HOW THE AIR MOVES AND HOW MUCH WE NEED 1 7 



however, the air grows cooler as it leaves the lamp, and 

 the feather comes down again. 



Breathe on your hand, and you will see that your 

 breath is warmer than the air in the room. You will 

 know from this that your breath rises, too. 



Since all this warm air goes to the top of the room as 

 fast as it can, the air up there must be warmer than it is 

 on the floor. To find 

 out about this, you 

 might set a thermom- 

 eter on the floor for 

 fifteen minutes and see 

 how high the mercury 

 goes. Now put the 

 same thermometer on 

 top of the bookshelves 

 for another fifteen min- 

 utes and look at the mercury again. In winter you will find 

 quite a difference between the two places, for at that time 

 hot air is sent into the room and it rises to the ceiling first. 



In summer, w r hen the windows are open, there is not 

 so much difference between floor and ceiling air as during 

 the winter. 



I stopped my writing just now to make an experiment 

 in the warm room where I sit. I opened the door into 

 the cold hall a narrow crack, then lighted a match and 

 held it close to the crack in the door, near the floor. 



DRAFT INTO THE ROOM 



