62 



MECHAiaCS. 



Fig. 66, 



\llllllllllllllHlllllHlliiliili 



Where a cord is passed over a single fixed Avheel, as in 

 fig. Q6, or over two or more wheels, no power is gained, 

 the moving force being the same in 

 iliiiiiiiliiliillll'iiii'J/ velocity as the weight. Such pulleys 

 are sometimes, however, of use by 

 altering the direction of the force. 

 The latter is applied with advantage 

 to unloading or pitching hay by 

 means of a horse-power, saving much 

 time and labor, as explained on a 

 future page. 



Among the many applications of 

 the pulley, one is shown in the ac- 

 companying figure (fig. 67) rep- 

 resenting Packer's Stone Lifter^ for 

 raising large boulders from the soil, 

 weighing from one to four and five 

 tons, and afterwards placing them in 

 walls. It is also employed for tearing out small or partly 

 decayed stumps. 



The usefulness of tho pulley depends mainly upon its 

 lightness and port- 

 able form, and the 

 facility with which 

 it may be made 

 to operate in al- 

 most any situation. 

 Hence it is much 

 used in building, 

 and is extensively 

 applied in the rig- 

 ging of ships. In 



Pulley Willi no ina-ease of 

 jMwer. 



Fiff. 07 



Packer's Stone Lifter. 



the computation of its power there is a large drawback, 

 not taken into account in the preceding calculation, which 

 mateiially lessens its advantage; this is the friction of 

 the wheels and blocks and the stiffness of the cordage, 



