THE PULLEY. 



273 



power to balance or overcome a great weight. In directing the power against 

 the resistance, it is often necessary to use two fixed pulleys. Thus, in eleva- 

 ting a weight A, fig. 3, to the summit of a building, by the strength of a horse 

 moving below, two fixed pulleys, B and C, may be used. The rope is carried 

 from A over the pulley B ; the rope passes, and returning downward, is brought 

 under C, and finally drawn by the animal on the horizontal plane. In the same 

 manner sails are spread, and flags hoisted on the yards and masts of a ship, by 

 sailors pulling a rope on the deck. 



Fig. 4. 



( By means of the fixed pulley a man may raise himself to a considerable 

 height, or descend to any proposed depth. If he be placed in a chair or 

 bucket attached to one end of a rope, which is carried over a fixed pulley, by 

 laying hold of this rope on the other side, as represented in fig. 4, he may, at 

 will, descend to a depth equal to half of the entire length of the rope, by con- 

 tinually yielding rope on the one side, and depressing the bucket or chair by 

 his weight on the other. Fire-escapes have been constructed on this principle, 

 the fixed pulley being attached to some part of the building. 



A single moveable pulley is represented in fig. 5. A cord is carried from a 

 fixed point F, and, passing through a block B, attached to a weight W, passes 

 over a fixed pulley C, the power being applied at P. We shall first suppose 

 the parts of the cord on each side the wheel B to be parallel ; in this case, the 

 whole weight W being sustained by the parts of the cords B C and B F, 

 and these parts being equally stretched, each must sustain half the weight, 

 which is therefore the tension of the cord. This tension is resisted by the 

 power at P, which must therefore be equal to half the weight. In this ma- 

 chine, therefore, the weight is twice the power. 



Fig. 5. 



Fig. 6. 



If the parts of the cord B C and B F be not parallel, as in fig. 6, a greater 

 power than half the weight is therefore necessary to sustain it. To determine 

 the power necessary to support a given weight, in this case take the line B 

 A In the vertical direction, consisting of as many inches as the weight consists 

 of ounces ; from A draw A D parallel to B C, and A E parallel to B F ; the 

 VOL,. II. 1 8 



