita corpori moto,
a force inherent in it by its being in motion, but that it may
continue to follow the impelled and retiring body, and ex¬
ert on it a force inherent in itself, whether in motion or at
rest. According to these philosophers, therefore, all mov¬
ing forces are of that kind which has been named solicita¬
tion, such as gravity. We shall know it afterwards by the
more familiar and descriptive name of acceleratmg or re¬
tarding force.
14. The exertions of mechanical forces are differently Action, re¬
termed, according to the reference that we make to thesistance,
result. If in boxing or wrestling I strike, or endeavour tore*acti°n*
throw my antagonist, I am said to act; but if I only parry
his blows, or prevent him from throwing me, I am said to
resist. This distinction is applied to the exertions of me¬
chanical powers. When one body, A, changes the motion
of another, B, we may consider the change in the motion
of B either as the indication and measure of A’s power of
producing motion, or as the indication and measure of A’s
resistance to the being brought to rest, or having its motion
any how changed. The distinction is not in the thing it¬
self, but only in the reference that we are disposed, by other
considerations, to make of its effect. They may be distin¬
guished in the following manner: If a change of motion
follow when one of the powers ceases to be exerted, that
power is conceived as having resisted. The whole lan¬
guage on this subject is metaphorical. Resistance, effort,
endeavour, &c. are words which cannot be employed in
mechanical discussions without figure, because they all
express notions which relate to sentient beings; and the
unguarded indulgence of this figurative language has so
much affected the imagination of philosophers, that many
have almost animated all matter. Perhaps the word re¬
action, introduced by Newton, is the best term for ex¬
pressing that mutual force which is perceived in all the
operations of nature that we have investigated with suc¬
cess. As the magnet attracts iron, and in so doing is said
to act on it; so the iron attracts the magnet, and may be
said to re-act on it.
15. With respect to the difficulty that has been object-We need
ed to the opinion of those who maintain that all the me-not sup-
chanical phenomena are produced by the agency of at'p?s^iCstl0n
tracting or repelling forces; namely, that this supposesaaHaels‘
the bodies to act on each other at a distance, however
small those distances may be, which is thought to be ab¬
surd, we may observe, that we may ascribe the mutual
approaches or recesses to tendencies to or from each other.
What we call the attraction of the magnet may be consider¬
ed as a tendency of the iron to the magnet, somewhat si¬
milar to the gravitation of a stone toward the earth. We
surely (at least the unlearned) can and do conceive the
iron to be affected by the magnet, without thinking of any
intermedium. The thing is not therefore inconceivable,
which is all that we know about absurdity; and we do not