THE PULLEY. 



277 



undue proportion of the weight, while other parts become nearly, and some- 

 times altogether slack. Besides these defects, the rope is so liable to derange- 

 ment by being thrown out of the grooves, that the pulley can scarcely be con- 

 sidered portable. 



For these and other reasons, this machine, ingenious as it unquestionably 

 is, has never been extensively used. 



In the several systems of pulleys just explained, the hook tO ( which the fixed 

 block is attached supports the entire of both the power and 'weight. When 

 the machine is in equilibrium, the power only supports so much of the weight 

 as is equal to the tension of the cord, all the remainder of the weight being 

 thrown on the fixed point. 



If the power be moved so as to raise the weight, it will move with a velocity 

 as many times greater than that of the weight, as the weight itself is greater 

 than the power. Thus in fig. 7, if the weight attached to the lower block as- 

 cend one foot, six feet of line will pass over the pulley A, according to what 

 has been already proved. Thus the power will descend through six feet, 

 while the weight rises one foot. But, in this case, the weight is six times the 

 power. 



When two or more ropes are used, pulleys may be combined in various ways 

 so as to produce any degree of mechanical effect. If to any of the systems 

 already described, a single moveable pulley be added, the power of the ma- 

 chine would be doubled. In this case, the second rope is attached to the hook 

 of the lower block, as in fig. 11, and, being carried through a moveable pulley 



Fig. 11. 



Fig. 12. 



attached to the weight, it is finally brought up to a fixed point. The tension 

 of the second cord is equal to half the weight ; and therefore the power P, by 

 means of the first cord, will have only half the tension which it would have if 

 the weight were attached to the lower block. A moveable pulley thus applied 

 is called a runner. 



Two systems of pulleys, called Spanish bartons, having each two ropes, are 

 represented in fig. 12. The tension of the rope P A B C in the first system 

 is equal to the power ; and therefore the parts B A and B C support a portion 

 of the weight equal to twice the power. The rope E A supports the tensions 

 of A P and A B ; and therefore the tension of A E D is twice the power. 

 Thus the united tensions of the ropes which support the pulley B is four times 



