Fire and Heat 



97 



becomes hot in due course. The result is not due to conduction 

 of heat, which has just been considered, but to another method 

 of heat transfer. 



Recall the facts observed in the experiment with the box 

 and candle, and in those experiments illustrating the changes 

 in unit volume of air under the influence of heat and cold. 

 You recognize at once 

 that the air in the box 

 near the burning candle 

 became warm, expanded, 

 and was then forced 

 away by the colder and 

 denser parts of the air 

 in and surrounding the 

 box. The same thing 

 occurs in the case of the 

 warming of the air of a 

 room by fireplace or 

 stove. The air adjacent 

 to the fire becomes warm, 

 expands, and loses dens- 

 ity. It is then forced 

 upward by the denser 

 part of the air, thus 

 promoting a circulation 

 that eventually equalizes 

 the temperature of all 

 the air of the room. The 

 heat is thus seen to be 

 carried away from the 

 fire by the air currents. 



FIG. 30. Showing convection currents in 

 heating a house with a furnace. Trace the 

 course of the warm air to and through each 

 room illustrated. 



Exercise : Convection currents in air. By means of the smoke 

 from a burning joss stick trace the course of air currents in a room 

 heated by a stove. They will be found to follow a course similar 

 to that of the currents produced in the box-and-candle experiment. 



