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THE CIVIL liNGINEER AND AilCMlTECrs JOURNAL. 



[April, 



spirit as well as the leller — depRi'ture from which principle for priviite ends 

 is wron^j, atul, bec>*nse wrDri;;, iftcxpedient in the long run. 



"On tilts ground il Is conleudfd that those who purposely exaggerate or 

 diminish, or ollurwise distort llie real facts which are proper to be intro- 

 duced into the specification <lefeat their own ends and greatlj injure them- 

 selves by blunting their sense of rectitude." 



Treatise on Mechanics. By J. F. Heathkr, B.A. London : JoLn 

 Weale, 1S17; royal Svo. No. 1. pp. 48. 



To detect and expose error — no less tlian to supply correct in- 

 formation on al! subjects connected with niecli.inical science — is the 

 constant endeavour of the condiictcrs of this Journal; and in no 

 instance are we more forcibly reminded of the responsibility of our 

 position, than when called upon to analyse the merits of educational 

 works professedly adapted to further tlie ends we have in view. 

 Mr. Heather's treatise is peculiarly of this character; — his claims on 

 public attention rest mainly on a profession of elementary preciseness 

 of style, as will be seen by the following quotation from his preface — 

 which will likewise serve to indicate the general nature and plan of 

 the publication: — 



" In putting forth a work in parts, it is not usual to make any prefatory 

 remarks, until the whole be conipleied ; but as I shall introduce into the 

 treatment of the su) jeet, in its earliest stage, some new enunciations of 

 important principles, and shall endeavour to show that considerable improve- 

 ments can be made upon the manner in which this subject has been hand- 

 led, by even its greatest masters, I have thought it more courteous to my 

 readers, thus early to call their attention to the inSuence which these prin- 

 ciples will exercise throughout the subject. 



My endeavour has been, in the first place, to attempt, with what success 

 my readers must judge, to give clear and distinct definitions of the terms 

 thereafter to be employed ; and, in the next, to confine their use, on all 

 occasions, strictly to the sense in which they have been originally defined," 



In reply to all this, we are sorry to be compelled to state that the 

 success of Mr. Heather li.is been in an inverse proportion to his pre- 

 tensions, that his definitions are not clear and distinct, and thai he 

 has lamentably tailed to prove — so far, at least, as he himself is con- 

 cerned — that "considerable improvements can be made upon the 

 manner in which this subji ct has been handled by even its greatest 

 masters." We do not deny that Mr. Heather may be capable of 

 clearly apprehending physicd principles ; but we do most positively 

 assert, that he is utterly incapable of putting forth his couceptions 

 either correctly or in a manner intelligihle to those among his readers 

 who may have taken up the subject of mechanics for the first time. 

 His phraseology is in.iccurate in the extreme ; — terms constantly 

 occur to which no definite meaning has previously been assigned; — 

 his defiiiiiions are either old and well-known forms clothed iu a new 

 and looser garb, — or when original, generally iiicoirect. 



Lest, however, we be accused of undue severity, we proceed to 

 give extracts from the number before us, — pouiting out the various 

 inaccuracies and fallacies as they occur. 



The first four paragraphs of the introduction being purely meta- 

 physical, are, perh,.ps, not strictly within the province of a physical 

 critique. We must, however, object to the assumption of the im- 

 mutability of the law ot nature, as derived from the imuiutabilitv of 

 their Divine Author. The same face that was smiling and beautiful 

 at fifteen is wrinkled at fifty; — the same leaf that was green in June 

 is brown in November ; — the universe is iu a coiilinual state of 

 change. Why, then, should the l.iws that govern this varying world 

 be themselves euvaryiogi W hy miglit i,ol the purposes of Creation 

 demand that they too should be subject to time, and thai by an im- 

 mutable decree ot the Creator? 



"While a certain determinate point with respect to a body, always pre- 

 serves the same dutanees from ilie olijeets which surround it, the body is 

 said to be at rest; ami, when these distances undergo successive variations. 

 It is said to be in motion." 



This definition is neither new nor complete; it is incomplete because 

 it is purely geuiiietnc.d, and excludes all idea of the mechanical cou- 

 sequences of motion. Suppose tile e.irtli the only body iu space — 

 neither sun nor planets existing, to which to reierits motion; — then, 

 according to the above (U hnuioti, any point on its surlace maybe 

 said to be at rest. But the vaiiation ot gravity at that point (sup- 

 posed neither of the poles;, arising from the centrifugal force, de- 

 monstrates that there miisi be a motion of rotation ot the earth about 

 an axis, and, consequently, that the point in question is absolutelv 

 moving, though, relatively to the other pai ts ol the earth, at rest. 



" Bodies, however ditfereut in volume, upon which the same force pro- 



duces the same effects, are said to contain the same quantity of matter. 

 The quantity ol matter in a body is called its mass. Also, the greater the 

 mass of a body, the greater the number of particles it is said to contain." 



