206 C.’s Reply to D. (Serr. 
actual temperatures of bodies in contact, as upon the parts of 
the paths of the particles where the collision took place. Accord- 
ing to the well-known facts, however, the communication of 
temperature depends simply upon the real temperatures of the 
bodies in contact. On the other hand, if it were true that the 
particles of bodies had such a motion as D. supposes, and would 
not return till they received an inward motion from collision with 
other particles in vacuo, where they could receive no such inward : 
motion, the particles would altogether fly off and be dissipated ; 
which is no less contrary to fact. 
The remainder of D.’s reasoning on this subject rests upon the 
truth of Mr. H.’s theory of the laws of collision of hard bodies ; 
and I shall now proceed to examine the answer D. has attempted 
to give to my former arguments upon that part of the subject. 
D. commences his observations in the Annals for May last, 
p- 357, by attempting to show that absurd consequences would 
follow, according to the usually admitted theory of collision of 
bodies, from propositions which I have made, or admitted to be 
true ; the reasoning in this instance will be found to be as nearly 
as the different kind of argument will admit, of the same nature 
as that upon which I have already observed. Before, he mis- 
stated the obvious meaning and expressions ; here he will be 
found to have misstated the no less obvious consequences. 
D. states, “ He allows that bodies act with a force equal to 
their momentum, and, therefore, as one consequence, that the 
force with which a hard fixed plane, and a hard ball moving per- 
pendicularly upon it,come in contact, is equal to the momentum 
of the ball.” Again C. grants that “the mtensity of the force 
with which two hard balls moving in opposite directions come 
in contact is equal to the sum of their momenta.” Admitting, 
therefore, that the three momenta in these two cases are respect- 
ively equal, it is evident by what C. himself allows to be true, 
“ that the intensity of the collision in the latter case is double 
the former.” ‘ It is on all hands allowed, | believe, in the case 
of perfectly hard bodies, that the changes of motion have at least 
the same ratio as these intensities. For instance, if a certain 
intensity of stroke produce a certain change of motion, double, 
treble, Xc. that intensity will generate a double, treble, &c. 
change of motion.” Most obviously the consequences of this 
reasoning is, that as the intensity of the collision in the case of 
the two balls coming in contact is duuble the intensity of the col- 
lision in the case of the one ball striking perpendicularly upon + 
the hard fixed plane, the change of motion is also double. Con- 
sequently, if when the one ball strikes perpendicularly upon the 
plane, the motion of the one ball is destroyed ; when the two balis | 
come into contact, double that motion, or the motion of the two 
balls, is destroyed. Instead of these consequences which are so 
direct and conclusive, and which accord with what was stated 
in.my former paper, D. proceeds: “Therefore, in the case of the 
