186 



THE BEASON WHY. 



1 Better is a poor and a wise child, than an old and foolish king who will no 

 more be admonished." ECCLESIASTES iv. 





819. Why does a striped marble appear to have a greater 

 number of stripes when rolling, than ichen at rest ? 



Because the stripes are presented in rapid succession to the 

 eye ; and as the eye receives fresh impressions of stripes before 

 the previous impressions have passed away, the stripes appear 

 multiplied. 



Fig. 26. MARBLE AT REST 



Fig. 27. MARBLE ROLLING. 



820. Why does a marble rebound when dropped upon the 

 pavement ? 



Because the force of its fall to the earth compresses the atoms of 

 which the marble is composed ; and the atoms then exert the force 

 of elasticity to restore themselves to their former condition ; and 

 by the exercise of this force the marble is repelled, or thrown 

 upward from the pavement. Although a marble may be made of 

 very hard stone, yet that stone may be elastic, and possess, though 

 in a much less degree, the same kind of elasticity which causes 

 the India-rubber ball to rebound from the earth. 



821. Why does a marble, assuming it to be impelled with 

 equal force, roll further on ice than on pavement, and further 

 on pavement than on a pebble walk? 



Because the friction is greater upon pavement than upon ice, 

 and greater upon a pebble walk than upon pavement. 



822. How many forces contribute to stay the progress of 

 rolling marble ? 



