J 



Pulley. 

 TpT 



and the length of this lever is 



It will hence be perceived, that when a weight is to be raised 

 by a heavy lever, employed as in figure 81, a particular length 

 is necessary in the lever, in order that the force may act to the 

 greatest advantage, and that a given effect may be produced 

 with the least possible force ; and that a greater or less length 

 would be attended with a less of power. There is accordingly 

 a difference in this respect between a heavy lever and a lever- 

 without weight. 



Of the Pulley. 



115. A pulley is a solid circle or wheel having a groove 

 farmed round its circumference, and an axis passing perpendicu- 

 larly through its centre, and through a case or frame work 

 called the 6/oc/c. The several parts taken together, are some- 

 times called the block, and sometimes simply the pulley. 



The different kinds of pulleys may be reduced to two, the 



fixed, and the moveable. 



i 

 The fixed pulley is that in which the power and the weight Fig. 82^ 



(or resistance to be overcome), are both applied according to 83> 

 directions that are tangents to the circumference of the pulley. 



In the moveable pulley, the weight or resistance is applied Fig. 84, 

 at the centre, or in a direction passing through the centre or 85 ' 8 * 

 axis of the pulley. 



This machine, considered in a general point of view, is sus- 

 ceptible of two sorts of motion ; one by which the rope passing 

 through the groove of the pulley, changes its place without alter- 

 ing the position of the body of the pulley, the other is such that 

 the body of the pulley changes its situation at the same time. 

 Thus a state of equilibrium requires two different conditions. 

 The first is that the two parts of the rope which embraces the 



