112 THE ATMOSPHERE AND ITS SERVICE TO MAN 



part of the room. A fan placed as near the ceiling as 

 possible will force this warm air down to a lower level, and 

 in this way make all parts of the room more nearly uniform 

 in temperature. This often proves an effective remedy for 

 cold floors. In winter the air near windowpanes is often 

 reduced below its dew point and films of ice form inside 

 the panes. This can be prevented by using a fan to keep 

 a fresh supply .of warm air moving across the glass. Most 

 merchants have learned to apply this principle in keeping 

 their display windows clear in severe weather. 



Ventilation. The movement of air caused by its heat- 

 ing and cooling provides a means for ventilating rooms and 

 buildings in winter. In warm weather we do not have to 

 be persuaded to keep our windows open; but when winter 

 comes, many people become careless about ventilating their 

 houses. Health requires that a person have pure, normally 

 moist air to breathe. Sleeping rooms as well as living 

 rooms must be constantly supplied with outdoor air. The 

 old notion that night air was harmful is contrary to the 

 truth. Fresh air day and night is essential to the main- 

 tenance of health. 



Several ways have been devised for ventilating large 

 buildings and for maintaining proper air conditions, but 

 these require mechanical means for driving or for draw- 

 ing the air into the building, and are not suitable for 

 dwellings. 



Houses heated by hot-air furnaces in which the cold air 

 flue is properly cared for (Figure 56) need only a provision 

 for the exit of hot, stale air. An open grate or fireplace in 

 which there is a fire, or a window in each room opened slightly 

 at the top will accomplish this. 



