MOTION is now generally considered as incapable of definition, being merely a simple ideaor notion received by the senses. The ancients, however, thought differently. Some of them defined it to be a passage out of one state into another, which conveys no idea to him who is ignorant of the nature of motion. The Peripatetic definition has been mentioned elsewhere, and shown to be wholly unintelligible, as well as the celebrated division, by the same sect, of motion into four classes belonging to the three categories, quality, quantity, and where (see Metaphysics). The Cartesians, too, amongst the moderns, pretend to define motion by calling it a passage or removal of one part of matter, out of the neighbourhood of those parts to which it is immediately contiguous, into the neighbourhood of others. Borelli defines motion to be the successive passage of a body from place to place. Others say that it is the application of a body to different parts of infinite and immoveable space; and Mr Young, in his Essay on the Powers of Nature, has given as a definition of motion, change of place.

We have elsewhere offered our opinion of every possible attempt to define motion; but as the author of the last-quoted definition has endeavoured to obviate such objections as ours, candour requires that he be heard for himself. “It is said,” he observes, “by some, that change implies motion, and therefore cannot be a part of its definition, being the very thing defined. To this I answer, We are speaking of the sensible idea of motion, as it appears [7:15:574] to our sight; now changes do appear to our view, and to all our senses, which give us no idea of motion. Changes in heat or cold; in colour, flavour, smell, sound, hardness, softness, pain, pleasure; in these, and many other ideas, changes do not produce ideas like that produced by a ball rolling or a stone falling. We may perhaps ultimately trace them to motion, but to insensible motions; to motions which arise only in reflection, and constitute no part of the actual idea of change. We can therefore conceive of change without conceiving at the same time of motion. Change is a generic idea, including many species; motion, as a sensible idea, is a species of that genus. Change is therefore a necessary part of the definition of motion; it marks the genus of the thing defined. Motion is a change; but as there are many species of change, which of those species is motion? The answer is, it is a change of place. This marks the species, and distinguishes it from change of colour, of temperament, and figure.”

This is the ablest defence of an attempt to define motion that we have ever seen; and at first view the definition itself appears to be perfect. Aristotle, the prince of definers, considers a definition^[1. See Dr Reid’s account of Aristotle’s logic, in Lord Kames’ Sketches of Man. ] as “a speech declaring what a thing is.” Every thing essential to the thing defined, and nothing more, must be contained in the definition. Now the essence of a thing consists of these two parts: first, what is common to it with other things of the same kind; and secondly, what distinguishes it from other things of the same kind. The first is called the genus of the thing; the second, its specific difference. The definition, therefore, consists of these two parts.

In obedience to this rule, the definition under consideration seems to consist of the genus, signified by the word change, and of the specific difference, denoted by the words of place. But does the speech change of place really declare what motion is? We cannot admit that it does? as, in our apprehension, a change of place is the effect of motion, and not motion itself Suppose a lover of dialectic undertaking to define the stroke by which he saw his neighbour wounded with a bludgeon; what should we think of his art were he to call it a contusion on the head? He might say that contusion is a general term, as contusions may be produced on the arms, on the legs, and on various parts of the body; and as there are many species of contusion, if he were asked which of those species was the stroke to be defined, he might answer, “a contusion on the head.” Here would be apparently the genus and specific difference; the former denoted by contusion, and the latter by the words on the head. But would this be a definition of a stroke? No, surely. A contusion on the head may be the effect of a stroke, but it can no more be the stroke itself, than a blow can be a bludgeon, or a flesh wound the point of a sword. Equally evident it is, that a change of place cannot be motion; because every body must have been actually moved before we can discern, or even conceive, a change of its place.

The act of changing the place would perhaps come nearer to a definition of motion; but so far would it be from “a speech declaring what motion is,” that we are confident a man who had never by any of his senses perceived a body in actual motion, would acquire no ideas whatever from the words “act of changing place.” He might have experienced changes in heat, cold, smell, and sound; but he could not possibly combine the ideas of such changes with the signification of the word place, were he even capable of understanding that word, which to us appears to be more than doubtful. See Metaphysics.

The distinctions of motion into different kinds have been no less various, and no less insignificant, than the several definitions of it. The moderns who reject the Peripatetic division of motion into four classes, yet consider it themselves as either absolute or relative. Thus we are told, that absolute motion is the change of absolute place, and that its celerity must be measured by the quantity of absolute space which the moving body passes through in a given time. Relative motion, on the other hand, is a mutation of the relative or vulgar place of the moving body, and has its celerity estimated by the quantity of relative space passed through.

Now it is obvious, that this distinction conveys no ideas without a further explanation of the terms by which it is expressed; but that explanation is impossible to be given. Thus, before we can understand what absolute motion is, we must understand what is meant by absolute place. But absolute place is a contradiction; for all place is relative, and consists in the positions of different bodies with regard to one another. Were a globe in the regions of empty space to be put in motion by Almighty Power, and all the rest of the corporeal world to be soon afterwards annihilated, the motion would undoubtedly continue unchanged; and yet, according to this distinction, it would be at first relative, and afterwards absolute. That the beginning of such a motion would be perceptible, and the remainder of it imperceptible, is readily granted; but on this account to consider it as of two kinds, is as absurd as to suppose the motion of the minute hand of a clock to be affected by our looking at it.

