THE ELEMENTS OF MACHINERY. 



103 



great weight. Thus, if we wish to apply the strength of a horse to Uft a 

 heavy weight to the top of a building, we should find it a difficult matter to 



Fig. 81. 



accomplish directl}', since the horse exerts his 

 strength mamly, and to the best advantage, in 

 drawmg horizontally ; but by changing the di- 

 rection of the power of the horse, by an ar- 

 rangement of fixed pulleys, as is represented 

 in Fig. 81, the weight is lifted most readily, 

 and the horse exerts his power to the best ad- 

 vantage. 



223. A fixed pulley is most 

 ^ifr a"pllS: 'useful for changing the direc- 

 tions of fixed tion of power, and for apply- 

 pulleys ? 

 *^ ' mg power 



advantageously. 

 By it a man standing on the ground can raise 

 a weight to the top of a builduig. A curtain, a flag, or a sail, can be readily 

 raised to an elevation by a fixed pulley, without ascending with it, by draw- 

 ing down a cord running over the pulley. 



whatisamov- 224. A MoVABLE PuLLEY differs from a 

 able puuey? g^g^j puUej in being attached to the weight ; 

 it therefore rises and falls with the weight. 



Fig. 82 represents a movable pulley, B, associated, as it Fig. 82. 

 most commonly is, with a fixed pulley, C. The movable pulley, 



E, is often called a " Runner." 

 225. In the fixed pulley, Fig. 80, it wUl be 



readily seen that to move the weight, "W, at 

 one end of the cord, passing over the pulley, a 

 greater weight must be applied at P, for if P 

 is only equal to W, they will balance one an- 

 other. I^ however, we fasten one end of the cord to a fixed support, as at 



F, Fig. 82, and pass it under the groove in the movable pulley B, to which 

 the weight, W, we desire to raise is attached, and then carry it over the fixed 

 pulley C, we may lift a force of 100 pounds at W by an application of 50 

 pounds at P. To understand tliis, we must remember that the weight W is 

 supported by the cords B F and B C on each side of the movable pulley B; 

 and as each are equally stretched, the weight must be equally divided be- 

 tween them ; or, in other word.«;, the point of support, F. sustains half tha 

 weight, and the power, P, the other half A person, therefore, puUing at I^ ' 

 will raise the weight by exerting a force equal to its half But the cord at P 

 must move through two feet to raise the weight W one foot. 



"When still greater power is required, pulleys are compounded into a system 

 containing two more single pulleys, called Blocks, and these again are com- 

 bined in a compound svi^tem of fixed and movable pulleys. 



A single movable pifiley may be so arranged that the power will sustain 

 three times its own weight Such an arrangement is represented in Fig. 83. 



What is the ad- 

 vantage gained 

 by tlie use of a 

 movable pul- 

 ley? 



