THE PULLEY. 



THE class of simple machines which present themselves to our attention at 

 this time, is that which is called the cord. If a rope were perfectly flexible, 

 and were capable of being bent over a sharp edge, and of moving upon it with- 

 out friction, we should be enabled by its means to make a force in any one direc- 

 tion overcome resistance, or communicate motion in any other direction. Thus 

 if P, fig. 1, be such an edge, a perfectly flexible rope passing over it would be 



Fig. i. 



capable of transmitting a force S F to a resistance Q B, so as to support or 

 overcome B, or by a motion in the direction of S F to produce another motion 

 in the direction B Q. But as no materials of which ropes can be constructed 

 can give them perfect flexibility, and as, in proportion to the strength by which 

 they are enabled to transmit force, their rigidity increases, it is necessary, in 

 practice, to adopt means to remove or mitigate those effects which attend im- 

 perfect flexibility, and which would otherwise render cords practically inappli- 

 cable as machines. 



When a cord is used to transmit a force from one direction to another, its 

 stiffness renders some force necessary in bending it over the angle P, which 

 the two directions form ; and if the angle be sharp, the exertion of such a force 

 may be attended with the rupture of the cord. If, instead of bending the rope 

 at one point over a single angle, the change of direction were produced by suc- 

 cessively deflecting it over several angles, each of which would be less sharp 



