THE SCREW. 



71 



times as great as the moving power. If it be moved 

 by a lever twenty times as long as the diameter of the 

 Fig. 76. screw, here is another increase of twenty 



times in force. Multiplying 20 by 20 

 gives 400, the whole amount gained by 

 this combination, and by which a man 

 applying one hundred pounds in force 

 could exert a pressure equal to twenty 

 tons. About one-third or one-fourth of 

 this should, however, be deducted for 

 friction. 



When the screw is combined with the 

 wheel and axle (fig. 77), it is capable of exerting great 

 power, which may be readily calculated by multiplying 

 the power of the screw and its lever into the power of 

 the Avheel and axle. 



Screw and lever 

 combined. 



THE KNEE-JOINT POWER. 



The Jcnee-jomt or toggle-joint is usually regarded as a com- 

 pound lever, and consists of two rods connected by a tum- 



1. The outer end of one of 



the levers is fixed to 



^ a solid beam, and the 



1= -" 



ing joint, as represented in fi 



Fig. TT. 



Fig. 78. 



other connected with 

 a movable block. 

 When the joint a is 

 forced in the direc- 

 tion indicated by the 

 arrow, it produces a 

 powerful pressure 

 upon the movable 

 block, which in- 

 creases as the lever approaches a straight line. This is 

 easily understood by the rule of virtual velocities, for the 

 force moves with a velocity many times greater than the 



Screw, lever, and wheel 

 ccmMned: 



Knee-joint power. 



