204 



ELEMENTARY SCIENCE 



(hot-air jacket) which surrounds the fire-box, while many 

 flues lead away from it. Since air expands as it is heated, it 

 is evident that the area of the cross-section of the feed- 

 flue does not have to be so large as the combined areas of 

 the exit-flues; the right difference between them will be 

 the difference between the volume of the 

 air when it enters the hot-air jacket and its 

 volume when it leaves. Many hot-air fur- 

 naces do not work well for the reason that 

 the feed-flue is not large enough in propor- 

 tion to the exit-flues; often the efficiency 

 of such a furnace will be greatly increased 

 by adding another cold-air flue. 



Then there is the question of whether the 

 air which feeds the furnace shall be drawn 

 from outdoors or indoors. That drawn from 

 outdoors has the advantage of freshness. 

 That drawn from indoors has the advantage 

 of economy; it is already partly warmed; 

 this arrangement has the advantage of in- 

 suring better circulation of air in the rooms 

 from which it is drawn. Usually both of 

 these sources of air are used; some is drawn in from out- 

 doors, and some from the rooms above the furnace. This 

 is the reason why you find in some rooms both cold-air and 

 hot-air registers, the use of the former being simply to fur- 

 nish an exit, the draft being into rather than out of it. 



A hot-water heating system, like a hot-air system, depends 

 first on the distribution of heat by convection currents. 

 The movements of convection currents of water may be 

 nicely demonstrated by means of such an apparatus as that 

 shown in Fig. 76; this shows clearly the principle of a hot- 



Fio. 76. Diagram 

 showing the prin- 

 ciple of the hot- 

 water heating 

 system. Explain 

 this. 