Leaving these unintelligible distinctions, therefore, we now come to consider a question of a very abstruse nature, but much agitated amongst philosophers, namely, What is the original source of motion in the creation? Is it natural to matter, or are we to ascribe it to the immediate and continual agency of some immaterial being? The former has been strenuously argued by the Cartesians, and the latter by the Newtonians. The arguments of the former, founded upon the chimerical hypothesis of vortices and the original construction of matter, were evidently inconclusive; and the hypothesis of Sir Isaac Newton, who asserted that it was naturally incapable of motion, appeared more probable. To account for the quantity of motion in the universe, therefore, it became necessary to have recourse either to the Deity, or to some subordinate spiritual agent; and this became the more necessary, as the doctrine of an absolute vacuum in the celestial spaces, that is, throughout the incomparably greatest part of the creation, was one of the fundamental maxims of the system. As it was absolutely denied that matter existed in these spaces, and as it was plain that the celestial bodies affected one another at immense distances, the powers of attraction and repulsion were naturally called in as the sources of motion by their impulse upon inert and sluggish matter. And these being admitted, a speculation ensued concerning their nature. Spiritual, it was confessed, they were; but whether they were to be accounted the immediate action of the Divine Spirit himself, or that of some subordinate and inferior spirit, was a matter of no little dispute. Sir Isaac Newton, towards the latter part of his life, began to relax somewhat of the rigidity of his former doctrine, and allowed that a very subtile medium, which he called ether, might be the cause of attraction and repulsion, and thus of all the phenomena of nature. Since his time the multitude of discoveries in electricity, the similarity of that fluid to fire and light, with the vast influence it exerts on every part of the creation with which we are acquainted, have rendered it extremely probable that the ether mentioned by Sir Isaac is no other than the element of fire, “the most subtile^[2. * Siru, No. 153, &c. ] and elastic of all bodies, which seems to pervade and expand itself throughout the whole universe[7:15:575]. Electrical experiments show that this mighty agent is everywhere present, ready to break forth into action, if not restrained and governed with the greatest wisdom. Being always restless and in motion, it actuates and enlivens the whole visible mass; is equally fitted to produce and to destroy; distinguishes the various stages of nature, and keeps up the perpetual round of generations and corruptions, pregnant with forms which it constantly sends forth and re-absorbs. So quick in its motions, so subtile and penetrating in its nature, so extensive in its effects, it seemeth no other than the vegetative soul or vital spirit of the world.

“The animal spirit in man is the instrument both of sense and motion. To suppose sense in the corporeal world would be gross and unwarranted; but locomotive faculties are evident in all its parts. The Pythagoreans, Platonists, and Stoics, held the world to be an animal; though some of them have chosen to consider it as a vegetable. However, the phenomena do plainly show, that there is a spirit that moves, and a mind or providence that presides. This providence, Plutarch saith, was thought to be in regard to the world what the soul is in regard to man. The order and course of things, and the experiments we daily make, show that there is a mind which governs and actuates this mundane system as the proper and real agent and cause; and that the inferior instrumental cause is pure ether, fire, or the substance of light, which is applied and determined by an infinite mind in the macrocosm or universe, with unlimited power, and according to stated rules, as it is in the microcosm with limited power and skill by the human mind. We have no proof, either from experiment or reason, of any other agent or efficient cause than the mind or spirit. When, therefore, we speak of corporeal agents or corporeal causes, this is to be understood in a different, subordinate, and improper sense; and such an agent we know light or elementary fire to be.”

That this elementary fire, absorbed and fixed in all bodies, may be the cause of the universal principle of gravity, is made sufficiently evident by numberless experiments. Homberg having calcined in the focus of a burning glass some regulus of antimony, found that it had gained one tenth in weight, though the regulus, during the whole time of the operation, sent up a thick smoke, and thereby lost a considerable part of its own substance. It is vain to allege that any heterogeneous matter floating in the air, or that the air itself, may have been hurried into the mass by the action of the fire, and that by this additional matter the weight was increased; for it is known experimentally, that if a quantity of metal be even hermetically secured within a vessel of glass to keep off the air and all foreign matter, and the vessel be placed for some time in a strong fire, it will exhibit the same effect. “I have seen the operation performed,” says Mr Jones,^[3. Essay on the First Principles of Natural Philosophy. ] “on two ounces of pewter filings, hermetically scaled up in a Florence flask, which in two hours gained fifty-five grains, that is, nearly a seventeenth. Had it remained longer in the fire, it might probably have gained something more; as, in one of Mr Boyle’s experiments, steel filings were found to have gained a fourth.

“ Of accounting for these effects there are but two possible ways. 1. If the quantity of matter be the same, or, in the case of calcination, be somewhat less, after being exposed to the action of the fire, while the gravity of the whole is become greater; then does it follow that gravity is not according to the quantity of matter, and of course is not one of its properties. 2. If there be an increase of the mass, it can be imputed to nothing but the matter of light or fire entangled in its passage through the substance, and so fixed in its pores, or combined with its solid parts, as to gravitate together with it. Yet it is certain, from the phenomenon of light darting from the sun, that this elementary fire does not gravitate till it is fixed in metal, or some other solid substance. Here then we have a fluid which gravitates, if it gravitate at all, in some cases and not in others; so that which way soever the experiment be interpreted, we are forced to conclude that elementary or solar fire may be the cause of the law of gravitation.”

