208 C.’s Reply to D. [Srpr. 
momentum of the other.” The body which ts at rest before the 
stroke, yields to the force (of course not among its parts but 
altogether), and consequently does not receive the whole inten- 
sity. It is evident that the intensity of the stroke, according to 
the sense in which D. and Mr. H. use the term, must depend not 
only upon the momentum of the striking body, but the resist- 
ance of the body which receives the blow. When the resistance 
is equal to the whole force of the striking body, there the body 
struck receives the whole momentum; but in proportion as the 
resistance is less, the motion received by the resisting body is 
also less. The general proposition, however, which D. attri- 
butes to me, I never laid down, and his statement that I did so 
is absolutely false. Having, however, ascribed to me an asser- 
tion which I never made, he derives from it a consequence 
equally unsupported by fact. “ But C. tells us,” D. says, “ the 
one body after the stroke remains at rest on the plane ; therefore, 
the other body striking the quiescent one likewise remains at 
rest after the stroke.” When the moving body strikes the hard 
fixed plane, the resistance is equal to the momentum of the mov- 
ing body ; but the resistance of the quiescent body is not equal 
to that momentum; and it cannot rationally be contended, that 
because when the resistance is equal to the momentum, the body 
remains at rest after the stroke; therefore, when the resistance 
is less than the momentum, the body also remains at rest. Yet 
D. not only assumes that it is so, but insinuates that itis acon- — 
clusion of mine; although my former paper contains nothing 
from which any such inference can fairly be drawn; and he 
knows that I have endeavoured to support the laws of collision 
of bodies which have been laid down by former mathematicians, 
by which the consequences are totally different. That D. was 
aware of this is evident from what follows in his paper. ‘ Now,” 
says D. in the sentences immediately succeeding that which I 
have just quoted, ‘though this agrees with Mr. H.’s theory, it 
is decidedly at variance with the old. The old theory makes the 
two bodies after the stroke to go on together, and hence the 
collision deprives the striking body of only part, not of the whole, 
of its motion. C. has consequently embraced views in direct 
opposition to the theory he means to advocate.” It is certainly 
extraordinary that any writer should venture to make such wilful 
misstatements. [can only expose them. 1 cannot descend to 
apply to them the only names which would be their appropriate 
designation. I must leave them to that disgust which every 
honourable mind must feel on perceiving them. 
I must, however, consider the length to which I am led by 
exposing these misstatements one by one, and I shall pass on to 
that which D. would call demonstration, having put the supposed 
reasoning in the form of mathematical propositions. 
“If two perfectly hard and equal balls at rest be similarly 
