256 



Introduction to the Study of Science 



Exercise. Assuming that the weight is not given, how would 

 you find it? How would you find one unknown length? Explain 

 the relation between " pull of earth " and " push of stump," Fig. 73. 



s 



PULL OF o 

 EARTH 2 

 ON MAN 



PULL OF EARTH 

 ON PLANK 

 40 LB. 



PULL OF 

 EARTH 



FIG. 73. Showing the relation of weights and arms of the lever ; and the 

 relation of the downward pull and the upward push. The weights may rep- 

 resent a man and a boy. 



To get a clear and definite idea of the principle of the lever, plan 

 and carry out a series of experiments with a meter or yardstick and 

 different weights. Make problems involving the relation of weight 

 and force and the length of the weight and 

 force arms. Study the trip balance with its 

 movable weight on the arm. Note that the 

 weights are made to apply at definite centers. 

 Study also a chemist's balance. What is the 

 importance of having the centers exceedingly 

 fine and hard? (Fig. 74.) 



Adaptations of the lever. The simple 

 form of the lever used as a lift or a pry 

 (Fig. 75) has been adapted to many dif- 

 ferent uses and converted into important 

 parts of many tools or machines. A 

 small crowbar bent somewhat at the 

 fulcrum resembles a tack puller, and by 

 a little more bending may be converted 



i nt a Claw hammer > a nail P ller ' OT ther 



similar tools. Examine some common 



FIG. 



74. A Roman 



d t arL'of 



the lever. 