That it is likewise in many cases the cause of repulsion, is known to every one who has seen it fuse metals, and convert water and mercury into elastic vapour. But there is a fact recorded by Mr Jones, which seems to evince that the same fluid, which, as it issues from the sun, exhibits itself in the form of light and heat, is in other circumstances converted into a very fine air, or cold ether, which rushes very forcibly towards the body of that luminary. “As a sequel to what has been observed,” says he, “concerning the impregnation of solid substances with the particles of fire, give me leave to subjoin an experiment of Μ. de Stair. He tells us, that upon heating red lead in a glass whence the air was exhausted, by the rays of the sun collected in a burning glass, the vessel in which the said red lead was contained burst in pieces with a great noise. Now, as all explosions in general must be ascribed either to an admission of the air into a rarefied space, or to what is called the generation of it, and as air was not admitted upon this occasion, it must have been generated from the calx within the vessel; and certainly was so, because Dr Hales has made it appear that this substance, like crude tartar, and many others, will yield a considerable quantity of air in distillation. What went into the metal therefore as fire, came out of it again as air; which in a manner forces upon us conclusions of inestimable value in natural philosophy, and such as may carry us very far into the most sublime part of it.”

One of the conclusions which the ingenious author thinks thus forced upon us is, that the motions of the planets round the sun, as well as round their own axes, are to be attributed to the continual agency of this fluid, under its two forms of elementary fire and pure air. As fire and light, we know that it rushes with inconceivable rapidity from the body of the sun, and penetrates every corporeal substance, exerting itself sometimes with such force as nothing with which we are acquainted is able to resist. If it be indeed a fact, that this elementary fire, or principle of light and heat, afterwards cools, and becomes pure air, there cannot be a doubt but that under such a form it will return with great force, though surely in a somewhat different direction, towards the sun, forming a vortex, in which the planets are included, and by which they must of course be carried round the centre. Mr Jones does not suppose that the air into which the principle of light and heat is converted, is of so gross a nature as our atmosphere. He rather considers it as cool ether, just as he represents light to be ether heated; but he maintains that this ether, in its aërial form, though not fit for human respiration, is a better pabulum of fire than the air which we breathe.

This theory seems exceedingly plausible; and the author supports it by many experiments. He has not, indeed, convinced us that the solar light is converted or convertible into pure air; but he has, by just reasoning from undoubted facts, proved that the whole expanse of heaven, as far as comets wander, is filled not only with light, which is indeed obvious to the senses, but also with a fluid, which, whatever it may be called, supplies the place of the air in feeding the fire of these ignited bodies.

That the motion of the heavenly bodies should result from the perpetual agency of such a medium, appears to us [7:15:576] a much more rational hypothesis than that which makes them act upon each other at immense distances through empty space. But the hypothesis is by no means so complete a solution of the phenomena as some of its fond admirers pretend to think it. This fluid, whether called ether, heat, light, or air, is still material; and the question returns upon him who imagines that it is sufficient to account for gravitation, repulsion, magnetism, cohesion, and the like, What moves the fluid itself, and makes the parts of which it is composed cohere together? However widely it may be extended, it is incapable of positive infinity, and therefore may be divided into parts separated from each other; so that it must be held together by a foreign force, as well as a ball of lead or a piece of wax. As matter is not essentially active, the motion of this ether, under both its forms, must likewise be considered as an effect, for which we do not think that any propelling power in the body of the sun can be admitted as a sufficient cause. For how comes the sun to possess that power, and what makes the fluid return to the sun? We have no notion of power in the proper sense of the word, but as intelligence and volition; and, like the pious and excellent author of the Essay on the First Principles of Natural Philosophy, we are certain that the sun was never supposed to be intelligent.

Bishop Berkeley, who admits of light or ether as the instrumental cause of all corporeal motion, gets rid of this difficulty by supposing, with the ancients, that this powerful agent is animated. “According to the Pythagoreans and Platonists,” says his lordship,^[4. Siris i No. 277. ] “there is a life infused throughout all things; the πυg νοεgον, πυg τεχνι ϰ ό o ν, an intellectual and artificial fire, an inward principle, animal spirit, or natural life, producing and 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 juices 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.”

This life or animal spirit seems to be the same thing which Cudworth calls plastic nature, and which has been considered elsewhere. (See Metaphysics.) We shall therefore dismiss it at present, with just admitting the truth of the bishop’s position, “that if nature be supposed the life of the world, animated by one soul, compacted into one frame, and directed or governed in all its parts by one supreme and distinct intelligence, this system cannot be accused of atheism, though perhaps it may of mistake or impropriety.”

A theory of motion somewhat similar to that of Berkeley, though in several respects different from it, was not very long ago stated with great clearness, and supported with much ingenuity, in an Essay on the Powers and Mechanism of Nature, intended to improve, and more firmly establish, the grand superstructure of the Newtonian system. Mr Young, the author of the essay, admits, with most other philosophers of the present age, that body is composed of atoms which are impenetrable to each other, and may be denominated solid. These atoms, however, he does not consider as primary and simple elements, incapable of resolution into principles; but thinks that they are formed by certain motions of the parts of a substance immaterial and essentially active.

