200 On the Measure of 
force in any direction, there must be a corre- 
sponding change of figure, which may always 
be estimated upon the principles adopted in 
the preceding cases. 
In the 10th case, the quantity of motion of 
A (fig. 10) after collision is the same as that 
of the common centre of gravity of EK and F 
before collision. But the whole forces of EK 
and F are not exhibited in the quantity of 
motion of their common centre of gravity.— 
The motion of A, however, is the whole effect 
produced, and if we estimate its force by its 
mass into its velocity, we cannot account for 
the total loss of the forces of E and F; but if 
we estimate all the’ forces by the masses into 
the squares of their separate velocities, the 
agreement between the forces and their joint 
effect is obvious. gi 
I have already adverted (page 134) to a 
statement of a case of composition of motion 
made by M. Laplace, in which a hypothetical 
relation of the force of a body in motion to the 
square of its velocity is adopted, and where 
the supposed effects would be quite at variance 
with those of experience. It will perhaps 
be better understood with a reference to this 
10th case. | 
- M. Waplace says, “ La force peut étre 
