﻿Book Notices. ' 195 



How would you explain this from the statements concerniwj rest 

 as given under kinematics ?" The reference is, of course, to the fact 

 that we can have no knowledge except of relative motion, and the 

 confusion arises from neglect of the warning given by Clerk Maxwell, 

 in his little treatise on Matter and Motion, that in every mechanical 

 problem we must begin by defining the system which we mean to 

 consider. A similar oversight led Professor Newcomb into the 

 discovery of an elaborate mare's nest about the relativity of energy, 

 described in his paper in Vol. XXVII. of the Phil. Mag. Those who 

 wish to see the whole matter placed in the clearest light, together with 

 another reason why the absolute energy of a system can never 

 be known to us, and the considerations which render this of no 

 importance, should consult Maxwell's Matter and Motion. (§§ II, 

 III to XXX— I, CX). 



A confusion arising from the opposite error of neglecting to 

 remember that all force is of the nature of stress and that all energy 

 must be conceived as relative, or between parts of some system, leads 

 the writer to a somewhat severe handling of Potential Energy. Thus 

 (p. 36), a raised stone " before it starts has no velocity and, therefore, 

 no energy" ; and (p. 39) "At the extreme end of the swing does 

 the pendulum possess energy ?", to which the answer, No, is expected. 

 On p. 42, we have — " In such instances the body does not possess 

 actual energy, but only the possibility of acquiring it. It is said 

 to possess potential or possible energy" ; again (p. 97), " the amount of 

 Potential Energy relatively to a given poinf ; and finally (p. 98) 

 Potential Energy receives its quietus from the scathing epithet 

 " so-called." 



It is, no doubt, probable that all forms of Potential Energy may 

 be reduced ultimately to cases of relative motion ; but it seems 

 less confusing, meanwhile, to keep the felicitous term Potential 

 Energy to denote those forms of the capacity to do work which depend 

 on the relative position or condition of bodies relatively at rest, 

 without implying that the energy is in these cases less real ; while 

 kinetic covers the cases of energy due to relative motion. This is 

 a better distinction than that between possible and actual Qwevgy (p. 42). 



It is unfortunate, too, to exclude the idea of direction from the 

 terms velocity and acceleration, as is done in such phrases as 

 "accelerated as well as curved" (p. 56) "the motion (with uniform 

 velocity) may be over any path, either straight or curved." This 

 is certainly not a modern practice. 



It is more in accordance with modern fashion to discard as far 

 as possible the idea of force as a cause historically so important and 

 fruitful, and so harmless if properly safeguarded. How necessary this 

 conception is appears from the fact that after defining (p. 44) force as 

 " any tendency to acceleration," and rejecting the usual definition not 