As this notion is uncommon, and the offspring of a vigorous mind, we shall consider it more attentively hereafter. It is mentioned at present as a necessary introduction to the author’s theory of motion, of which he attributes both the origin and the continuance to the agency of this elementary substance pervading the most solid atoms of the densest bodies. Of every body and every atom he holds the constituent principles to be essentially active; but those principles act in such a manner as to counterbalance each other, so that the atom or body considered as a whole is inert, unless in so far as it resists the compression or separation of its parts. No body or atom can of itself begin to move, or continue in motion for a single instant; but, being pervious to the active substance, and coalescing with it, that substance, when it enters any body, carries it along with it, till, meeting some other body in the way, either the whole of the active substance lodged in the former body passes into the obstacle, in which case the impelling body instantly ceases to move; or else part of that substance passes into the obstacle, and part remains in the impelling body; and in this case both bodies are moved with a velocity in proportion to the quantity of matter which each contains, combined with the quantity of active substance by which they are respectively penetrated.

In order to pave the way for his proof of the existence of one uniform active substance, he observes, that “change being an essentially constituent part of motion, and change implying action, it follows that all motion implies action, and depends on an active cause. Every motion,” he continues, “has a beginning, a middle, and an end. The beginning is a change from rest to motion; the middle is a continuance in motion; the end is a change from motion to rest.” He then proceeds to show, that the beginning of motion is by an action begun; the continuance of motion by an action continued; and the end of motion by a cessation of action.

“The first of these positions is admitted by every body. That the continuance of motion is by an action continued, will be proved, if it shall be shown that the continuance of a motion is nothing different from its beginning, in regard to any point of time assumed in the continued motion. Now the beginning of motion,” he says, “consists in the beginning of change of place. But if any given portions of time and of space are assumed, a body beginning to move in the commencement of that time, and in the first portion of the space assumed, then and there begins that particular motion: And whether before the body began to move in that space it was moving in other spaces and times, has no relation to the motion in question; for this being in a space and time altogether distinct, is a distinct motion from any which might have preceded it immediately, as much as from a motion which preceded it a thousand years before. It is therefore a new motion begun; and so it may be said of every assumable point in the continued motion. The term continued serves only to connect any two distinct motions, the end of one with the beginning of the other, but does not destroy their distinctness.”

He then proceeds to combat, which he does very successfully, the arguments by which the more rigid Newtonians endeavour to prove that a body in motion will continue to be moved by its own inertia, till stopped by some opposite force. Having done this, he establishes the contrary conclusion by the following syllogisms:

“I. Whatever requires an active force to stop its motion, is disposed to move;

But every body in motion requires an active force to stop its motion; [7:15:577] Therefore every body in motion is disposed to move.

“II. Whatever is disposed to motion is possessed of action;

But a body in motion is disposed to continue in motion; Therefore a body in motion is possessed of action.

Thus it appears that the middle part of any motion is action equally with the beginning.

“The last part of motion is its termination. It is admitted that all motion is terminated by an action contrary to the direction of the motion. It is admitted, too, that the moving body acts at the time its motion is destroyed. Thus the beginning and the end of any uniform motion are confessed to be actions; but all the intermediate continuation which connects the beginning with the end is denied to be action. What can be more unaccountable than this denial? Is it not more consonant to reason and analogy, to ascribe to the whole continued motion one uninterrupted action? Such a conclusion true philosophy, we think, requires us to make.

“To move or act, is an attribute which cannot be conceived to exist without a substance. The action of a body in motion is indeed the attribute of the body, and the body relatively to its own motion is truly a substance, having the attribute or quality of motion. But the body being a name signifying a combination of certain ideas, which ideas are found to arise from action, that action which is productive of those ideas the combination of which we denominate body, is of the nature of an attribute as long as it is considered as constituted of action. To this attribute we must necessarily assign its substance. The actions which constitute body must be actions of something, or there must be something which acts. Whence, then, is this active something, from whose agency we get the idea of body, or whose actions constitute body? Is it not sufficient that it is something active? A name might be surely given it, but a name would not render the idea more clear. Its description may be found in every sensation; it is colour to the eye, flavour to the palate, odour to the nose, sound to the ear, and feeling to the touch; for all our sensations are but so many ways in which this active something is manifested to us. A substratum of solidity philosophers have imagined to exist, and have in vain sought to find. Our active substance is the substratum so long sought for, and with so little success. We give it a quality by which it may be perceived; it acts. One modification of action produces matter, another generates motion. These modifications of action are modes of the active substance, whose presence is action. Matter and motion constitute the whole of nature. There is therefore throughout nature an active substance, the constituent essence of matter, and immediate natural agent in all effects.”

By an argument which we do not think very conclusive, our author determines this active substance to be unintelligent. “In our sensations individually, not discovering,” says he, “the traces, not seeing the characters of intelligence, but finding only action present and necessary, our inferences go no farther than our observations warrant us to do; and we conclude in all these things an action only, and that action unintelligent.” Having given our opinion of real agency elsewhere (see Metaphysics), we shall not here stop to examine this reasoning. We may, however, ask, whether all our sensations individually be not excited for a certain end. If they be, according to our author’s mode of arguing in another place, then the exciting agent should be an intelligent being. By this we are far from meaning to deny the reality of a secondary or instrumental cause of sensation which is destitute of intelligence. We are strongly inclined to think that there is such a cause, though our persuasion results not from this argument of our author’s. In our opinion, he reasons better when he says, “that a subordinate agent constructed as the matter of creation, invested with perpetual laws, and producing agreeably to those laws all the forms of being, through the varieties of which inferior intelligences can, by progressive steps, arrive ultimately at the Supreme Contriver, is more agreeable to our ideas of dignity, and tends to impress us with more exalted sentiments, than viewing the Deity directly in all the individual impressions we receive, divided in the infinity of particular events, and unawful, by his continual presence in operations to our view insignificant and mean.”

