MOTION.
Are the Physical Forces modes of Motion?--Motion
defined--Philosophical Views of Motion, and the Principles to
which it has been referred--Motions of the Earth and of the Solar
System--Visible Proofs of the Earth’s Motion on its Axis--Influence
of the proper Motions of the Earth on the Conditions of
Matter--Theory of the Conversion of Motion into Heat, &c.--The
Physical Forces regarded as principles independent of Motion,
although the Cause and often apparently the Effects of it.
Many of the most eminent thinkers of the present time are disposed to
regard all the active principles of nature as “modes of motion,”--to
look upon light, heat, electricity, and even vital force, as phenomena
resulting from “change of place” among the particles of matter; this
change, disturbance, or motion, being dependent upon some undefined
mover.[2]
The habit of leaving purely inductive examination for the delusive
charms of hypothesis--of viewing the material world as a metaphysical
bundle of essential properties, and nothing more--has led some eminent
philosophers to struggle with the task of proving that all the
wonderful manifestations of the great physical powers of the universe
are but modifications of motion, without the evidence of any antecedent
force.[3]
The views of metaphysicians regarding motion involve many subtle
considerations which need not at present detain us. We can only
consider motion as a change of place in a given mass of matter. Now
matter cannot effect this of itself, no change of place being possible
without a mover; and, consequently, motion cannot be a _property_ of
matter, in the strict sense in which that term should be accepted.[4]
Motion depends upon certain external disturbing and directing forces
acting upon all matter; and, consequently, as every mode of action
is determined by some excitement external to the body moved, motion
cannot, philosophically, be regarded otherwise than as a peculiar
affection of matter under determinable conditions. “We find,” says Sir
Isaac Newton, “but little motion in the world, except what plainly
flows from either the active principles of nature, or from the command
of the willer.”[5]
Plato, Aristotle, and the Pythagoreans, supposed that throughout all
nature an active principle was diffused, upon which depended all the
properties exhibited by matter. This is the same as the “plastic
nature” of Cudworth,[6] the “intellectual and artificial fire” of
Bishop Berkeley;[7] and to these all modes of motion were referred.
Sir Isaac Newton also regards the material universe and its phenomena
as dependent upon “_active principles_”--for instance, the cause of
gravity--whereby the planets and comets preserve their motions in their
orbits, and all bodies acquire a degree of motion in falling; and the
cause of fomentation--whereby the heart and blood of animals preserve
a perpetual warmth and motion--the inner parts of the earth are kept
constantly warmed--many bodies burn and shine--and the sun himself
burns and shines, and with his light warms and cheers all things.
The earth turns on its axis at the rate of more than 1,000 miles an
hour, and passes around the sun with the speed of upwards of 68,000
miles in the same time.[8] The earth and the other planets of our
system move in ellipses around a common centre: therefore their motion
cannot have been originally communicated merely by the impressed force
of projection. Two forces, at least, must have operated, one making
the planets tend directly to the centre, and the other impelling them
to fly off at a tangent to the curve described. Here we have a system
of spheres, held by some power to a great central mass, around which
they revolve with a fearful velocity. Nor is this all; the Solar System
itself, bound by the same mystic chain to an undiscovered centre, moves
towards a point in space at the rate of 33,550,000 geographical miles,
whilst our earth performs one revolution around the sun.[9]
The evidence of the motion of the Earth around its axis, as afforded
by the swinging of a pendulum or the rotation of a sphere, is too
interesting to be omitted. In mechanical philosophy, we have two
terms of the same general meaning--the conservation of the plane
of vibration--and the conservation of the axis of rotation. For
the non-scientific reader, these terms require explanation, and in
endeavouring to simplify this as much as possible, we must ask the
indulgence of the Mechanical Philosopher. Let us fix in the centre
of a small round table an upright rod, having an arm extending from
its top, to which we can suspend a tolerably heavy weight attached
to a string. This is our piece of apparatus: upon the table draw a
chalk line, along which line we intend our pendulum to swing, and
continuing this line upon the floor, or by a mark on the wall, our
arrangements are complete. Raise steadily the bob of our pendulum,
