ATTRACTION. 



ATTWOOD-S MACIUNK. 



740 



but during man than a century and half, and by philosopher* of 

 evrnl countries not by men prejudiced in favour of Newton, but 

 UK contrary; for it wa long before hi* doctrine! found their way over 

 to the continent. and the dispute about the invention of fluxions had 

 U* ths found^UoM ol a strong anU-Newtonian prejudice. We may 



Ne 



obMrre, also, that England, where the veneration of Newton amounted 

 almost to idolatry, baa done much less toward* the dereloptnent of his 

 system than either France, Germany or Italy ; so that the Newtonian 

 system waa really fully established by those who had every national 

 *nd penonal bias to endeavour to overturn it. This it is necessary to 

 sate, because it is frequently asserted that the prevailing system is 

 sustained by the name and authority of Newton. 



We shall now give some account of the disputes about the word 

 attraction ; but, first, we shall show how it was used by Newton. In 

 his ' Optics,' he says, " attraction may be performed by impulse, or 

 some other means. I use that word, to signify any force by which 

 bodies tend towards one another." In his ' Principia,' he thus speaks 

 of y rarity : " Thus far I have explained the phenomena of the heavens 

 and the sea by the force of gravity ; but I have not yet assigned the 

 cause of gravity .... The reason of these properties I have not yet 

 deduced from phenomena, and I do not invent hypotheses. For what- 

 ever is nut deduced from phenomena is called hypathait ; and hypo- 

 theses, be they metaphysical, physical, of occult qualities, or mechanical, 

 have no place in experimental philosophy .... It is enough that 

 gravity really exists, and acts according to laws laid down by us ; and 

 suffices to explain all the motions of the heavens and the sea," 



The repeated use of the words not yet (nondum], would lead ua to 

 oppose that Newton thought that the cause of attraction might be 

 discovered ; and the sentence next following our preceding quotations 

 hows that he leaned towards the notion of a highly subtle fluid, 

 which was afterwards the kypoUtetit of those who constituted him their 

 opponent : " Something might be added about that most subtle spirit 

 which pervades and lies hid in all dense bodies ; by force and action 

 of which the particles of bodies mutually attract at the smallest dis- 

 tances, Ac. . . . But this cannot be explained in few words ; neither 

 is there a sufficient number of experiments by which the laws of 

 action of this spirit can be accurately determined and shown." (' Prin- 

 cipia,' SchoL Gen. at the end). 



Again, in the Optics, Newton dwells upon the same distinction 

 between a phenomenon and ita cause, and says that attraction may be 

 caused by an impulse or some other unknown cause. But once for all, 

 both against Newton and his opponents, we must observe, that an 

 invisible fluid leaves the difficulties of the question where it found 

 them. If this fluid have the common properties of matter, what is 

 there to explain the mutual repulsion of its particles ? Must they 

 have a fluid to cause that phenomenon, and so on ad iitjinitum, or must 

 an unknown cause of repulsion take the place of an unknown cause of 

 attraction t 



Leibnitz called attraction an occult quality, and a miracle. The first 

 term was the horror of the continental philosophers about his time. 

 Their predecessors had attributed various properties to matter which 

 could not be proved by experiment, which were justly called occult (or 

 hidden). In their desire to be rid of all such, succeeding philosophers 

 would not only abolish the qualities of matter which they hod 

 invented, over which of course they had absolute power, but they 

 tried also to abolish their own ignorance of the causes of the tentible 

 qualities of matter. They would not have occult causes, and Leibnitz 

 plainly confounds occult quality with occult cause. But it is needless to 

 dwell upon the fact that the ultimate causes of all qualities are occult. 

 When Newton adopted the word attraction, he did not take up and 

 fix the meaning of a word which till his time had been ambiguous ; 

 still less, as some have asserted, did he retain a mystical meaning, 

 which his followers afterwards cleared from absurdity. At and before 

 the time of Newton, the word attraction was frequently used ; for 

 example, in the P-ngHA translation of J. B. Porta, 1668, where to 

 " attract " is used for to " draw forth," in opposition to " compound " 

 or " lay together." But the philosophic use of the word is more con- 

 spicuous in Sir K. Digby's "Treatise on Bodies,' 1669, where it is 

 said that wherever " the first cause of the motion proceeds from that 

 body towards which the motion is made," the effect is "properly 

 called attraction," which is illustrated by the case of fire and air, in 

 which, though there is an intermediate cause assigned by himself 

 namely, that the fire rarefies the contiguous air, which therefore 

 ascends, and the surrounding air rushes in to supply its place the 

 author says that the fire attract* the air. 



The objections made to the Newtonian attraction have been, with 

 one or two exceptions, the work of those who had obviously not read 

 Newton, or any geometrical work on the subject. We must take them 

 in clssses, and describe them as far as our limits will allow us to do. 



1. We have those contained in axioms, which are either unproved 

 or unmeaning, such as "matter cannot act where it it not." Those 

 who bring this forward should explain the three hard words which 

 they have put in italics ; and we should then see whether this be self- 

 evident or not. They should also remember that the celebrated 

 immatorialism of Berkeley is, in several ways, an attack upon the 

 word matter of exactly the same kind of argument as their own upon 

 attraction ; so that, in fact, they must assume a principle as to matter 

 which they immediately proceed to oppose as to attraction. 



in Bioo. Dry. ; IMMATERIALUM.] Again, in speaking of the place where 

 matter it, they assume that the boundary of impenetrability is the 

 same as the boundary of colour ; a thing not only unproved, but from 

 several circumstances unlikely. 



