Gravitation 



29 



19. You may use a kind of soap that is full of invisible little air 



bubbles ; if you do, the soap will float on top of the water. 



20. When you drop a dry dishcloth into water, it floats until all 



the pores are filled with water ; then it sinks. 



SECTION 5. How things are kept from toppling over: 

 Stability. 



Why is it harder to keep your balance on stilts than on 

 your feet? 



Why does a rowboat tip over more easily if you stand up in it ? 



In Pisa, Italy, there is 

 a beautiful marble bell 

 tower which leans over 

 as if it were just about 

 to fall to the ground. 

 Yet it has stood in this 

 position for hundreds 

 of years and has never 

 given a sign of top- 

 pling. The foundations 

 on which it rested sank 

 down into the ground on 

 one side while the tower 

 was being built (it took 

 over 200 years to build 

 it), and this made it tip. 

 But the men who were 

 building it evidently 

 felt sure that it would 

 not fall over in spite of 

 its tipping. They knew 



the law of stability. 



FIG. 13. The Leaning Tower of Pisa. 