This active substance, or secondary cause, our author concludes to be neither matter nor mind. “Matter,” says he, “is a being, as a whole quiescent and inactive, but constituted of active parts, which resist separation, or cohere, giving what is usually denominated solidity to the mass. Mind is a substance which thinks. A being which should answer to neither of these definitions, would be neither matter nor mind, but an immaterial, and, if I may so say, an immental substance.” Such is the active substance of Mr Young, which, considered as the cause of motion, seems not to differ greatly from the plastic nature, hylarchical principle, or vis genitrix, of others. The manner in which it operates is indeed much more minutely detailed by our author than by any other philosopher, ancient or modern, with whose writings we have any acquaintance.

“Every thing,” he says, “must be in its own nature either disposed to rest or motion; consequently the active substance must be considered as a being naturally either quiescent or motive. But it cannot be naturally quiescent, for then it could not be active, because activity, which is a tendency to motion, cannot originate in a tendency to rest. Therefore the active substance is by nature motive, that is, tending to motion. The active substance is not solid, and does not resist penetration. It is, therefore, incapable of impelling or of sustaining impulse. Whence it follows, that as it tends to move, and is incapable of having its motion impeded by impulse, it must actually and continually move; in other words, motion is essential to the active substance.

“In order that this substance may act, some other thing upon which it may produce a change is necessary; for whatever suffers an action, receives some change. The active substance, in acting on some other thing, must impart and unite itself thereto; for its action is communicating its activity. But it cannot communicate its activity without imparting its substance; because it is the substance alone which possesses activity, and the quality cannot be separated from the substance. Therefore the active substance acts by uniting itself with the substance on which it acts. The union of this substance with bodies is not to be conceived of as a junction of small parts intimately blended together and attached at their surfaces; but as an entire diffusion and incorporation of one substance with another in perfect coalescence. As bodies are not naturally active, whenever they become so, as they always do in motion, it must be by the accession of some part of the active substance. The active substance being imparted to a body, penetrates the most solid or resisting parts, and does not reside in the pores without, and at the surfaces of the solid parts. For the activity is imparted to the body itself, and not to its pores, which are no parts of the body; therefore, if the active substance remained within the pores, the cause would not be present with its effect, but the cause would be in one place and the effect in another, which is impossible.

“Bodies, by their impulse on others, lose their activity in proportion to the impulse. This is matter of observation. Bodies which suffer impulse acquire activity in proportion to the impulse. This also is matter of observation. In impulse, therefore, the active substance passes out of the impelling body into the body impelled. For since bo [7:15:578] dies in motion are active, and activity consists in the presence of the active substance, and by impulse bodies lose their activity, therefore they lose their active substance, and the loss is proportional to the impulse. Bodies impelled acquire activity; therefore acquire active substance, and the acquisition is proportioned to the impulse. But the active substance lost by the impelling body ought to be concluded to be that found in the other; because there is no other receptacle than the impelled body to which the substance parted from can be traced, nor any other source than the active body whence that which is found can be derived. Therefore, in impulse, the active substance ought to be concluded to pass from the impelling body to the body impelled. The flowing of such a substance is a sufficient cause of the communication of activity, and no other rational cause can be assigned.

“The continued motion of a body depends not upon its inertia, but upon the continuance of the active substance within the body. The motion of a body is produced by the motion of the active substance in union with the body. It being evident, that since the active substance itself does always move, whatever it is united to will be moved along with it, if no obstacle prevent. In mere motion, the body moved is the patient, and the active substance the agent. In impulse, the body in motion may be considered as an agent, as it is made active by its active substance. While the active substance is flowing out of the active body into the obstacle or impelled body, the active body will press or impel the obstacle. For while the active substance is yet within the body, although flowing through it, it does not cease to impart to the body its own nature, nor can the body cease to be active because not yet deprived of the active substance. Therefore, during its passing out of the body, such portion of the active substance as is yet within, is urging and disposing the body to move, in like manner as if the active substance were continuing in the body; and the body being thus urged to move, but impeded from moving, presses or impels the obstacle.

“We see here,” says our author, “an obvious explanation of impulse; it consists in the flowing of the motive substance from a source into a receptacle ;” and he thinks, that although the existence of such a substance had not been established on any previous grounds, the communication of motion by impulse does alone afford a sufficient proof of its reality.

He employs the agency of the same substance to account for many other apparent activities in bodies, such as those of fire, electricity, attraction, repulsion, elasticity, &c. All the apparent origins of corporeal activity serve, he says, to impart the active substance to bodies; “and where activity is without any manifest origin, the active substance is derived from an invisible source.”

Our limits will not permit us to attend him in his solution of all the apparent activities in bodies; but the orbicular motions of the planets have been accounted for in so many different ways by philosophers ancient and modern, and each account has been so little satisfactory to him who can think, and wishes to trace effects from adequate causes, that we consider it as our duty to furnish our readers with the account of this phenomenon which is given by Mr Young.

