224 
the principles themselves, but against the indefinite language in 
which the} 7 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 :* — “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 hinds 
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 a 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 realty be 
counteracted by another force; as, for example, if the stone be 
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 manifesta- 
* Correlation of Physical Forces, fifth edition, preface, p. x. 
j- Lessons in Elementary Mechanics, second edition, 1871, pp. 7 and 8. 
