HOW THE AIR MOVES AND HOW MUCH W r E NEED 19 



Put ten drops of ink into a glass of water and it will 

 mix so quickly that you will have rather poor stuff for 

 drinking. Put in fifty drops and the water will be so 

 black that nobody would think of drinking it. This is 

 about the way that impure air from the lungs of fifty 

 children mixes with 

 pure air in a school- 

 room and spoils it for 

 breathing. As this 

 mixing goes on all the 

 time, we need to know 

 just how much fresh 

 air we ought to have 

 to keep the whole of it 

 pure enough to breathe. 



You might measure 

 something that is one 

 foot long, one foot high, 

 and one foot wide. A 

 box as big as that will hold one cubic foot of air. 



Now men who know about it say that, if possible, a 

 child should have two thousand cubic feet of fresh air 

 every hour. This means that, if you could use the box for 

 a spoon, you would have to put two thousand spoonfuls of 

 air every hour from out of doors into the room where you 

 are. More than that, you would have to make room for it 

 by dipping just as much impure air out of the room. 



THIS HOLDS ONE CUBIC FOOT 