and set it free, so that its plane of vibration is along the line
which has been marked. As the pendulum is swinging firmly along this
line, slowly and steadily turn the table round. It will then be seen
that the pendulum will still vibrate in the direction of the line we
have continued onward to the wall, but that the line on the table is
gradually withdrawn from it. If we had no upright, we might turn the
table entirely round, without in the slightest degree altering the
line along which the pendulum performs its oscillations. Now, if from
some elevated spot, say, from the centre of the dome of St. Paul’s,
a long and heavy pendulum is suspended, and if on the floor we mark
the line along which we set the pendulum free to vibrate, it will be
seen, as in the experiment with the table, that the marked line moves
away from under the pendulum. It continues to vibrate in the plane it
first described, although the line on the earth’s surface continues
to move forward by the diurnal rotation around the axis. Similar to
this is the law of the conservation of the axis of rotation. If a
common humming-top, the spindle of which is its axis of rotation, is
set spinning obliquely, it will be seen that the axis will continue
to point along the line it took at the commencement of motion. By
placing a heavy sphere in a lathe, resting its projecting axial points
on some moveable bearings, and then getting the sphere into extremely
rapid motion, one of the bearings may be removed without the mass
falling to the ground. The rapidity of motion changes so constantly and
quickly the position of the particles which have a tendency to fall,
that we have motion balanced against the force of gravitation in a
striking manner; and we learn, from this experiment, the explanation
of the planetary and stellar masses revolving on their axis at a speed
sufficient to maintain them without support in space. A mass of matter,
a sphere or a disc, carefully balanced, is fixed in gymbals such as we
employ for fixing our compass needles, and it is set by some mechanical
contrivance in rapid rotation. The position of the axis of rotation
remains unaltered, although the earth is moving; and thus, by this
instrument,--called the gyroscope,--we can determine, as with the
pendulum, the motion of the earth around its axis; and we learn why,
during its movement around the sun, its axis is undeviatingly pointed
towards one point in space, marked in our Heavens as the Polar Star.
In addition to these great rotations, the earth is subjected to other
motions, as the precession of the equinoxes and the nutation of its
axis. Rocking regularly upon a point round which it rapidly revolves,
whilst it progresses onward in its orbit, like some huge top in
tremulous gyration upon the deck of a vast aërial ship gliding rapidly
through space, is the earth performing its part in the great law of
motion.
The rapidity of these impulses, supposing the powers of the physical
forces were for a moment suspended, would be sufficient to scatter the
mass of our planet over space as a mere star-dust.
Limiting, as much as possible, the view which opens upon the mind as we
contemplate the adjustments by which this great machine, our system, is
preserved in all its order and beauty, let us forget the great movement
of the whole through space, and endeavour to consider the effect of
those motions which are directly related to the earth, as a member of
one small group of worlds.
We cannot for a moment doubt, although we have not any experimental
proof of the fact, that the proper motions of the earth materially
influence the conditions of the matter of which it is formed. Every
pair of atoms is, like a balance, delicately suspended, under the
constant struggle which arises from the tendency to fly asunder,
induced by one order of forces--centrifugal force--and the efforts of
others, gravitation and cohesion, to chain them together. The spring is
brought to the highest state of tension--one tremor more, and it would
be destroyed.
We cannot, by any comparison with the labours of the most skilful human
artisan, convey an idea of the exquisite perfection of planetary
mechanics, even so far as they have been discovered by the labours of
science; and we must admit that our insight into the vast machinery has
been very limited.
All we know is the fact that this planet moves in a certain order, and
at a fixed rate, and that the speed is of itself sufficient to rend
the hardest rocks; yet the delicate down which rests so lightly upon
the flower is undisturbed. It is, therefore, evident that matter is
endued with powers, by which mass is bound to mass, and atom to atom;
these powers are not the results of any of the motions which we have
examined, but, acting in antagonism to them, they sustain our globe in
its present form.
