VENTILATION, HEAT, AND CLOTHING 



93 



change of air is very necessary where people are together 

 in a tight room. 



170. Ventilation. Changing the air of a room is called 

 ventilation. In large buildings it is often done by fans 

 and pumps, but in most buildings it is done by natural 



Diagram of the natural ventilation of a room. 

 The arrows indicate the direction of the air currents. 



currents of air. Breathed air is warmed in the lungs. 

 Warm air is lighter than cold air and so rises to the ceil- 

 ing. If the upper part of the window is opened, the 

 warmed air will pass out while the fresh air will enter by 

 cracks in the doors and lower parts of the windows. On 

 a cold day the difference between the warmed and cold 

 air is very great. This causes a strong current ; but on 



