SIMPLE MACHINES 347 



through a pulley at the top of the pole, and so run it up into posi- 

 tion, than it would be to shin up the pole and fasten it in place. 



When we use two pulleys in combination, especially if each 

 has several wheels over which the rope may run, we gain a 

 mechanical advantage. There is shown in Figure 189 a combina- 

 tion of two pulleys, each with two wheels. It is evident now 

 that the weight to be raised is supported by four strands of rope, 

 while the one you pull on in passing over the pulley merely gives, 

 as before, the advantage of a change in direction of the power 



FIG. 1 88 FIG. 189 



FIGS. 188-89: FIG. 1 88. A single-wheeled pulley. FIG. 189. Double pulleys 



applied. A fourth of the weight is borne by each strand of rope. 

 To raise the weight a given distance, the power must move through 

 four times that distance. Therefore, the power applied will be 

 only one-fourth as great as the weight plus whatever is required 

 to overcome the friction of the system. Divide, then, the weight 

 to be raised by the number of strands of rope between the pulleys 

 excepting the one to which the power is applied to obtain the 

 power required to raise the weight. If there is one wheel in 

 each pulley of such a block and tackle (as a combination of 

 pulleys is called) there will be two strands of rope not counting 

 the one on which the pull is exerted, and the weight raised will 



