OF THE MOTORY APPARATUS IN MAN. 127 



The bones represent levers, which move on a fixed point, 

 called the point of support. The force which puts the lever 

 in action is the power, and that which opposes its displace- 

 ment is the resistance. Finally, the name of arm of the 

 lever of the power, and arm of the lever of the resistance, 

 is given to the distance separating the point of support 

 from that to which is applied one or other of these forces. 



Now the length of these arms of the lever has a powerful 

 influence over the force required to produce an equilibrium 

 to a given resistance. Observe the mechanism of the balance 

 called steel-yard (Fig. 73). The beam is divided into two 

 unequal parts by the point of sup- 

 port a. At the extremity of one @ 

 of the branches (r), which is very 



short, is placed the resistance, that r -4 * t ., ,., ^ f 



is, the object to be weighed ; and 3 if 



on the other ( p) slides any weight ! 



which produces the equilibrium to 73 



a weight or resistance, always the 



more considerable that it is further removed from the point of 



support, and that we elongate, in consequence, the arm of the 



lever of the power, the lever of resistance remaining always 



the same. 



On the same principle it is that we can raise a so much 

 greater weight with the arm flexed instead of extended. It 

 is a question (see the figure) of shortening or lengthening the 

 respective arms of the lever to or from the point of support. 

 By mechanics we learn that, in order to establish a perfect 

 equilibrium (or to weigh the body as in the figure) in any 

 lever, it is necessary that the resistance and the power (the 

 weight and the weighed body) be reciprocally proportional to 

 the length of the arms of the lever; that is to say, that 

 multiplied by these arms of the lever respectively, they both 

 give the same product. 



Thus, to produce an equilibrium to a resistance (r) equal 

 to 10, applied to the extremity of a lever (a b) of a length 

 equal to 20, it is necessary that the power (p),i? it be applied 

 to the same point (b), and consequently equally distant from 

 the point of support a, be also equal to 10 ; but if it were 

 applied to the point c, to produce the same effect it must be 

 equal to 20 ; for the resistance equal to 10 being multiplied 

 by the length of its arm of the lever 20, will give as a product 

 200 ; and on the other hand, the arm of the lever of the 



