246 NATURE STUDY AND AGRICULTURE 



tion of the heated air near the stove ? Can you account for 

 these movements of the air ? You have already seen that 

 heat expands air. Is heated air heavier or lighter than 

 colder air ? What does the cold air really do with the light 

 air? 



Fill a tumbler two thirds full of very cold water. Heat 

 some water almost to the boiling point and put a few drops 

 of red or black ink in it. Now make a paper tube by 

 rolling up a sheet of paper. Hold this tube in the middle 

 of the glass of water with the end on the bottom of the glass. 

 Now pour some of the warm colored water into the tube. 

 . Watch it as it begins to come out at the bottom of the tube. 

 Slowly lift the tube. Why does the colored water come to 

 the top of the glass ? Why did the ice come to the top of the 

 glass of water in a former experiment ? In the same way 

 that the ice floats in the water, and the warm water floats 

 on the cold water, so the warm air floats on heavier, colder 

 air. Or we may say the cold air buoys up, or holds up, or 

 even pushes up the lighter air. Can air ever be perfectly 

 quiet in a room in which one portion is a little warmer 

 than another portion ? What will be the direction of this 

 movement, from light to heavy air, or from heavy to light? 

 With a little thinking the pupils will see that the move- 

 ment must always be from the heavy toward the light. 



How about air out of doors? What makes it move? 

 When there is a difference of temperature there must be 

 difference in pressure, hence movement of air, or wind. 

 Other things as well as temperature may make the air 

 lighter in some places than in others. But, whatever the 

 cause, difference of pressure results in movement. 



The method of heating by which the heated bodies 



