134 ZOOLOGY. 



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), if 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 



Fig. 87. 



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

 power (c d) being only equal to 10, thia must be multiplied 

 by a force equal to 20 to give the same product, 200. Finally, 

 by placing the former still nearer to the point of support at d, 

 a force must be given, but equal to 100, for its arm of 

 the lever will no longer be more than 2, and 2 X 100 = 200. 

 The disposition of the levers has as much influence over 

 the rapidity of the movements produced as over their force ; 

 and if by employing a power comparatively feeble we may 

 thus overcome a much stronger resistance, we may also, by 

 employing moving force having a certain quickness, obtain, 



