ee" 
‘Moving Force.” 251 
direction BC, and’ A’and B ‘to strike their 
respective pegs at the'same ‘instant. The 
result will be as follows.’ C will remain un- 
moved, A and B will be brought to rest in the 
same time, and the peg P will be found to 
have penetrated C twice as far as it has been 
penetrated by O. This case appears to me to 
admit of the same explanation as some’ of 
those which we have already examined. It is 
considered by many, however, to show dis- 
tinctly, that the forces of A and B are equal. 
If we confine our attention solely to the cir- 
cumstance of C remaining at rest, we must 
no doubt conclude, that the opposite forces of 
A and B are equal; but if we attend to all the 
results of the experiment, we cannot con- 
sistently, draw that conclusion. It has often 
been asserted by the advocates on both sides of 
this question, that we can judge of forces only 
by their effects ; yet it has been contended by 
M. D’Alembert,* and by many other gcod 
writers on dynamics, that the estimation of 
forces by their total effects, involves a meta- 
physical question which ought not to be mixed 
with experimental investigations of physical 
facts. -It may be safely. affirmed, however, 
that nothing can be more strictly grounded upon 
_* Traité de Dynamique, Disc. Prélim. p. 22. 
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