MOTION, LAWS OF. 



MOTION, LAWS OF. 



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Ute between the writer and himself, M to motion. Hence we are led 

 to the conclusion that all the relatioai of matter to matter remain 

 unaltered, if the whole system be made to move, provided that the 

 .... motion be communicated to all its parts. And though wo do 

 not, absolutely speaking, know what rest is, since no point of the 

 earth, nor of any heavenly body, can be shown to be at rest, yet since 

 we see that the relations remain unaltered when the velocity of a whole 

 system is changed, we are led to conclude that the same laws which 

 prevail when all the parts of a system have the same motion, would 

 also prevail if the whole system were at rest; the ground of our pre- 

 sumption being, that the laws remain unaltered under any alteration of 

 the common motion which it is in our power to make. 



Let us now suppose that the carriage, instead of moving in a right 

 line, is carried on a curved road, say a simple circle. It is no longer 

 observed that loose objects in the carriage have a tendency to repose 

 relatively to the carriage itaeli If the motion become sufficiently 

 rapid, or the friction of the substances on which they stand be suffi- 

 ciently small, they will endeavour to move outwards, or from the" 

 centre of the circle of motion. This phenomenon can be made a con- 

 sequence of the laws of motion, when the latter have obtained their 

 simplest form ; we do not at present enter into this subject further 

 than to point out that it is only of rectilinear motions we can predicate 

 any law as descriptive of what is inherent in matter. We have, it is 

 true, already spoken of circular motions in taking into account those of 

 the earth; but it must be remembered, firstly, that the circles in 

 question are so huge, that a small arc of any one is nearly a straight 

 line ; secondly, that we hare been obliged to advert to this tendency 

 outwards, which is the reason of the diminution of weight (or of much 

 the greater part of it) detected from the oscillations of a pendulum 

 which is earned towards the equator. 



This second law of motion (for such it is called, though it must be 

 deduced first when the earth's motion is considered) may be thus 

 stated : If there be two or more causes of motion, taking place in two 

 different right lines, whether inherent in the body or external to it, 

 their effects do not interfere, nor does either diminish or augment the 

 effect of the other. If, for instance, the body A be subject to two 



actions, one of which, being entirely in thn direction A n, would bring 

 the body to B in a given time, and the other, entirely in the direction 

 A c, would bring it to c in the same given time ; then the body will 

 move from A to D, precisely as it would have done if, moving along A B 

 in the manner first specified, the line A B had been translated with its 

 extremity A moving in the second manner specified, the said line A B 

 not changing its direction. 



The most simple and general method of stating this law is as 

 follows : The distance of a point from a straight line or plane, 

 measured in any given direction, and as it will be at the end of a given 

 time, U not affected by the action, during that time, of any causes of 

 motion, provided they act in the direction of, or parallel to, that straight 

 line or plane ; or no force, in a given direction, can produce motion to 

 or from a line in that direction. Thus if a ball were thrown up in 

 till air, in such a manner that it would mount 50 feet in one second, 

 no imaginable koruontal current or whirlwind, however much it might 

 alter the actual course of the ball, would prevent its rising 50 feet in 

 the second. The statement of the law by Newton, namely, that when 

 a force acts upon a body in motion, the change of motion which it 

 produces is in the direction and proportional to the magnitude of the 

 force which acts, is perhaps rather too vague to give a distinct notion 

 to learners. 



From the law just enunciated, we may learn that bodies upon the 

 earth, moving with the earth, have the properties of bodies at rest 

 with respect to all motions that are to be estimated relatively to the 

 earth : at least upon the supposition that the curvature of the motions 

 of the earth is not sufficiently great to produce a sensible effect We 

 have then to inquire what is the natural state of matter on the earth f 

 Can it preserve any motion of itself, or does every motion gradual!; 

