462 LEVERAGE OF BONES. 
of the power-arm of the lever is increased or diminished, that 
of the weight-arm remaining the same. 
613. Now in order that there may be an equilibrium, or 
balancing between the power and the weight, it is necessary 
that they should be inversely proportional to the lengths of 
their respective arms ; that is, the power multiplied by the 
length of its arm, should be always equal to the weight 
multiplied by the length of its arm. ‘Thus, to balance a 
certain resistance 7, equal 
to 10, and applied at the 
end of a lever a b (fig. 
217), whose length we 
shall call 20, it is neces- 
sary that a force p, ap- 
pj plied at the same point, 
Fakes 8 and consequently at the 
pcp same distance from the 
fulcrum a, should also be equal to 10; but, if the power be 
applied at the point c, which is at only half the distance from 
the fulcrum a, it must be doubled in amount, or equal to 20,— 
since it must be sufficient, when multiplied by its distance 10 
from the fulcrum, to make 200, which is the product of the 
resistance 10 and its distance from the fulerum 20; and in 
like manner, if the power be applied at d, where its distance 
from the fulcrum is only 2, its amount must be 100, in order 
that its product with the distance at which it is applied may 
be equal to 200. Hence, when a muscle is applied near the 
fulcrum, while the resist- 
ance is at a distance from 
it, so that the bone be- 
comes a lever of the 
“third order,” its force 
must be proportionably 
greater. 
614. But this arrange- 
ment greatly increases the 
rapidity of the motion 
Fig. 218 which is the consequence 
rice of the muscular action. 
For let us suppose that the muscle p (fig. 218) acts upon the — 
lever a r, in such a manner that its point of insertion ¢ tra- 
