28 Common Science 



all the air out and leave vacuums. They're air-tight and 

 they are strong enough to resist the air pressure." But the 

 third man said, " Why, you've got to have some air in them 

 to buoy them up. Cork would be all right, but it isn't as 

 light as air ; so air would be the best thing to fill them with." 



Which way would the floats have worked best ? 



Application 7. A little girl was telling her class about 

 icebergs. " They are very dangerous," she said, " and ships 

 are often wrecked by running into them. You see, the sun 

 melts the top off them so that all there is left is under water. 

 The sailors can't see the ice under water, and so their ships 

 run into it and are sunk." Another girl objected to this; 

 she said, " That couldn't be ; the ice would bob up as fast 

 as the top melted." " No, it wouldn't," said a boy. " If 

 that lower part wasn't heavier than water, it never would 

 have stayed under at all. And if it was heavier at the be- 

 ginning, it would still be heavier after the top melted off." 



Who was right? 



Inference Exercise 

 Explain the following : 



1 1 . When you wash dishes, a cup often floats on top of the water, 



while a plate made of the same sort of china sinks to the 

 bottom of the pan. 



12. If you put the cup in sidewise, it sinks. 



13. The water in the cup, when lying on its side, is exactly as high 



as the water in the dish pan. 



14. If you put a glass into the water, mouth first, the water cannot 



get up into the glass ; if you tip it a little, there are bubbles 

 in the water and some water enters the glass. 



15. If you let a dish slip while you are wiping it, it crashes to 



the floor. 



1 6. It is much harder to hold a large platter while you are wiping 



it than it is to hold a small butter plate. 



17. If you set a hot glass upside down on the oilcloth table 



cover, the oilcloth bulges up into it when the hot air and 

 steam shrink and leave a partial vacuum within the glass. 



18. If you spill any of the dishwater on the floor, it flattens out. 



