224 



the principles themselves, but against the indefinite language in 

 whicli they have from time to time been expressed. There is 

 probably no term employed in physics that has been more mis- 

 applied, and in its misuse has led to greater confusion of ideas, 

 than ^' force/^ 



16. Mr. Justice Grove writes thus i"^ — '^Physical science treats 

 of matter, and what I shall term its affections, namely, Attrac- 

 tion, Motion, Heat, Light, Electricity, Magnetism, Chemical 

 Affinity ; when these react upon matter they constitute Forces.^^ 

 Attraction undoubtedly constitutes a force, but motion can mean 

 nothing else than the act of changing the position occupied in 

 space, and how that act can be held to constitute a force it is 

 not easy to understand. Heat, Light, and the rest, in acting or 

 reacting upon matter, constitute not forces, but forms or kinds 

 of energy. 



17. Professor Balfour Stewart^ avoids any definition of force, 

 but the illustrations given involve the above commonly received 

 definition. Thus, in the case of a stone resting on the edge of 

 a cliff that author writes : — ''Whilst the stone lay on the top of 

 the cliff the force with which the earth attracted it was coun- 

 teracted by an opposite force, namely, the resistance of the 

 support on which the stone was placed. Now, the ''resistance 

 of the support is obviously not 2i force, but a statical pressure, 

 and differs totally from its opponent, the force of gravitation, 

 in that the one is capable, and the other incapable, of producing 

 motion. 



18. It is easy to put a case in which one force may really be 

 counteracted by another force; as, for example, if the stone bo 

 suspended either from one end of a spring of which the other 

 end is fixed, or by an elastic cord, then elastic force is opposed 

 to gravitation, and both are really forces, for both are capable 

 of producing motion. 



19. Professor Ball, in a recent treatise on Experimental Me- 

 chanics, states, very dogmatically, that the true definition of a 

 force is that which " tends to produce or destroy motion.^^ If 

 that be so, every obstacle to the movement of a body is a 

 "force,^^ which is obviously absurd. Subsequently he terms 

 friction a force,^' in strict accordance, doubtless, with the 

 language of his definition, but not in accordance with generally 

 received ideas on the subject. 



20. Mr. Moore, in reference to the confusion of the terms 

 employed by writers on physics, quotes from Professor Bain 

 that " Inert matter in motion is force under every mauifesta- 



Correlation of Physical Forces, fifth edition, preface, p. x. 

 f Lej^sons in Elementary Mechanics, second edition, 1871, pp. 7 ^nd 8. 