Are there other motions to which these powers can be referred? We
know of none. That absolute rest may not exist among the particles of
matter is probable. Electrical action, chemical power, crystalline
aggregation, the expansive force of heat, and many other known
agencies, are in constant operation to prevent it. It must, however,
be remembered, that each and every atom constituting a mass may be
so suspended between the balanced forces, that it may be regarded as
relatively at rest.
Theory imagines Motion as producing Force--a body is moved, and its
mere mechanical change of place is regarded as generating heat; and
hence the refinements of modern science have advanced to the conclusion
that motion and heat are convertible. Admitting that the material atoms
of which this world is formed are never in a state of quiescence, yet
we cannot suppose any gross ponderable particle as capable of moving
itself; but once set in motion, it may become the secondary cause of
motion in other particles.[10] The difficulties of the case would
appear to have been as follows:--Are heat, light, electricity, &c.,
material bodies? If they are material bodies--and heat, for example, is
the cause of motion--must not the calorific matter move itself--or if
it be not self-moving, by what is it moved? If heat is material, and
the primary cause of motion, then matter must have an innate power of
moving; it can convert itself into active force, or be at once a cause
and an effect, which can scarcely be regarded as a logical deduction.
We move a particle of matter, and heat is manifested; the force being
continued, light, electricity, and chemical action result; all,
as appears from a limited view of the phenomena, arising out of
the mechanical force applied to the particle first moved.[11] This
mechanical force, it must be remembered, is external to the body moved,
and is, in all probability, set up by the movement of a muscle, acted
upon by nerves, under the influence of a will.
The series of phenomena we have supposed to arise admit of an
explanation free of the hypothesis of motion, and we avoid the
dangerous ground of metaphysical speculation, and the subtleties of
that logic which rests upon the immateriality of all creation. This
explanation, it is freely admitted, is incomplete: we cannot distinctly
correlate each feature of the phenomena, combine link to link, and thus
form a perfect chain; but it is sufficiently clear to exhibit what we
do know, and leave the unknown free for unbiassed investigation.
Each particle, each atom of that which conveys to our senses the only
ideas we have of natural objects--ponderable matter--is involved
in, or interpenetrated by, those principles which we call heat and
electricity, with probably many others which are unknown to us; and
although these principles or powers are, according to some law, bound
in statical equilibrium to inert matter, they are freely developed
by an external excitement, and the disturbance of any one of them,
upsetting the equilibrium, leaves the other power equally free to be
brought under the cognizance of human sense by their effects.
When we come to an examination of the influences exerted by these
powers upon the physical earth, the position, that they must be
regarded as the causes of motion rather than the effects of it, will
be further considered. At present it is only necessary to state
thus generally the views we entertain of the conditions of matter
in connection with the imponderable forces and mechanical powers.
The conversion, as it has been called, of motion into heat, in the
experiments of Count Rumford and Mr. Joule,[12] are only evidences that
a certain uniformity exists between the mechanical force applied, and
the amount of heat liberated. It does not appear that we have any proof
of the conversion of motion into physical power.
It is necessary, to a satisfactory contemplation of the wonderful
properties of matter, and of the forces regulating the forms of
the entire creation, that we should be content with regarding the
elementary bodies which chemistry instructs us form our globe, as
tangible, ponderable atoms, having specific and distinguishing
properties. That we should, as far as it is possible for finite minds
to do so, endeavour to conceive the powers or forces--gravitation,
molecular attraction, electricity, heat, light, and the principle which
determines all chemical phenomena--as manifestations of agencies which
hold a place between the most subtile form of matter and the hidden
principles of vitality, which is still vastly inferior to the spiritual
state, which reveals itself dimly in psychological phenomena, and
arrives at its sublimity in the God of the universe.
FOOTNOTES:
[2] “Motion, therefore, is a change of rectilinear distance between
two points. Allowing the accuracy of this definition, it appears that
two points are necessary to constitute motion; that in all cases, when
we are inquiring whether or no any body or point is in motion, we must
recur to some other point which we can compare with it; and that if a
single atom existed alone in the universe, it could neither be said to
be in motion nor at rest.
