164 



GENERAL SCIENCE 



they are able to make use of the various and exhaustless 

 forces of nature. Among the simple machines employed 

 in a wide range of usefulness is the pulley which is a modified 

 form of a lever. Any full understanding of pulleys involves a 

 discussion of levers. 



When a force (P) is employed to overcome a resistance or 

 lift a weight (W) by use of a lever, it may be considered that 

 the lever is essentially a rigid bar turning about a pivot or an 

 axis. This point of support is known 

 as the fulcrum (F). It is not neces- 

 sary that levers be straight, although 

 they are usually represented in books 

 by straight lines. 



P may be on the side of the fulcrum 

 opposite W as in first class levers, or 

 W may be between F and P as in 

 second class levers, or P may be be- 

 tween F and W as in third class levers 1 . 

 The power arm (Pa) may be con- 

 sidered as a radius of the arc traversed 

 by P around F as a centre. This 

 means that the straight line represent- 

 ing the actual length of the power arm of a bent lever may 

 be largely outside the body. The weight arm (Wa) } too, 

 may lie outside the lever. 



It is worth while to represent these three kinds of levers as 

 described, using straight lines upon which lettered positions 

 are assigned for the effort, the resistance, and the fulcrum. 

 The correctness of these constructions should then be verified 

 by reference to drawings in some text in Physics. 



In all cases the movement of a lever is one around the 



1 Archimedes, the Greek philosopher (287-212 B.C.) is said to have asserted 

 that he could lift (overturn) the earth if he but had a place for a fulcrum 

 somewhere out in space off the earth's surface. 



FIG. 55. A bent lever. 

 P F, Power arm; W F, 

 weight arm. 



