MECHANICS. 



VII. 



8. How is equilibrium maintained with 

 this machine ? 



9. Give the rule for finding the equili- 

 brium of this machine. 



10. How is the effect of this machine 

 governed 1 



11. Do we gain or lose by increasing 

 the size of a wheel t 



LESSON VII. 



BEFORE we can understand the pulley, we must consider what it consists of. Now, 

 if we examine a pulley attentively, we shall see that it is nothing more than a wheel 

 and axle, with the multiplied cord ; so that, in fact, it is a compound machine. It is 

 therefore evident, that we must understand what the multiplied cord really is, before 

 we can consider the uses and mode of action of the pulley. The multiplied cord has 

 sometimes been called the funicular machine, or rope machine, the term being derived 

 from the Latin word funis, a rope. You observe that I rest the ends of this rope upon 

 the backs of two chairs. [Experiment 4.] It is not in a straight line, and will not be 

 so, whatever force may be employed in stretching it, because its own weight will 

 prevent it. Suppose that I hang a pound weight in the centre of the rope [performs 

 the Experiment], the chairs, which are the resisting points, are dragged towards each 

 other ; thus proving, that a very small force, if properly applied, will be sufficient to 

 overcome a great resistance a practical fact that seamen avail themselves of when 

 bracing their sails. We know that ropes are not perfectly flexible, and that, as their 

 strength increases, their rigidity is in proportion ; if they were flexible, we should be 

 able to dispense with the assistance of the pulley in many instances. For example, if 

 we could bend a rope over a sharp edge (F), as in Fig. 22, and 

 the rope could be moved without friction, we should be enabled 

 to transmit a force in any one direction, overcome resistance, 

 or impart motion in another direction. The force P 0, could 

 be transmitted to T W, so as to support or overcome the resist- 

 ance (W) ; or, a motion in the direction of O P could be 

 made to produce another motion in the direction W T. But, as 

 the cord is not perfectly flexible, the angle sharp, and friction 

 exists, the force employed would cause the cord to snap ; and, 

 therefore, we find it is necessary to employ a curved surface for 

 If the weight is only to be sustained, then a simple curved 



surface will answer the purpose ; but if the cord has to move over the curved surface, it 

 will be subject to friction, and corresponding wear; therefore, it has been found more 

 advisable to cause the surface on which the rope runs to move with it, which is the 

 case in the common pulley. 



QUESTIONS. 



Fig. 22, 



the cord to pass over. 



83. T. What is a pulley 1 



P. It consists of a round thin disc or 

 wheel grooved at its edges, called a sheave, 

 turning upon an axis or pivot, passing 

 through its centre at right angles with its 

 plane, the whole being enclosed in a case 

 called a block. The ropes and cords are 



called a tackle ; and, when the whole 

 machine is complete, and in working order, 

 it is called a block and tackle. 



84. T. How are pulleys divided 1 

 P. Into fixed and movable. 



85. J. What are fixed pulleys 1 



