124 On the Measure of 
collision. But that is only one feature of the 
case. If we examine all the results after col- 
lision, we shall find that the motion of A is not 
the same as it would have been if it had been 
struck by a mass equal to E+F, having the 
same velocity as the common centre of gravity 
of E and F before collision. If, however, we 
reckon the forces as the masses into the squares 
of their absolute velocities, we shall (if they be 
‘perfectly elastic) always find that whatever 
force is lost by the striking balls, is gained by 
that which is struck. 
12. Let four equal balls A, B, D, E, (fig.12.) 
revolve about their common centre of gravity, 
C. Let A and B be connected by a rod of no 
sensible inertia, and D and E by a similar 
rod, but unconnected with A and B. Let the 
distance of the centres of gyration of A and B 
be twice that of D and E, and let D and E 
make two revolutions while A and B make 
one. Ifthe balls and rods be elastic, and the 
velocity of each ball 10, and if the rod con- 
necting A and B be struck by the balls D and 
E at their centre of gyration, the velocity of 
A and B after the stroke will be 14, and that 
of Dand E will be 2. If the balls and rods be 
non-elastic, the velocity of A and B after the 
stroke will be 12, and that of D and E, 6. 
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