302 HOW MACHINES LIGHTEN LABOR 



Pulleys. The pulley, another of the machines, is merely 

 a grooved wheel around which a cord passes. It is sometimes 

 more convenient to move a load in one direction rather than in 

 another, and the pulley in its simplest form enables us to do 

 this. In order to raise a flag to the top of a mast, it is not 

 necessary to climb the mast, and so pull up 

 the flag. The same result is accomplished 

 by attaching the flag to a movable string, 

 somewhat as in Figure 170, and pulling from 

 below. As the string is pulled down, the 

 flag rises. 



If we employ a stationary pulley, as in 

 Figure 170, the force required to balance the 

 load is as large as the load itself. The only 

 advantage is that a force in one direction 

 produces motion in another direction. Such 

 a pulley is known as a fixed pulley. 



Movable pulleys. By the use of a mov- 

 able pulley, we are able to support a heavy 

 weight by a small force. In Figure 171, the 

 spring balance supports only one half the 

 entire load, the remaining half being borne 

 by the hook to which the string is attached. 



Kia. 170. By means of The weight is divided equally between the 

 a fixed pulley, a force r i_ !_ i. 



in one direction pro- two P arts ot the string which passes around 



duces motion in the the pulley, so that each strand bears only 



opposite direction. i i r r i i i 



one half of the burden. 



We have seen in our study of the lever and the inclined 

 plane that an increase in force is always accompanied by a 

 decrease in distance, and in the case of the pulley we naturally 

 look for a similar result. If you raise the balance (Fig. 171) 12 

 feet, you will find that the weight rises only 6 feet; if you 

 raise the balance 24 inches, you will find that the weight rises 



