Encyclopaedia Britannica > Volume 8, DIA-England

346
DYNAMICS.
Dynamics, moving with any velocity, strike another equal ball which
is at rest, the struck ball moves with half the velocity of
the other. And it is farther remarkable that the striking
body always loses as much motion as the struck body
gains. This universal and remarkable fact seems to have
given rise to a confused or indistinct notion of a sort of
transference of motion from one body to another. The
phraseology in general use on this subject expresses this
in the most precise terms. The one ball is not said to
cause or produce motion in the other, but to communicate
motion to it; and the whole phenomenon is called the
Communi- communication of motion. We call this an indistinct no¬
cation of tion ; for surely no one will say that he has any clear con-
a^ood1 n°tcePt*on We can form the most distinct notion of
pression. " ^ie communication of heat, or of the cause of heat; of the
communication of saltness, sweetness, and a thousand
other things; but we cannot conceive how part of that
identical motion which was formerly in A, is now infused
into B, being given up by A. It is in our attempt to
form this notion that we find that motion is not a thing,
not a substance which can exist independently, and is
susceptible of actual transference. It appears in this
case to be a state, or condition, or mode of existence, of
which bodies are susceptible, which is producible, or (to
speak without metaphor) causable, in bodies, and which
is the effect and characteristic of certain natural qualities,
properties, or powers. We are anxious to have our read¬
ers impressed with clear and precise notions on this sub¬
ject, being confident that such, and only such, will carry
them through some intricate paths of mechanical and phi¬
losophical research.
Inherent 10. The remarkable circumstance in this phenomenon is,
force is the that a rapid motion, which requires for the effecting it
distinctive the action of a pressing power, continued for a sensible,
c aracterofan(j freqUent]y a long time, seems to be affected in an in-
^ ’ stant by impulsion. This has tended much to support
the notion of the actual transference of something former¬
ly possessed exclusively by the striking body, inhering in
it, but separable, and now transfused into the body stricken.
And now room is found for the employment of metaphor,
both in thought and language. The striking body affects
the body which it thus impels: it therefore possesses the
•power of impulsion, that is, of communicating motion. It
possesses it only while it is in motion. This power, there¬
fore, is the efficient distinguishing cause of its motion,
and its only office must be the continuation of this mo¬
tion. It is therefore called the inherent force, the force
inherent in a moving body, vis insita corpori moto.
This force is transfused into the body impelled ; and
therefore the transference is instantaneous, and the impell¬
ed body continues its motion till it is changed by some
other action. All this is at first sight very plausible ; but
a scrupulous attention to those feelings which have given
rise to this metaphorical conception should have produced
very different notions. I am conscious of exertion in or¬
der to begin motion on a slide; but if the ice be very
smooth, I am conscious of no exertion in order to slide
along. My power is felt only while I am conscious of ex¬
erting it; therefore I have no primitive feeling or notion
of power wdiile I am sliding along. I am certain that no
exertion of power is necessary here. Nay, I find that I
cannot think of my moving forward without effort other¬
wise than as a certain mode of my existence. Yet we
imagine that the partizans of this opinion did really de¬
duce it in some shape from their feelings. We must con¬
tinue the exertion of walking in order to walk on; our
power of walking must be continually exerted, otherwise
we shall stop. But this is a very imperfect, incomplete,
and careless observation. Walking is much more than
mere continuance in progressive motion. It is a continu¬
ally repeated lifting our body up a small height, and al-Byn
lowing it to come down again. This renewed ascent re-
quires repeated exertion.
11. We have other observations of importance yet toand
make on this force of moving bodies, but this is not the be hi
most proper occasion. Meanwhile we must remark, thatVgr
the instantaneous production of rapid motion by impulset!lan
has induced the first mechanicians of Europe to maintainsion'
that the power or force of impulse is unsusceptible of any
comparison with a pressing power. They have asserted
that impulse is infinitely great when compared with pres¬
sure ; not recollecting that they held them to be things
totally disparate, that have no proportion more than weight
and sweetness. But these gentlemen are perpetually en¬
ticed away from their creed by the similarity of the ultimate
results of pressure and impulse. No person can find any
difference between the motion of two balls moving equally
swift, in the same direction, one of which is descending
by gravity, and the other has derived its motion from a
blow. This struggle of the mind to maintain its faith,
and yet accommodate its doctrines to what we see, has
occasioned some other curious forms of expression. Pres¬
sure is considered as an effort to produce motion. When
a ball lies on a table, its weight, which they call a power,
continually and repeatedly endeavours (mark the meta¬
phorical word and thought) to move the ball downward.
But these efforts are ineffectual. They say that this in¬
effectual power is dead, and call it a vis mortua : but the
force of impulsion is called a vis viva, a living force.
But this is very whimsical and very inaccurate. If the
impelling ball falls perpendicularly on the other lying on
the table, it will produce no motion, any more than gra¬
vity wfill; and if the table be annihilated, gravity becomes
a vis viva.
We must now add, that in order to prove that impulse ArguU
is infinitely greater than pressure, these mechanicians turnindis t
our attention to many familiar facts which plead stronglyan(lj “•
in their favour. A carpenter will drive a nail into a boardclu5i’
with a very moderate blow of his hammer. This will re¬
quire a pressure which seems many hundred times greater
than the impelling effort of the carpenter. A very mode¬
rate blow will shiver into pieces a diamond which would
carry the weight of a mountain. Seeing this prodigious
superiority in the impulse, how shall they account for the
production of motion by means of pressure; for this mo¬
tion of the hammer might have been acquired by its fall¬
ing from a height; nay, it is actually acquired by means
of the continued pressure of the carpenter’s arm. They
consider it as the aggregate of an infinity of succeeding
pressures in every instant of its continuance; so that the
insignificant smallness of each effort is compensated by
their inconceivable number.
On the whole, we do not think that there is clear evi-Nod
dence that there are two kinds of mechanical force essen-ence
tially different in their nature. It is virtually given up bytwee 'es‘
those who say that impulse is infinitely greater than pres-?^"
sure. Nor is there any considerable advantage to be ob¬
tained by arranging the phenomenon under those two
heads. We may perhaps find some method of explaining
satisfactorily the remarkable difference that is really ob¬
served in the two modes of producing motion; namely,
the gradual production of motion by acknowledged pres¬
sure, and the instantaneous production of it by impulse.
Indeed we should not have taken up so much of our read¬
ers’ attention with this subject, had it not been for some
inferences that have been made from these premises, which
meet us in our very entry on the consideration of first prin¬
ciples, and that are of extensive influence on the whole
science of mechanical philosophy, and, indeed, on the
whole study of nature.