280 



THE PULLEY. 



From its portable form, cheapness of construction, and the facility with 

 which it may be applied in almost every situation, the pulley is one of the 

 most useful of the simple machines. The mechanical advantage, however, 

 which it appears in theory to possess, is considerably diminished in practice, 

 owing to the stiffness of the cordage and the friction of the wheels and blocks. 

 By this means, it is computed that in most cases so great a proportion as two 

 thirds of the power is lost. The pulley is much used in building, where 

 weights are to be elevated to great heights. But its most extensive applica- 

 tion is found in the rigging of ships, where almost every motion' is accom- 

 plished by its means. 



In all the examples of pulleys, we have supposed the parts of the rope sus- 

 taining the weight, and each of the moveable pulleys, to be parallel to each 

 other. If they be subject to considerable obliquity, the relative tensions of 

 the different ropes must be estimated according to the principle applied in 

 figure 6. 



