IiV POPULAR PHYSICS. 17 



I would lower it, since the cold of winter shortens the pen- 

 dulum, and this movement of the bob would counteract that 

 change. 



16. IVliy is the pendulum-bob generally made flat ? 



To decrease the friction of the air. 



It '. IVJiat beats off the time in a ivatch ? 



The vibration of the balance-wheel. 



IS. Is solved in the book. 



19. What should be the length of a pendulum at New 

 York to vibrate half-seconds ? 



(1 sec.) 2 : (~|- sec.) 2 :: 39.1 in. : x 9. 7 + inches. 



To vibrate quarter-seconds ? 



(1 sec.) 2 : ( sec.) 2 :: 39.1 in. : x = 2. 4 + inches. 

 To vibrate hours ? 



(1 sec.) 2 : (3600 sec.) 2 : : 39.1 in. : x = 7997.7 miles.* 



20. What is the proportionate time of vibration of two 

 pendulums, 16 and 64 inches long, respectively ? 



According to the 2d law of pendulums, 

 Time of vib. of 1st : Time of vib. of 2d : : /16 : *Qi : : 4 : 8 : : 1 : 2. 



21. Why, when you are standing erect against a ivall, 

 and a piece of money is placed between your feet, can you 

 not stoop fonvard and pick it up ? 



By leaning forward you bring the center of gravity in front 

 of your feet, and, as on account of the wall, you can not 

 throw any part of your body back to preserve the balance, you 

 fall forward. 



22. If a toiver ivere 198 feet high, with tvhat velocity 

 ivould a stone, dropped from the summit, strike the 

 ground ? 



& = 2gh (See p. 64, foot-note.) 

 v* = 64x198 

 v = 112.5 feet. 



* Nearly the diameter of the earth. 



