168 



MECHANICS 



wheels of a clock are allowed to turn slowly by means of 

 a pendulum. Now we are going to see why a clock can 

 be regulated by the length of its pendulum. 



Experiment 70. The Pendulum. 

 Apparatus: String, stones of various sizes, rule. 

 a. Hang up a small stone by a string 25 centi- 

 meters long and count the number of swings per minute. 

 Tie on a large stone in place of the small stone, keeping 1 

 the length of the pendulum the same. Does the weight 

 of the stone make any difference in the number of swings 

 per minute? The time that it takes a pendulum to swing" 

 from one side and return again to its 

 first position is called the time of 

 vibration. 



b. Hang up a stone, making the 

 string 100 centimeters long and count 

 the vibrations per minute. How does 

 the number compare with the result 

 obtained in (a) above? If you want a 

 clock to go faster would you lengthen 

 or shorten the pendulum? The illustra- 

 tion shows a clock pendulum. The pend- 

 ulum is kept in motion by the push oi 

 the projections on the wheel, but the 

 time is regulated by the length of the 

 pendulum. Do you think that a clock 

 would go faster or slower in hot 

 weather? Explain. See Section 18. 



65. The Lever. 



A stick, supported at a point between 