2. We have those who would substitute pure hypothetical causes, 

 such as Newton declines entering into, to explain the phenomenon of 

 attraction. One writer requires no more than that all bodies should 

 be composed of two distinct sets of particles, the one set of water, the 

 other of some volatile fluid from which he thinks he deduces attr.. 

 another is satisfied with an efflux and reflux of a fluid from and to the 

 sun, to cause what he denominates the centripetal and centrifugal 

 forces : evidently confounding the nature of the two in a manner 

 which could not have been done by any person who had read Newton. 

 A third fills the whole universe with streams of matter which are 

 always pawing through every point in every direction. On all these 

 we shall only observe, that, in their attempts to produce an explanation 

 of the phenomenon, they admit the phenomenon itself, v 

 that Newton contended for. 



8. We have those who leave out of view the main fact, that N 

 explains phenomena as they really are, and who treat the raultt ax 

 hypothetical, as well as the principle. " Let the idea," says one writer, 

 "of particles of matter attracting each other be impressed u].n the 

 mind, and it will then dilate upon their mutual actions, calculate the 

 density of substances composed by them, whirl them at pleasure in 

 empty space, and show in what manner their motions will be dis- 

 turbed by the actions of each upon the other." Hut it is here forgotten 

 that the " whirls " alluded to were not made " at pleasure," but they 

 were " whirls " actually taking place, which were examined in order to 

 gee how they did whirl. Newton laid by his theory of attraction for 

 years, as a forgotten thing, because he found that, with the received 

 notions of the earth's magnitude, it would not give the moon the 

 motion which she is actually found to possess : it was only when h 

 received the more accurate measurement of Picard that he resumed his 

 inquiry. Did he whirl his planets " at pleasure ?" 



4. Another class of objectors cannot conceive how attraction can be, 

 and therefore they reject it. This argument is wholly unanswerable, 

 because it is impossible to see on what part of the subject it bears, or 

 how it is shown to be unreasonable to admit nothing as proved, except 

 what can be conceived and accounted for. Nothing, except an abso- 

 lute contradiction in terms, can be rejected on this ground. 



5. All the above objections have been at one time or other advanced 

 by men of knowledge : there remains one class more, nain 



men, who, being ignorant of mechanics, deduce from wrong reasonings 

 results which are not found in the heavens, on which they deny the 

 truth of the principle. To this class, we are happy to say, personal 

 aspersion, and imputations of intentionally misleading others, have 

 been for the moat part confined. The common mistake is a confusion 

 between the words velocity and force ; being much the same as if they 

 confounded the drops which are pouring into a cistern for the time 

 being, with the whole body of rain in the cistern itself. We quote 

 another instance. A certain traveller remarks that it cannot be that 

 the sun attracts a planet, at the very time when the planet is flying off 

 from it. "What more could it do, if it were really repelled?" He 

 does not see that the same argument applies to a stone thrown up 

 into the air; and moreover, that what it could do more, if really 

 repelled, would be to describe a conrej; curve, instead of one 

 concave towards the centre of force. To those who have any acquaint- 

 ance with mechanics it is unnecessary to say anything upon such 

 objections : to others who have not, we recommend, if they form an 

 "pinion upon this question, which it is noways necessary they should 

 do, to follow either those who have studied it, or those who have not, 

 whichever they have found most advantageous in the common business 

 of life. 



ATTRACTION, ELECTIVE. [CHF.HICAL AFFINITY.! 

 ATTRACTION, ELECTRICAL. [ELECTRICITY.] 

 ATTWOOD'S MACHINE. When a constant or uniform force acts 

 upon a mass, it produces equal accessions of velocity in equal times, 

 and the whole distances described are as the squares of the t 

 that is, whatever length is described in the first second, four times as 

 much is described in the first two seconds, nine times as much in the 

 first three seconds, and so on ; that is, the length described during the 

 first second being called 1, that described during the second second is 

 S, that during the third 5, and so on. [ACCELERATING. FORCE.] Where 

 the weight of a mass is the pressure applied, and the mats ittelf only is 

 moved, that is, where a body falls freely in vacua, the velocity 

 created in every second is found to be 32J feet, and the spaces 

 described in successive seconds are 16,^ feet, three times 16]>, feet, five 

 times 16^, Ac. These are distances too great on which to try experi- 

 ments; and Attwood's machine is a method of contriving systems 

 which shall move under constant forces of less amount, so that the 

 space described during four or five seconds shall not require a very 

 great fall. The principle made use of is one which is well known in 

 mechanics, namely, that if a pressure A, acting uniformly upon a mass B, 

 produce a certain velocity per second, it will only produce half that 

 velocity when acting on a mass twice as great as B, Ac., and will pro- 

 duce twice as much velocity in a mass half as great as B, *c. Suppose, 

 for instance, weights of six and seven pounds hang over a pulley, the 

 weight and friction of which are for the present neglected ; if both 



