308 
second, it will continue rising until the attraction of gravitation which is 
continually pulling it downwards and diminishing its progress upwards, at 
last arrests it, and its velocity upwards becomes nothing ; it comes to rest 
at a certain point. If a shelf be there placed under it to support it, the ball is 
then said to have acquired a certain amount of potential energy, or energy of 
position. What does that mean ? It means simply that if it be allowed to 
descend again from that point to the earth, it will in its descent acquire 
exactly the same amount of energy which was expended in propelling it, and 
that is a fact which no experiment or proof in any way can controvert. 
Again : — 
“ Contradiction the third. Motion, by the theory, may be trans- 
ferred from one body to another, remaining the same motion still. It may 
reverse its direction, and be the same motion, if its rate be the same.” 
Certainly not ; no one can say that motion in one direction is the same as 
motion in an opposite direction. I do not know any author who has ever stated 
that, and it seems to me to arise from a misapprehension of the theory which 
the author is endeavouring to combat. Then, in another passage, Professor 
Birks has called potential energy the amount of force which would be 
expended in bringing a body from an infinite distance to the place it 
occupies. And he goes on to say : — 
“ Contradiction the fifth. The Potential Energy supplies another element 
of confused thought and metaphysical incongruity, as striking as the last.” 
If the definition he has already given be correct, it is true that there is an 
element of confused thought and metaphysical incongruity, but that I fear is 
the fault of his definition of potential energy. Then we have this passage : — 
“ But how can we conceive a particle of motion, which is not a thing that 
moves, but an abstract quality or relation, pushing or pulling another particle 
of the same force ?” 
We cannot, of course, conceive a particle of motion. Motion is a change of 
place, and a particle of motion has no meaning. No one that I know of ever 
attempted the use of these expressions. 
Professor Birks. — You will find them used both by Mill and Spencer. 
Mr. Brooke. — Then we have this passage : — 
“ Three fundamental errors have already been pointed out, which contra- 
dict the first principles of clear dynamical reasoning : that statical pressures 
are not forces, that friction is not a force, and that one body in motion can 
move another without the intervention of any force whatever.” 
I would hardly go into that, but if the definition which Professor Birks has 
given us is to be generally accepted, then anything that changes the con- 
ditions of a body is force. Certainly friction is a force. This table is a force, 
as it arrests the falling of this book to the ground. But it appears to me 
that this involves a contradiction in terms which is unsuitable to the real 
meaning of the word, which I think had much better be considered and de- 
fined in the way that I have elsewhere defined it. I will now only make 
one or two further remarks. There is one point personally affecting myself 
