14 ELEMENTS OF GENERAL SCIENCE 



in a room containing a heated stove or radiator. Since the 

 air in the vicinity of the source of heat is lighter, volume 

 for volume, than that in the other parts of the room, it fol- 

 lows that the lower air near the stove does not have so much 

 weight resting upon it as does that in other parts of the room. 

 It is therefore pushed upward and finally reaches the ceiling, 

 when it spreads out, is cooled, and descends slowly in the 

 cooler parts of the room, only to flow across the floor toward 

 the source of heat, where it will again be warmed and be 

 pushed up. Thus the circulation continues. Currents of air 

 that are caused by differences of temperature, such as the cur- 

 rents of which we have been speaking, are called convection 

 currents. 



12. Hot-air furnaces. The circulation of air in hot-air heat- 

 ing apparatus such as is commonly used in private houses is 

 maintained by convection currents. The heater is located in 

 the lowest part of the house, usually the basement, and is sur- 

 rounded by a jacket which incloses a heating chamber. Cold 

 air is admitted to this chamber from outdoors through a large 

 pipe, is expanded by heating, is pushed up through the pipes 

 leading from the top (why ?) of the air chamber to the various 

 rooms of the house, and escapes out of doors through available 

 openings. In some cases the apparatus is so arranged as to 

 take cold air from within the house, as from the front hall, 

 instead of from without, but this arrangement is not desir- 

 able, as in this case the same air is passed through the rooms 

 repeatedly, while by the other arrangement fresh air from out- 

 doors is supplied continually. It will also be found of interest 

 in this connection to determine the reason for the circulation 

 of water in the pipes of hot-water heating plants. 



13. Chimneys. The main purpose of a chimney is to sup- 

 ply a draft ; that is, to cause the air to pass rapidly through 

 the fire and make it burn more vigorously. The chimney must 

 also remove the smoke. Since the air in the chimney is heated, 

 it is not so heavy as the outside air. The outside air, therefore, 



