288 MUSCLES 



and a small body a long distance without varying the 

 total value of the work done. Thus with ten foot pounds 

 of energy we may raise a ten-pound weight one foot or a 

 one-pound weight ten feet. 



Men have contrived various pieces of apparatus called 

 machines by means of which weights, and the distances 

 thej?^ are moved, may be varied without changing the total 

 amount of energy required. One of the simplest of these 

 machines is the lever. 



Laws of the lever. — A lever is a rigid bar turning about 

 a fixed point or pivot. There are three classes of levers 

 known respectively as first, second, and third class levers. 



I II III 



/ ■•. ■•. •. . ', 



Fig. 124 — Lerers ; I, lever of the first class ; II, lever of the second class ; III, 

 lever of the third class ; W, weight ; F, fulcrum, or pivot ; P, power. 



In a first-class lever the pivot is between the weight to be 

 raised and the force applied to raise it. (See Fig. 127.) In 

 the second-class lever the weight is between the pivot and 

 the power, while in the third class the power is between the 

 pivot and the weight. In each of these classes the points 

 nearest the pivot evidently move a less distance than those 

 farther away, and by arranging at different distances from 

 the pivot the power and weight applied, it is possible to 

 make a smaU power balance a large weight and vice versa. 