“The space which we call quiescent is in general the earth’s surface;
yet we well know, from astronomical considerations, that every point of
the earth’s surface is perpetually in motion, and that in very various
directions: nor are any material objects accessible to our senses which
we can consider as absolutely motionless, or even as motionless with
regard to each other; since the continual variation of temperature to
which all bodies are liable, and the minute agitations arising from the
motion of other bodies with which they are connected, will always tend
to produce some imperceptible changes in their distances.”--_Lectures
on Natural Philosophy, &c._, by Thomas Young, M.D. Edited by the Rev.
P. Kelland. 1845.
[3] “The position which I seek to establish in this essay is, that
the various imponderable agencies, or the affections of matter which
constitute the main objects of experimental physics, viz., heat,
light, electricity, magnetism, chemical affinity, and motion, are all
correlative, or have a reciprocal dependence;--that neither, taken
abstractedly, can be said to be the essential or proximate cause of the
others; but that either may, as a force, produce, or be convertible
into, the other:--thus heat may mediately or immediately produce
electricity, electricity may produce heat, and so of the rest....
Although strongly inclined to believe that the five other affections of
matter, which I have above named, are, and will ultimately be, resolved
into modes of motion, it would be going too far at present to assume
their identity with it: I, therefore, use the term force, in reference
to them, as meaning that active force inseparable from matter, which
induces its various changes.”--_On the Correlation of Physical Forces_,
by W. R. GROVE, Esq., M.A., F.R.S.
[4] When discussing the hypothesis of Hobbes--_that no body can
possibly be moved but by a body contiguous and moved_--Boyle asks:--
“I demand how there comes to be local motion in the world? For either
all the portions of matter that compose the universe have motion
belonging to their natures, which the Epicureans affirmed for their
atoms, or some parts of matter have this motive power, and some have
not, or else none of them have it; but all of them are naturally
devoid of motion. If it be granted that motion does naturally belong
to all parts of matter, the dispute is at an end, the concession quite
overthrowing the hypothesis.
“If Mr. Hobbes should reply that the motion is impressed upon any of
the parts of matter by God, he will say that which I most readily grant
to be true, but will not serve his turn, if he would speak congruously
with his own hypothesis. For I demand whether this Supreme Being
that the assertion has recourse to, be a corporeal or an incorporeal
substance? If it be the latter, and yet the efficient cause of motion
in bodies, then it will not be universally true that whatever body is
moved is so by a body contiguous and moved. For, in our supposition,
the bodies that God moves, either immediately or by the intervention
of any other immaterial being, are not moved by a body contiguous,
but by an incorporeal spirit.”--_Some Considerations about the
Reconcileableness of Reason and Religion_: Boyle, vol. iii. p. 520.
[5] Boyle has some ingenious speculations on this point:--
“That there is local motion in many parts of matter is manifest to
sense, but how matter came by this motion was of old, and is still,
hotly disputed of: for the ancient Corpuscularian philosophers (whose
doctrine in most other points, though not in all, we are the most
inclinable to), not acknowledging an author of the universe, were
thereby reduced to make motion congenite to matter, and consequently
coeval with it. But since local motion, or an endeavour at it, is not
included in the nature of matter, which is as much matter when it rests
as when it moves; and since we see that the same portion of matter may
from motion be reduced to rest, and after it hath continued at rest, so
long as other bodies do not put it out of that state, may by external
agents be set a moving again; I, who am not wont to think a man the
worse naturalist for not being an atheist, shall not scruple to say
with an eminent philosopher of old, whom I find to have proposed among
the Greeks that opinion (for the main) that the excellent Des Cartes
has revived amongst us, that the origin of motion in matter is from
God; and not only so, but that thinking it very unfit to be believed,
that matter barely put into motion, and then left to itself, should
casually constitute this beautiful and orderly world; I think also
further, that the wise Author of things did, by establishing the laws
of motion among bodies, and by guiding the first motions of the small
parts of matter, bring them to convene after the manner requisite
to compose the world; and especially did contrive those curious and
elaborate engines, the bodies of living creatures, endowing most of
them with the power of propagating their species.”--_Considerations and
Experiments touching the Origin of Forms and Qualities_: Boyle’s Works,
vol. ii. p. 460. Edinburgh. 1744.