The question which has been so long agitated, “Whence is the origin of motion ?” our author considers as implying an absurdity. “It supposes,” says he, “that rest was the primitive state of matter, and that motion was produced by a subsequent act. But this supposition must ever be rejected, as it is giving precedency to the inferior, and inverting the order of nature.” The substance which he holds to be the basis of matter is essentially active, and its action is motion. This motion, however, in the ori-. ginal element, was power without direction, agency without order, activity to no end. To this power it was necessary that a law should be superadded; that its agency should be guided to some regular purpose, and its motion conspire to the production of some uniform effects. Our author shows, or endeavours to show, by a process of reasoning which shall be examined elsewhere, that the primary atoms of matter are produced by the circular motion of the parts of this substance round a centre; and that a similar motion of a number of these atoms around another centre common to them all, produces what in common language is called a solid body ; a cannon ball, for instance, the terrestrial globe, and the body of the sun, &c. In a word, he labours to prove, and with no small success, that a principle of union is implied in the revolving or circulating movements of the active substance.

“But we may also assume,” he says, “a priori, that a principle of union is a general law of nature; because we see in fact all the component parts of the universe are united systems, which successively combine into larger unions, and ultimately form one whole.” Let us then suppose the sun, with all his planets, primary and secondary, to be already formed for the purpose of making one system, and the orbits of all of them, as well as these great bodies themselves, to be pervaded by the active substance, which necessarily exists in a state of motion, and is the cause of the motion of every thing corporeal. “ If to this motion a principle of union be added, the effect of such a principle would be a determination of all the parts of the active substance, and of course all the bodies to which it is united, towards a common centre, which would be at rest, and void of any tendency in any direction. But this determination of all the parts of the system towards a common centre tends to the destruction both of the motion of the active substance and of the system; for should all the parts continually approximate from a circumference towards a centre, the sun and planets would at last meet, and form one solid and quiescent mass. But to preserve existence, and consequently motion, is the first law of the active substance, as of all being; and it cannot be doubted, that to preserve distinct the several parts of the solar system, is the first law given to the substance actuating that system. The union of the system is a subsequent law.

“When the direct tendency of any Inferior law is obviated by a higher law, the inferior law will operate indirectly in the manner the nearest to its direct tendency that the superior law will permit. If a body in motion be obliquely obstructed, it will move on in a direction oblique to its first motion. Now the law of union, which pervades the solar system, being continually obstructed by the law of self-preservation, the motion of the active substance, and of the bodies to which it is united, can be no other than a revolving motion about the common centre of approach, towards which all the parts have a determination. But when this revolution has actually taken place, it gives birth to a new tendency, which supersedes the operation of the law of self-preservation. It has been shown, that the motion essential to the active substance required to be governed by some law, to give being to an orderly state of things. Now, there are motions simple and motions complex; the more simple is in all things first in order, and out of the more simple the more complex arises in order posterior. The most simple motion is rectilineal; therefore a rectilineal motion is to· be considered as that which is the original and natural state of things, and consequently that to which all things tend. It will follow from hence, that when any portion of active substance in which the law of union operates, has in the manner above explained been compelled to assume a revolving motion, that is, a motion in some curve, a tendency to a rectili
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neal motion will continually exist in every part of the revolving portion, and in every point of the curve which it describes during its revolution. And this rectilineal tendency will be a tendency to recede from the centre in every point of the revolving orbit, and to proceed in a tangent to the orbit at each point. These two tendencies, if not originally equal, must necessarily in all cases arrive at an equality. For the tendency towards the centre, called the centripetal tendency, that is, the law of union, operating first, if we suppose the motion approaches the centre, the tendency to recede from it, called the centrifugal tendency, will have its proportion to the centripetal continually increased as thecrbit of revolution grows less, so as ultimately to equal the centripetal tendency, and restrain the motion from its central course, at which point it will no longer seek the centre, but revolve round it.”

As our author holds that every atom of matter is formed by the motion of parts of the active substance, and every body formed by the motion of atoms; so he maintains, not only that the sun, moon, earth, planets, and stars, are penetrated by the same substance, but that each is the centre of a vortex of that substance, and that of these vortices some are included within others. “The subtile revolving fluid, the centre of whose vortex the earth occupies, not only surrounds, but pervades the earth, and other vortices their earths, to their centres; and the earth and planets are by its revolutions carried around on their own axes. The earth is an active mass, and all its component masses are severally as well as collectively inactive; but the earth and all its parts have various collective and separate movements, imparted from the fluid which surrounds, pervades, and constitutes it. Being immersed together with its proper surrounding sphere or vortex in the larger sphere or vortex of the sun, it is carried thereby in a larger orbit about the sun, at the same time that by the revolution of its proper sphere it rotates on its own axis.”

Such is the most complete view which our limits will permit us to give of Mr Young’s theory of motion. To the philosopher who considers experiment as the only test of truth, and who in all his inquiries employs his hands more than his head, we are fully aware that it will appear in no better light than as “the baseless fabric of a vision.” Even to the intellectual philosopher who is not frightened at the word metaphysics, we are afraid that such an active substance as∙the author contends for, will appear as inadequate to the production of the phenomena of gravitation and repulsion as the material ether of Mr Jones and his followers. A being void of intelligence, whether it be material or immaterial, quiescent or motive, cannot be the subject of law, in the proper sense of the word. The laws of which Mr Young speaks as necessary to regulate the motions of the active substance, must be mere forces, applied by some extrinsic and superior power. And since motion, as it is essential to the active substance, is power without direction, agency without order, activity to no end; since it is of such a nature, that from its unguided agitations there could result neither connection, order, nor harmony; it follows that those extrinsic forces must be perpetually applied, because what is essential to any substance can never be destroyed or changed as long as the substance itself remains.