 slacken and die out, by the mere inability of matter to maintain it 

 without the application of external causes ? On this point we have 

 only strong presumptions, which would be by themselves insufficient 

 Our first step would be to conclude, from what we actually see, tba 

 rest U the natural state of matter, and one to which it always approaches 

 however great a cause of motion be applied, unless that external cause 

 or some part of it, be maintained. On looking further however we 

 find that terrestrial matter, immediately on its being put in motion 

 noountera causes of retardation. The resistance of the air, and the 

 friction of the basU on which the substances rest, are easily shown to 

 less in the motion of bodies which encounter them. The more nearly 

 these are removed, the longer does motion continue. It is certain then 

 that these resistance contribute in a great degree to the destruction o 

 motion; bat it i not therefore to be imm.Hi.ti.ly assumed that there 



s no other cause. If we grant that a perfectly smooth ball, lying upon 

 n i n- 1< -finitely extended plane without friction, and not in contact 

 with any atmosphere, would move for a long time without any sensible 

 iminution of the rate with which it was made to set out, we grant 

 uite enough to explain all that we see, without the necessity of imp- 

 osing that the motion would continue for ever. Huw then can we 

 tablish the first law of motion (so called), which is thus stated, that 

 matter will retain it* state of rest, or of motion, for any length of time, 

 lowever great, until acted upon by some external cause ? We must 

 lere appeal to the results of the application of this law, which have 

 icrcr, in any one instance, exhibited any reason to suspect that it is 

 inly approximately true. Throughout the long period of astronomical 

 listory, no one of the heavenly bodies has shown any diminution of 

 U motion, or any of the consequences which would arise if the n> 

 lad a tendency to wear itself out. We shall not here go into the 

 letails of these consequences ; the conclusion in, that the state which 

 matter, independently of external bodies, has been created capable of 

 maintaining, is not merely rest, but also uniform motion in a straight 

 ine ; so that it has no^nore tendency of itself to part with any of its 

 velocity, nor to move slower or faster than it was first made to move, 

 ,han it has to set itself from rest into motion. A great many, perhaps 

 most, of the mistakes which have been made by writers against the 

 Newtonian theory of attraction, have arisen from want of proper con- 

 ception of the neutral state of matter. Maintenance of velocity and 

 direction has been to them a proof of the existence of external causes 

 maintained in action ; whereas it proves nothing, but that there was at 

 some time or other an external cause which acted for a longer or 

 shorter time : the external cause steps in when the velocity changes, or 

 ,he direction, or both, and not till then. 



Properly considered, the immense number of different states which 

 matter retains, namely, either absolute rest, or any degree of velocity 

 whatsoever, is as wonderful and mysterious a law as that of the 

 attraction of matter upon matter, without any apparent intermediate 

 agent. That matter should, without any perceptible maintainer, keep 

 one rate of motion and one direction until acted on from without, 

 is as difficult to admit, as that the mere presence of other matter 

 should change that motion and that direction. What should teach 

 blind atoms to draw straight lines in preference to circles or spirals ? 

 Have they the fundamental conceptions, according to some, or the 

 powers of perception and inference, according to others, by which 

 reasoning minds know or discover the simplicity of a straight line ? 



These two consequences of observation, namely, the law of its 

 existence, by which matter can retain certain states, if no other matter 

 interfere, and that by which it can change the state of other matter, 

 its own at the same time undergoing another change, should never be 

 allowed to be separated. There are two classes of philosophical 

 speculators (for no religious question need be allowed to enter), whose 

 system introduces no difficulty into the details of mechanical philosophy 

 which did not enter into its principles. The first consists of those 

 theists who look upon the maintenance of the creation to be the con- 

 sequence of the same power as that which first created, and who 

 consider that one moment's cessation of a sustaining power, of the 

 same quality, so to speak, as the creative, would be the annihilation of 

 all things : the second consists of atheists, who will of course find no 

 more difficulty in the maintenance of the universe than in its first con- 

 struction. But a great confusion of ideas is introduced into all 

 fundamental questions which relate to matter, by the existence of a 

 sect which we suspect greatly to outnumber either of the former two, 

 and whom we may call believers in the Creator and not in the Main- 

 tainer. These, whatever they may think of the God of the moral 

 world, imagine that the God of the material became inactive and 

 quiescent as soon as matter was created, and endowed with certain 

 powers, or made subject to certain laws. These laws, which are really 

 their minor deities, carry on the business of the universe, and they 

 can abstract the idea of God altogether from the continuance of the 

 existence of matter, though not from its first creation. Among them 

 may bo found many of the literal interpreters of the Mosaic account 

 [MOTION or THE EAHTII], who hold strictly that the Creator "rested 

 from his work," and left matter to its " laws." except on certain rare 

 interpositions. Many of this sect have admitted the laws of motion, 

 and, among others, the power of matter to maintain its motion, 

 because there was an appearance of inactive sameness, or want of 

 change, in the permanence of rest, or permanence of direction and 

 velocity. But tlicy have been startled by the entrance of attract i-m, 

 and have disputed its possibility on account of the absence of tecond 

 causes sufficient for its explanation : however clearly it might be shown 

 that all the results of attraction are present among phenomena, they 

 would not allow their Jirtt cause to be awakened from the sleep in 

 which it was their pleasure to suppose him plunged, so far as matter 

 was concerned. Perhaps it is one of the most singular mental aber- 

 rations which ever was manifested, that at the time of the appearance 

 of the Newtonian doctrine, the first mechanical theory which rested 

 on the maintainer of the creation, at least until (which has not yet 

 happened) some good quiescent " second " cause was discovered that 

 doctrine was frequently charged with atheism. 



If the earth were supposed to be fixed, we might obviously (tli<-ML-h 

 not obliged to do so) begin from matter at rest, and establish first that 

 law of motion which usually stands first. 



