120 THE PRODUCTION AND USE OF HEAT 



7. The chimney should extend higher than any other por- 

 tion of the house or surrounding buildings. If this is not the 

 case, the wind is likely to blow over the buildings and down- 

 ward into the chimney with sufficient force to ruin its draft. 



8. All openings into a chimney should be completely closed 

 except those which are in actual use. Why? 



It is rarely possible to comply with all cf these rules. If 

 many of them are disregarded, however, there should be no 

 cause to regard the poor draft as mysterious. // the laws of 

 physics have been understood and regarded in constructing the 

 chimney, and if the chimney is kept reasonably free from soot, 

 there will be a good draft upward whenever there is a fire in 

 the stove, furnace, or fireplace. NATURE'S LAWS ARE INVARIABLE. 



APPLICATION OF CONVECTION CURRENTS TO ROOM HEATING 



138. Convection Currents in a Room Heated by Means of a 

 Stove. Convection currents play an important part in all 

 heating of rooms by means of stoves. The movements of 

 air in a stove-heated room can easily be determined by 

 experiment. 



Exercise 37. Air Currents about a Stove 



Close all windows and doors. Light some punk or a piece of cot- 

 ton cloth and test the currents of air by holding the torch in the fol- 

 lowing positions and observing the movement of the smoke: First, 

 above the stove; on each side of the stove, level with the top of it; 

 on each side of the stove and about 6 in. from the floor. Second, 

 hold the torch 6 in. or 1 ft. from the ceiling and about half way from 

 the stove to the window or outside wall; do this on each side of the 

 room. Third, hold the torch about 3 ft. from the floor and about 6 

 in. or 1 ft. from the window or outside wall. Fourth, hold the torch 

 6 in. from the floor and half way from the outside walls to the stove. 

 Show by means of a sketch the air currents as you found them. If 

 some of the walls of the room are inside walls, the circulation will 

 hardly be as perfect as it would be if they were all outside walls. 



The general circulation about the stove in a room having all 

 four of its walls outside walls is very simple. There is a 

 rising column about and above the stove. As this column 



