142 BREATHING THROUGH THE MOUTH. 



to have at least some indirect heat, so that the fresh air in- 

 troduced will be sufficiently heated. 



If the introduction of air is thus provided for, it is then 

 safe to put on double windows and make the cracks around 

 the door very tight. Without any special provision for the 

 renewal of the air, these cracks are the means of safety. 



In houses heated by furnaces, steam, or hot water, the floor 

 is likely to be warmer from the escape of heat from the heater 

 itself, and from pipes or air ducts under the floor. 



There is a very common misunderstanding as to the cold 

 felt near a window in cold weather. It seems that air is enter- 

 ing ; but a little reflection will show that even if the window 

 were air-tight this effect would be produced, for the air near 

 the window is cooled by losing heat to the outer air. The air 

 next to the window, thus cooled, is heavier, and falls to the 

 floor ; and if there is any source of heat in the room, this cold 

 air will pass along the floor to that source of heat, up from 

 the heating body to the ceiling, and across the ceiling, and so 

 on around again. There may thus be currents without any 

 appreciable change in the quality of the air. It is economy 

 to use double windows, and prevent the loss of heat through 

 the glass. So both economy and comfort suggest to us that 

 we reduce as much as possible cracks around doors and win- 

 dows, use double windows, make vestibules at entrances, and 

 build special ducts by which fresh air may enter, and heat it 

 properly on its way in. 



Breathing through the Mouth. We should breathe 

 through the nose, and not through the mouth. The nasal 

 passages are fitted for the introduction of the air (1) by being 

 narrow, but with an extended area ; (2) the lining membrane 

 is richly supplied with blood and (3) secretes an abundant sup- 

 ply of mucus. The air, coming through this narrow channel, is 

 warmed, and a large part of any dust it may contain is caught 



