64 PRACTICAL LESSONS IN SCIENCE. 



the fulcrum. Levers are divided into three classes, on the relative 

 positions of the fulcrum and the points of application of the 

 power and weight. If the fulcrum is between the points of appli- 

 tion it is a lever of the first class, as in Fig. 3. The ordinary 



steelyard and beam balances are illus- 

 p ?\ " trations of levers of this class. If the 



weight is between the power and the 



fulcrum, the lever is of the second class, as in Fig. 4. The 

 ordinary crowbar, when used for prying, is a lever of the first 

 class; when used for lifting, it is a lever of the second class. Two 

 persons carrying a weight hung from a pole are using a lever of 

 the second class, and a wheelbarrow is an illustration of a lever 



of this class. If the power is between P p 



the weight and fulcrum the lever is of Fio^ 



the third class, as in Fig. 5. The common fire tongs, sugar 

 tongs and sheep shears are pairs of levers of this class. When 

 we lift a weight with the hand the forearm is a lever of the third 

 class; the fulcrum is at the elbow and the power is applied 

 through the tendons which are inserted between the fulcrum and 



weight. Levers may be combined in 



p- ~ ~ w various ways. Fig. 6 shows a com- 



bination of a lever of the first class 

 and one of the second class. 



A lever consists of two arms along arm or power arm, which 

 is the distance from the fulcrum to the power, and a short arm 

 or weight arm which is the distance between the fulcrum and the 

 weight. Thus in Fig. 3, PF is the long arm and WF the short 

 arm, which is shorter than the power w * _* 

 arm, while in Fig. 5, the power arm, f""1 JTF 



PT ~~w r 



PF, is shorter than the weight arm A ' ^ 



WF. It may be shown by geometry 



that the distances PF and WF in Fig. 3, have the same relations 

 to each other as the perpendicular distances through which the 

 power and weight, respectively, move, so that work will equal 

 the product of the power by its arm, and the product of the 

 weight by its arm, and the forces will be in equilibrium when 



