THE FARM HOME 



313 



your thumb or the palm of your hand over the top of the 

 bottle and allow the bottle to cool. Cooling may be hastened 

 by putting the bottle in cold water, keeping your thumb 

 or hand firmly over the top. As the bottle and the air it 

 contains cool, you will feel a pressure on the back of your 

 thumb or hand, which will seem to be sucked into the 

 bottle. This fact indicates that, as the air in the bottle 



Figure 135. — Method of ventilating a Figure 136. — Method of ventilating a 



schoolroom when fresh air is taken schoolroom when fresh air is taken 



in through the wall near the ceiling. in under the stove, where it is heat- 



Air movement indicated by arrows. ed, then diffused through the room. 



Arrows indicate movement of air. 



cools, it contracts and occupies less space. It is evident, 

 then, that cool air occupies less space than warm air. 



How Air Moves. — Because air expands when it becomes 

 warm, it is Ughter in proportion to its bulk. The air moves 

 somewhat as a pair of balances, the heavier end going down 

 and the lighter end going up. When some of the air is 

 lieated and some remains cool, it gets out of balance, and 

 the heavier cool air pressing down around the lighter warm 

 air forces the warm air upward. Hence, what we call 

 draught in the chimney is caused by the heavier cool air 

 outside forcing, by its greater weight, the light, warm air 

 upward. 



Object of Ventilation. — The problem in ventilation is 

 not to remove the warm air from within buildings and let 

 in cool air, but to remove foul air and replace it with fresh 

 air without causing a draught. Thus the principles explain- 