This definition is sheer nonsense. What are we to unilerstand by 

 the word ettects ? Are statical or dynamical effects here alluded to i 

 if statical, behold the consequences of this certainly new definition. 

 Suppose one pound of coals supported by a scuttle, and another by 

 the siirf.ice of the eaith; — then the weight of the coals "produces 

 the same efTects" — that is, the same pressures — on the scuttle and the 

 earth, — trgo, the mass ot the scuttle is equal to the mass of the earth. 

 What Mr. Heather probably means is tins: — Any two bodies are said 

 to have equal masses, when equal velocities are generated in them in 

 the same time, by equal, single, and invariable imprcssi d forces, where 

 by equal impressed forces we mean forces that would cause the bodies 

 when at rest to exert equal pressures agaiust fixed plane surfaces 

 perpendicular to the direction of the forces. 



From tills definition of the word m.iss — combined with the fact, 

 that the dynamical measure of gravity is the same for all bodies — 

 we in er that the masses of bodies vary as their weights. As this 

 definition cannot be understood by the tyro until he be conversant with 

 the various measures of force, and the third law ol motion, il ought 

 to be deferred until those are explained. In the next number of the 

 Journal, we hope to lay before our readers a shot I account of the 

 measures of force, — the laws of motion, — and the me.iniiig of the 

 word mass, or quantity of matter. At present, we shall content our- 

 selves with stating where we believe Mr. HeathL-r to be incorrect, 

 without any attempt at emendaiiou — from which, iudeed, the limits 

 of a cursory review preclude us. 



"12. Any two forces which are in equilibrium, when applied to the same 

 material particle ot any body, in the same right liue, in uppuAiie airecuons, 

 are called equal forces. 



13. A force which produces the same etiect as two equal forces, applied 

 at the same point in the same direction, is said to be tnice one of these 

 forces : a force which produces the same effect as three, is said to be thme 

 times one of them ; and so on. 



14. We are thus enabled to measure all kinds of forces, by units selected 

 from the eliects produced by forces of any one kind ; and it is found most 

 convenient to select these units from the titects produced by the attractiuo 

 ol the earth upon bodies near its surface. We tind, m fact, that all bodies 

 near the surface of the earth have a tendency to tall towards its centre j 

 and when they do nut so fall, we are enat>led, in all cases, to trace out a sut- 

 fieieitt cause which counteracts, and thus holds in suspension, the etiect of 

 this lendeiiey ; but the moment we remove the counteracting cause, the 

 body liegins to fall, and continues to do so, untd it lueet with some uew ob- 

 struction. 



15. When this effect is entirely uncounteracted, the same velocity is always 

 generated in the same time in all bodies, whatever be their figures, volumes, 

 and masses. This force, then, is called gravity, and is measured by the 

 velocity generated in a second ol time ; and this measure is taken for the 

 unit ot measure of all other forces which are nut in equilibrium, and when 

 our oliject, consequently, is to find the relations between the lorccs and the 

 motions produced. 



16. When, however, we apply a force to a body in the opposite direction 

 to gia\ity, so as to be exactly in equilibrium with it, aud thus keep the body 

 at rest, in which case it is said to support the body, we lind that the force 

 so applied must be in exact proportion to the mass of the body. The etiect, 

 then, of gravity in counteracting the etfects of the other forces applied to a 

 body, when it is kept at rest, is called the weight of that body ; and, in the 

 iiivc--tigation of the relations suhsisiing between the maguiludes and circuiu- 

 stances ot action of forces iu eqnilibuum, the lorccs are measured by tbe 

 weights of the bodies which they will suppoit." 



'I'liis is a jumble of inextricable confusion; — the explanation of mea- 

 sures of force — a subject of the first importance — is disposed of in 

 about forty lints. One kind of loice is described as producing 

 effects two or three times as much as anothei ; while tbe uature of 

 the ert'ects, aud their susceptibility of measurement, are left entirely 

 to conjecture. There are many causes followed by eti'ects, which aie 

 not c.ipable of being measured. Alcoholic liquors produce eti'ects 

 which are not capable ol being measured. We cannot say that A is 

 three tunes as druuk as B. 



" Gravity, in fact, must be considered as acting upon every particle of 

 which a body is composed, and generating in each of these particles, in tbe 

 same time, precisely the same velucil}' ; and thus these pai tides neither 

 accelerate nor retard the motion of one another." 



—Another instance of the inaccuracy of our author. This assertion, 

 applied to rotating bodies, is absolutely uulrue. 



We have now arrived at the end of the ijtroductury chapter; — ti e 

 remainder of the nuinbet contains uolhiiig vei\ original or very lu- 

 corieel. There is a fierce attack, near the coiiclusiuii, on i^oissou, — 

 founded on a luisapprehensiun ol his uieauiiig ; and au impruveuient 



