462 



LEVEEAGE OP BONES. 



Fig. 217. 



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 r, 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- 

 plied at the same point, 

 and consequently at the 

 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 fulcrum 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 



rt ^ c t r it, so that the bone be- 

 comes a lever of the 

 "third order," its force 

 must be proportionably 



jr. 



614. But this arrange- 

 ment greatly increases the 

 rapidity of the motion 

 which is the consequence 

 Fig<218< 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 c tra- 