[6] Cudworth’s _Intellectual System_.
[7] “According to the Pythagoreans and Platonists, there is a life
infused throughout all things ... an intellectual and artificial
fire--an inward principle, animal spirit, or natural life, producing
or forming within, as art doth without--regulating, moderating, and
reconciling the various motions, qualities, and parts of the mundane
system. By virtue of this life, the great masses are held together in
their ordinary courses, as well as the minutest particles governed in
their natural motions, according to the several laws of attraction,
gravity, electricity, magnetism, and the rest. It is this gives
instincts, teaches the spider her web, and the bee her honey;--this
it is that directs the roots of plants to draw forth juice from the
earth, and the leaves and the cortical vessels to separate and attract
such particles of air and elementary fire as suit their respective
natures.”--Bishop Berkeley, _Siris_, No. 277.
[8] “The revolution of the earth is performed in a natural day,
or, more strictly speaking, once in 23h. 56' 4", and as its mean
circumference is 24,871 miles, it follows that any point in its
equatorial surface has a rotatory motion of more than 1,000 miles per
hour. This velocity must gradually diminish to nothing at either pole.
Whilst the earth is thus revolving on its axis, it has a progressive
motion in its orbit. If we take the length of the earth’s orbit at
630,000,000, its motion through space must exceed 68,490 miles in the
hour.”--Enc. Brit. art. _Physical Geography_.
[9] “Here then we have the splendid result of the united studies of
MM. Argelander, O. Struve, and Peters, grounded on observations made
at the three observatories of Dorpat, Abo, Pulkova, and which is
expressed in the following thesis:--The motion of the solar system in
space is directed to a point of the celestial vault situated on the
right line which joins the two stars π and μ _Herculis_, at a quarter
of the apparent distance of these stars, reckoning from π _Herculis_.
The velocity of this motion is such, that the sun, with all the bodies
which depend upon it, advances annually in the above direction 1·623
times the radius of the earth’s orbit, or 33,550,000 geographical
miles. The possible error of this last number amounts to 1,733,000
geographical miles, or to a _seventh_ of the whole value. We may then
wager 400,000 to 1 that the sun has a proper progressive motion, and
1 to 1 that it is comprised between the limits of thirty-eight and
twenty-nine millions of geographical miles.”--_Etudes d’Astronomie
Stellaire: Sur la Voie Lactée et sur les Distances des Etoiles Fixes_:
M. F. W. G. Struve. [A report addressed to his Excellency M. Le Comte
Ouvaroff; Minister of Public Instruction and President of the Imperial
Academy of Sciences at St. Petersburg.]
[10] “The first great agent which the analysis of natural phenomena
offers to our consideration, more frequently and prominently than
any other, is force. Its effects are either, 1st, to counteract the
exertion of opposing force, and thereby to maintain _equilibrium_; or,
2ndly, to produce _motion_ in matter,
“Matter, or that whatever it be of which all the objects in nature
which manifest themselves directly to our senses consist, presents us
with two general qualities, which at first sight appear to stand in
contradiction to each other--activity and inertness. Its activity is
proved by its power of spontaneously setting other matter in motion,
and of itself obeying their mutual impulse, and moving under the
influence of its own and other force; inertness, in refusing to move
unless obliged to do so by a force impressed externally, or mutually
exerted between itself and other matter, and by persisting in its state
of motion or rest unless disturbed by some external cause. Yet, in
reality, this contradiction is only apparent. Force being the cause,
and motion the effect produced by it on matter, to say that matter is
inert, or has _inertia_, as it is termed, is only to say that the cause
is expended in producing its effect, and that the same cause cannot
(without renewal) produce double or triple its own proper effect.
In this point of view, equilibrium may be conceived as a continual
production of two opposite effects, each, undoing at every instant what
the other has done,”?--See continuation of the argument in _Herschel’s
Discourse on the Study of Natural Philosophy_, page 223.
