336 ScientifiG Proceedings, Royal Dublin Society, 



XXXI.— ON THE DEFINITION OF FORCE AS THE CAUSE 

 OF MOTION, WITH SOME OF THE INCONVENIENCES 

 CONNECTED THEREWITH, By REV. MAXWELL H. 

 CLOSE, M.A. 



[Received August, 1882.] 



Force is still generally declared to be that which produces 

 motion or momentum in a mass. It will be sufficient to quote 

 two of the latest and most authoritative statements to this effect 

 both of which are given in the most comprehensive form : — 



" Definition of Force. — Force is whatever changes or tends to 

 change the motion of a body, by altering either its direction or 

 its magnitude." — (Clerk Maxwell, Theory of Heat, p. 83.) 



" The definition of Force x x may thus be given. — Force is 

 any cause which alters or tends to alter a body's state of rest or 

 of uniform motion in a straight line." — (Tait, Recent Advances 

 in Physical Science, p. 10.) 



We have the very same position in Thomson and Tait's 

 ''Treatise" and "Elements of Natural Philosophy" but it is 

 not there so completely expressed in a single sentence. 



It is most respectfully submitted that this definition is incon- 

 sistent with the meaning of ' ' Force" in modern kinetics, as 

 well as in statics and energetics, where it seems to be altoge- 

 ther out of place. Perhaps some might prefer to say that the 

 following, if correct, proves rather that " force" is still used to 

 mean two totally difierent things, viz. : force proper and impulse. 

 This, however, comes very nearly to the same as what we have 

 just said. Such looseness in the employment of so important a 

 word is equivalent to an objectionable definition of it, and quite 

 as inconvenient. 



Let us premise that we are not now raising any objection to the 

 above definition drawn from the metaphysics of cause. Our reply 

 is on the same plane of thought as the definition itself We shall 

 indeed have to appeal to what is involved in the notion of cause, 

 but are not concerned with any deeper question respecting it. 

 Clifibrd refers merely to the metaphysical objection to the above 

 definition of force, and Tait expressly declares that it is the only 

 objection. We now accept cause in its ordinary sense, but submit 

 that the cause of motion is Impidse and not force. 



