io6 Common Science 



" Cover the pan with something ; you'll let all the heat 

 out ! " Helen said. 



" No, you want as much heat to go through the water as 

 possible. Leave the lid off so that the heat can flow through 

 easily," said Rose. 



" The water will evaporate too fast if the lid is off, and all 

 the heat will be used up in making it evaporate ; it will take 

 it much longer to get hot without the lid," Louise argued. 



" That's not right," Rose answered. " Boiling water 

 evaporates fastest of all. We want this to boil, so let it 

 evaporate ; leave the lid off." 



What should they have done? 



Application 29. Two men were about to cross a desert. 

 They had their supply of water in canvas water bags that 

 leaked just enough to keep the outside of the bags wet. 

 Naturally they wanted to keep the water as cold as possible. 



" I'm going to wrap my rubber poncho around my water 

 bag and keep the hot desert air away from the water," said 

 one. 



" I'm not. I'm going to leave mine open to the air," the 

 other said. 



Which man was right ? Why ? 



Inference Exercise 

 Explain the following : 



151. When you go up high in an elevator, you feel the pressure of 



the air in your ears. 



152. Water is always flowing into Great Salt Lake ; it has no out- 



let ; yet it is getting more nearly empty all the time. 



153. A nail sinks while a cork floats in water. 



154. Steep hillsides are paved with cobblestones instead of asphalt. 



155. If you place one wet glass tumbler inside another you can 



pull them apart only with difficulty, and frequently you 

 break the outer one in the attempt. 



156. Sausages often break their skins when they are being cooked. 



157. A drop of water splashed against a hot lamp chimney 



cracks it. 



