140 UEATING AND VENTILATION. 



air. The heavier surrounding air presses the lighter air up- 

 ward. If there are outlets above and below, the heavier, 

 colder air will press in at any opening left below, and push 

 the lighter, warmer air out above. Thus, in the case of the 

 common stove we very well know that there are currents of 

 heated air rising above the stove. Children make whirligigs 

 and various toys to place in these up-currents above stoves. 

 Air is, at the same time, flowing toward the stove along the 

 floor and lower part of the room. Cold air can usually be 

 detected entering around the windows and doors, which presses 

 downward and toward the source of heat. The stove does not 

 do much to renew the air in the room except in this general 

 way; some heated air escapes at openings in the upper part 

 of the room, and some is passed out through the stove, taken 

 in as a draft. But in the main, the action of the heat of the 

 stove is to make a current of warm air up from the stove, 

 which current passes along the ceiling to the more distant 

 corners of the room, then descends, joining the cold air, and 

 repeating the round. 



In some cases a jacket is placed around a stove, and a duct 

 from the outer air connects with the lower part of the space 

 inside of the jacket and outside of the stove. Then as the air 

 heated by the stove rises, fresh air is drawn in from outside 

 to be warmed. In this case the direct heat from the stove 

 is shut off from the room. Heat radiates in straight lines. 

 When one holds out his hands beside a stove the heat he 

 receives is radiant heat. Most of the heat from a grate is 

 radiant heat. But in a jacketed stove the heating by air 

 currents is 'called heating by convection. Now a furnace is 

 practically a jacketed stove (almost always placed in a base- 

 ment). Furnaces have this good feature that they are all the 

 time sending fresh air into a room. 



Although in private dwellings heated by furnaces there is 



