96 Common Science 



down on it from above, it expands. So the air that rises 

 high and expands gets very cold. Consequently moun- 

 tains which reach up into this high, cold air are snow 

 Covered all the year round ; and aviators who fly high 

 suffer keenly from the cold. There are several reasons 

 for this coldness of the high air. This is just one of them. 



Application 26. Explain why air usually cools when it 

 rises; why high mountain tops are always covered with 

 snow. 



Inference Exercise 

 Explain the following : 



131. You should not fill a teakettle brim full of cold water when 



you are going to put it on the stove. 



132. It is harder to erase an ink mark than a pencil mark. 



133. Bearings of good watches, where there is constant rubbing 



on the parts, are made of very hard jewels. 



134. You feel lighter for an instant when you are in an elevator 



which starts down suddenly. 



135. When men lay cement sidewalks, they almost always make 



cracks across them every few feet. 



136. To cool hot coffee one sometimes blows on it. 



137. It is much easier to turn the latch of a door with the knob 



than with the spindle when the knob is off. 



138. Patent-leather shoes do not soil as easily as plain leather 



shoes. 



139. We use rubber bands to hold things together tightly. 



140. As air goes up it usually cools. 



SECTION 17. Freezing and melting. 



When water freezes in a pipe, why does the pipe burst? 

 What is liquid air ? 



Why does not the wire in an electric lamp melt when it is 

 red hot? 



Suppose we looked at a piece of ice through the imag- 

 inary microscope that shows us the molecules. The 