In the Edinburgh New Philosophical Journal, vol. xlv., will be found a
paper by Dr Robert Brown--“_Of the sources of motions upon the Earth,
and of the means by which they are sustained_,” which will well repay
an attentive perusal, as pointing to a class of investigation of the
highest order, and containing deductions of the most philosophic
description.
[11] Friction, it is well known, generates heat; by rapidly rubbing two
sticks together, the Indian produces their ignition; heat and light
being both manifested. Under every mechanical disturbance electrical
changes can be detected, and the action of heat in the combustion of
the wood is a chemical phenomenon.
[12] Count Rumford’s experiment consisted in placing a mass of metal
in a box of water at a known temperature, and, by employing a boring
apparatus, ascertaining carefully the increase of heat after a given
number of revolutions. He thus describes his most satisfactory
experiment:--
“Everything being ready, I proceeded to make the experiment I had
projected, in the following manner. The hollow cylinder having been
previously cleaned out, and the inside of its bore wiped with a clean
towel till it was quite dry, the square iron bar, with the blunt steel
borer fixed to the end of it, was put into its place; the mouth of the
bore of the cylinder being closed at the same time by means of the
circular piston through the centre of which the iron bar passed.
“This being done, the box was put in its place; and the joinings of
the iron rod, and of the neck of the cylinder with the two ends of
the box, having been made water-tight, by means of collars of oiled
leather, the box was filled with cold water (viz., at the temperature
of 60°) and the machine was put in motion. The result of this beautiful
experiment was very striking, and the pleasure it afforded me amply
repaid me for all the trouble I had had, in contriving and arranging
the complicated machinery used in making it. The cylinder, revolving
at the rate of about thirty-two times in a minute, had been in motion
but a short time, when I perceived, by putting my hand into the water
and touching the outside of the cylinder, that heat was generated, and
it was not long before the water which surrounded the cylinder began
to be sensibly warm. At the end of one hour, I found, by plunging a
thermometer into the water in the box (the quantity of which fluid
amounted to 18·77 lbs. avoirdupois, or 2-1/4 wine gallons), that
its temperature had been raised no less than 47°; being now 107° of
Fahrenheit’s scale. When thirty minutes more had elapsed, or one hour
and thirty minutes after the machinery had been put in motion, the heat
of the water in the box was 142°. At the end of two hours, reckoning
from the beginning of the experiment, the temperature of the water
was found to be raised to 178°. At two hours twenty minutes it was at
200°; and at two hours thirty minutes it _actually boiled_.”--_Inquiry
concerning the Source of the Heat excited by Friction_: Philosophical
Transactions, vol. lxxxviii. A.D. 1798.
“Mr. Joule brought a communication on the same subject before the
British Association at Cambridge, which was afterwards published in the
Philosophical Magazine, and from that journal the following notices are
extracted:--
“The apparatus exhibited before the Association consisted of a brass
paddle-wheel, working horizontally in a can of water. Motion could
be communicated to this paddle by means of weights, pulleys, &c. The
paddle moved with great resistance in the can of water, so that the
weights (each of four pounds) descended at the slow rate of about one
foot per second. The height of the pulleys from the ground was twelve
yards, and consequently when the weights had descended through that
distance they had to be wound up again in order to renew the motion of
the paddle. After this operation had been repeated sixteen times, the
increase of the temperature of the water was ascertained by means of a
very sensible and accurate thermometer.
“A series of nine experiments was performed in the above manner, and
nine experiments were made in order to eliminate the cooling or heating
effects of the atmosphere. After reducing the result to the capacity
for heat of a pound of water, it appeared that for each degree of heat
evolved by the friction of water, a mechanical power equal to that
which can raise a weight of 890 lbs. to the height of one foot, had
been expended.
“Any of your readers who are so fortunate as to reside amid the
romantic scenery of Wales or Scotland could, I doubt not, confirm my
experiments by trying the temperature of the water at the top and at
the bottom of a cascade. If my views be correct, a fall of 817 feet
will of course generate one degree of heat, and the temperature of the
river Niagara will be raised about one fifth of a degree by its fall of
160 feet.”--_Relation between Heat and Mechanical Power_: Philosoph.
Mag. vol. xxvii. 1845.