340 Scientific Proceedings, Royal Dublin Society. 



up ; but it is well to be reminded of their real character, though 

 we must continue to use them. 



(8.) To say that force is the cause of motion obscures the 

 distinction between force and impulse by making a very promi- 

 nent statement respecting force which is applicable only to 

 impulse. In some cases it would seem, perhaps, more correct to 

 say that the effect of this definition is to make " force " ambiguous, 

 by giving it two meanings, viz. : — force and impulse. Not long 

 ago " force" (or its equivalents in other languages) was used very 

 generally to mean energy, as well as force. This vagueness has 

 now nearly ceased, in this country at least, partly no doubt in 

 consequence of Tait's trenchant remonstrances against it ; but 

 the other still obtains to a greater extent than we are conscious of. 



The distinguished writers of the following statements, in which 

 "force" means impulse, could not possibly be guilty of the in- 

 accuracy of using " force " for energy. 



" It requires an equal and opposite application of force to set a 

 body in motion and to bring it again to rest." 



" Forces proper, or (their equivalents) quantities of momentum." 



" Change of motion is proportional to the impressed force." 

 (" Change " meaning, as the following context shows, amount of 

 change. This is a translation from Newton accepted by theauthors.) 



What we are now calling by Maxwell's name, "Impulse," 

 requires to have its own proper designation quite as much as 

 energy ; impulse and force are fully as disparate as energy and 

 force, and, if possible, even more so than surface and length. 

 Force being measurable in pounds, energy is gauged in foot- 

 pounds, and similarly impulse in second-pounds. 



It is frequently said and written that " force never accomplishes 

 any effect, but in some time," that time is an inevitable or essen- 

 tial condition of its acting, which is always implied, and therefore 

 need not be referred to, except when it is necessary to mention 

 the length of it given by the conditions ; since what the given 

 force can perform is proportional to the time of its acting. Now 

 in such statements force really means impulse ; it will be seen on 

 consideration that there is here a fallacy in " effect," and, as we 

 have already said, an impropriety in " acting," also that there is 

 a latent confusion between force and impulse of unit of time, 

 both of which are represented by /". It might seem that in the 

 equation of kinetics, Jt = mv, taken by itself, f should be inter- 