Forces producing order out of confusion can be applied only by a being possessed of intelligence; and if the immediate and perpetual agency of an intelligent being be necessary to regulate the motions of the active substance, that substance itself may be thought superfluous, and its very existence may be denied. Entia non sunt multiplicanda absque necessitate, is a rule of philosophizing which every man of science acknowledges to be just. And it will hardly be denied that the immediate and perpetual agency of an intelligent being upon Mr Jones’s ethereal fluid, or even upon the matter of solid bodies themselves, would be capable of producing every kind of motion, without the instrumentality of a substance which is neither mind nor matter.

Such, we conceive, are the objections which our metaphysical readers may make to this theory. Part of their force, however, will perhaps be removed by the ingenious manner in which our author analyzes matter into an immaterial principle. But so much of it remains that the writer of this article is inclined to believe that no mechanical account can be given of the motions of the heavenly bodies, the growth of plants, and various other phenomena which arc usually solved by attraction and repulsion. In the present age, philosophers in general are strangely averse to admitting on any occasion the agency of mind; yet as every effect must have a cause, it is surely not irrational to attribute such effects as mechanism cannot produce to the operation either of.intelligence or instinct. To suppose the Deity the immediate agent in the great motions of the universe, has been deemed impious; and it must be confessed that very impious conclusions have been deduced from that principle. But there is surely no impiety in supposing, with the excellent Bishop of Cloyne, that the fluid which is known to pervade the solar system, and to operate with resistless force, may be animated by a powerful mind, which acts instinctively for ends of which itself knows nothing. For the existence of such a mind, no other evidence, indeed, can be brought than what is afforded by a very ancient and very general tradition, and by the impossibility of accounting for the phenomena upon principles of mere mechanism. Perhaps some of our more pious readers may be inclined to think that the Supreme Being has committed the immediate government of the various planetary systems to powerful intelligences, or angels, who, as his ministers, direct their motions with wisdom and foresight. Such an opinion i§ certainly not absurd in itself; and it seems to be countenanced by an ancient writer, who, though not known by the name of a philosopher, knew as much of the matter as any founder of the most celebrated school.

To object to either of these hypotheses, as has sometimes been done, that it represents the government of the world as a perpetual miracle, betrays the grossest ignorance; for we might as well call the movements of the bodies of men and brutes, which are certainly produced by minds, miraculous. We do not affirm that either hypothesis is certainly true; but they are both as probable and as satisfactory as the hypothesis which attributes agency to attraction and repulsion, to a subtile ether, or to a substance which is neither mind nor matter. Were the immediate agency of intellect to be admitted, there would be no room for many of those disputes which have been agitated amongst philosophers, about the increase or diminution of motion in the universe; because an intelligent agent, which could begin motion as well as carry it on, might increase or diminish it as he should judge proper. If instinctive agency, or something similar to it, be adopted, there is the same room for investigation as upon the principles of mechanism; because instinct works blindly, according to steady laws imposed by a superior mind, which may be discovered by observation of their effects. As we consider this as by far the more probable hypothesis of the two, we find ourselves involved in the following question: If a certain quantity of motion was originally communicated to the matter of the universe, how comes it to pass that the original quantity, still remains? Considering the many opposite and contradictory motionswhich since the creation have taken place in the universe, and which have undoubtedly destroyed a great part of the original quantity, by what means has that quantity been restored?
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If this question can be resolved by natural means, it must be upon the principles of Newton; for, in every case where quantities and relations of quantities are required, it is the province of mathematics to supply the information sought; and all philosophers agree that Sir Isaac’s doctrine of the composition and resolution of motion, though in what respects the heavenly bodies it may have no physical reality, is so mathematically just, as to be the only principle from which the quantity of motion, or the force of powers, can in any case be computed. If we choose to answer the question, by saying that the motion left is restored by the interposition of the Deity, then we might as well have had recourse to him at first, and say that he alone is the true principle of motion throughout the universe.

