INTERESTING THINGS ABOUT THE AIR 



19 



It will also be found of interest in this connection to de- 

 termine the reason for the circulation of water in the hot- 

 water pipes by which a building is heated. 



17. Chimneys. The main purpose of a chimney is to supply 

 a draft ; that is, to cause the air to pass rapidly through the 



Fio. 18. Diagrams of two types of refrigerators 



Air passes downward over the ice to the bottom of the box, then upward and back 

 to the ice again. Where should articles be placed to be kept at lowest temperatures? 



fire and make it burn more vigorously. The chimney must 

 also remove the smoke. Since the air in the chimney is heated, 

 it is not so heavy as the outside air. The outside air there- 

 fore pushes through the furnace into the chimney and crowds 

 the warmer air upward with sufficient current to enable it to 

 carry the smoke with it. But the incoming cold air is heated 

 as it passes through the fire, so that it is lighter than the 



