MECHANIC POWERS. 41 



THE PULLEY. 



The pulley is a small wheel turning on an axis, 

 with a drawing rope passing over it : the small 

 wheel is usually called a sheeve, and is so fixed in a 

 box, or block, as to be moveable round a pin passing 

 through its centre. 



Pulleys are of two kinds. 1. Fixed, which do 

 not move out of their places. 2. Moveable, which 

 rise and fall with the weight. 



When a pulley is fixed, as Plate 2. fig. 5. two 

 equal weights, suspended to the ends of a rope 

 passing over it, will balance each other ; for they 

 stretch the rope equally ; and if either of them be 

 pulled down through any given space, the other 

 will rise through an equal space in the same time ; 

 and consequently, as the velocities of both are 

 equal, they must balance each other. This kind 

 of pulley, therefore, gives no mechanical advan- 

 tage ; so that you can raise no greater weight by 

 it than you could do by your natural strength. Its 

 use consists in changing the direction of the 

 power, and sometimes enabling it to be applied 

 with more convenience. By it, a man may raise a 

 weight to any point, without moving from the 

 place he is in ; whereas, otherwise, he would have 

 been obliged to ascend with the weight : it also 

 enables several men together to apply their strength 

 to the weight by means of the rope. 



The moveable pulley represented at A, (Plate l 2. 

 fig. 6.) is fixed to the weight W, and rises and 

 falls with it. In comparing this to a lever, the ful- 

 crum must be considered as at A (Fig. 6.) ; the 

 weight acts upon the centre c, and the power is 

 applied at the extremity of the lever D. The power, 