Before we are reduced to this dilemma, however, it is necessary, in the first place, to inquire whether there is or can be any real diminution of the quantity of motion throughout the universe? In this question the Cartesians take the negative side, and maintain that the Creator at the beginning impressed a certain quantity of motion on bodies, and that under such laws that no part of it could be lost, but the same portion of motion would be constantly preserved in matter; and hence they conclude, that if any moving body should strike on any other body, the former would lose no more of its motion than it communicated to the latter. Sir Isaac Newton takes the contrary side, and argues in the following manner: From the various compositions of two motions, it is manifest there is not always the same quantity of motion in the world; for if two balls, joined together by a slender wire, revolve with a uniform motion about their common centre of gravity, and at the same time that centre be carried uniformly in a right line drawn in the plane of their circular motion, the sum of the motions of the two balls, as often as they are in a right line, drawn from their common centre of gravity, will be greater than the sum of their motions when they are in a line perpendicular to that other. Hence it appears, that motion may be both generated and lost. But by reason of the tenacity of fluid bodies, and the friction of their parts, with the weakness of the elastic power in solid bodies, nature seems to incline much rather to the destruction than the production of motion; and, in reality, motion becomes continually less and less. For bodies which are either so perfectly hard or so soft as to have no elastic power, will not rebound from each other; and their impenetrability will only stop their motion. And if two such bodies equal to one another be carried with equal but opposite motions, so as to meet in a void space, by the laws of motion they must stop in the very place of concourse, lose all their motion, and be at rest for ever, unless they have an elastic power to give them a new motion. If they have elasticity enough to make them rebound with one fourth, one half, or three fourths, of the force they meet with, they will lose three fourths, one half, or one fourth, of their motion. And this is confirmed by experiments; for if two equal pendulums be let fall from equal heights, so as to strike full upon each other; if those pendulums be of lead or soft clay, they will lose all, or almost all, their motion; and if they be of any elastic matter, they will only retain so much motion as they receive from their elastic power.

Motion, therefore, being thus, in the opinion of this celebrated author, lost or absolutely destroyed, it is necessary to find some cause by which it may be renewed. Such renovation Sir Isaac attributes to active principles; for instance, the cause of gravity, whereby the planets and comets preserve their motions in their orbits, and all bodies acquire a great degree of motion in falling; and the cause of fermentation, whereby the heart and blood of animals preserve a perpetual warmth and motion, the inner parts of the earth are kept perpetually warmed, many bodies burn and shine, and the sun himself burnsand shines, and with his light warms and cheers all things.

Elasticity is another cause of the renovation of motion mentioned by Sir Isaac. “We find but little motion in the world,” says he, “except what plainly flows either from these active principles, or from the command of the wilier.”

With regard to the destruction or positive lossof motion, however, we must observe, that, notwithstanding the high authority of Sir Isaac Newton, it is altogether impossible that any such thing can happen. All moving bodies which come under the cognizance of our senses are merely passive, and acted upon by something which we call powers or fluids, and which are to us totally invisible. Motion, therefore, cannot be lost without a destruction or diminution of one of these powers, which we have no reason to think can ever happen. When two pendulums rush against each other, the motion is the mere effect of the action of gravity; and that action, which in this case is the power, continues to be the very same whether the pendulum moves or not. Could motion, therefore, be exhausted in this case, we must suppose, that by separating two pendulums to the same distance from each other, and then letting them come together, for a great number of times, they would at last meet with less force than before. But there is certainly not the least foundation for this supposition; and no rational person will take it into his head, that supposing the whole human race had employed themselves in nothing else from the creation to the present day but in separating pendulums and letting them stop each other’s motion, they would now come together with less force than they did at first. Power, therefore, which is the cause of motion, is absolutely indestructible. Powers may indeed counteract one another, or they may be made to counteract themselves; but the moment that the obstacle is removed, they show themselves in their pristine vigour, without the least symptom of abatement or decay.

Whether, therefore, we reckon the ultimate source of motion to be spiritual or material, it is plain that it must be to our conceptions infinite; neither will the phenomena of nature allow us to give any other explanation than we have done; for no power whatever can lose more than its own quantity, and it seems absurd to think that the Deity would create the world in such a manner that it should ultimately become immoveable, and then have recourse to unknown principles to remedy the supposed defect. On the principle we have just now laid down, however, the matter becomes exceedingly plain and obvious. The Creator at first formed two opposite powers, the action of which is varied according to the circumstances of the bodies upon which they act; and these circumstances are again varied by the action of the powers themselves in innumerable ways upon one another, and the approach of one body to another, or their receding to a greater distance. Where these powers happen to oppose each other directly, the body upon which they act is at rest; when they act obliquely, it' moves in the diagonal; or if the force acting upon one side is by any means lessened, the body certainly must move towards that side, as is evident from the case of the atmosphere, the pressure of which, when removed from one side of a body, will make it move very violently towards that side, and if we could continually keep off the pressure in this manner, the motion would assuredly be perpetual. We must not imagine that motion is destroyed because it is counteracted; for it is impossible to destroy motion by any means but removing the cause; counteracting the effect is only a temporary obstacle, and must cease whenever the obstacle is removed. Nature, therefore, having in itself an infinite quantity of motion, produces greater or lesser motions, according to the various action of the moving powers upon different bodies[7:15:581] or upon one another, without a possibility of the general stock being either augmented or diminished; unless one of the moving powers were withdrawn by the Creator, in which case the other would destroy the whole system in an instant. As to the nature of these great original powers, we must confess ourselves totally ignorant; nor do we perceive any data from which the nature of them can be investigated. The elements of light, air, &c. are the agents; but in what manner they act, or in what manner they received their action, can be known only to the Creator himself.

Perpetual MOTION, in Mechanics, a motion which is supplied and renewed from itself, without the intervention of any external cause; or it is an uninterrupted communication of the same degree of motion from one part of matter to another, in a circle or other curve returning into itself, so that the same momentum still returns undiminished upon the first mover.

The celebrated problem of a perpetual motion consists in the inventing of a machine which shall have the principles of its motion within itself. M. de la Hire has demonstrated the impossibility of any such machine, and shown that it amounts to this, namely, to find a body which is both heavier and lighter at the same time, or to find a body which is heavier than itself.