The Poetry of Science; or, Studies of the Physical Phenomena of Nature

CHAPTER II.

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 p and Herculis, at a quarter of the apparent distance of these stars, reckoning from p 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.

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