THE KNEE-JOINT POWER. 



Fig. 138. 



159 



Atkmb Automaton Reaper in operation 



the bunches more frequently where the crop is heavy, 

 or more remotely where it is light. 



SECTION IV. 



THE KNEE-JOINT POWER APPLIED TO MACHINES. 



The knee-joint or toggle-joint is usually regarded 

 Fig. 130. as a compound lever, and consists of two 

 rods cormected hy a tm-ning joint, as rep- 

 resented in Fig. 139. The outer end of 

 one of the levers is fixed to a soUd beam, 

 and the other connected with a movable 

 block. When the joint a is forced in the'' 

 direction indicated by the arrow, it pro- 

 Knee-jomt power, duces a powcrful prcssurc upon the mov- 

 able block, which increases as the lever approaches a 

 straight hne. This is easily understood by the rule of 

 virtual velocities, for the force moves with a velocity 

 many times greater than the power given to the block, 

 and this relative difference increases as the joint is 

 made straighter. 



This power is made use of in the lever printing-press, 

 where the greatest force is given just as the pressure 

 is completed. Another example occurs in the Lever 



