ADVANTAGES AND DISADVANTAGES OF HOT AIR 193 



and as it becomes hot, rises and passes through the pipes 

 to the various rooms. A study of the diagram on page 

 192 shows that nearly the same method of heat transfer 

 takes place in the furnace as in the case of the stove. 



Regulation of the air supply. It is well to remember 

 that air when heated increases in volume from five per 

 cent in cool to over twenty per cent in extremely cold 

 weather. For this reason less cold air should be let into 

 the furnace through the cold air flue in very cold weather. 

 The small slide used to regulate this volume of air (see 

 S in diagram) should be adjusted according to the weather 

 outside, otherwise much coal may be wasted in order 

 to keep the rooms warm. Some houses have a system 

 by which the warm air after circulating through the 

 rooms is sent to the furnace to be warmed again. This 

 is more economical of heat, but does not give as good 

 ventilation. 



Advantages and disadvantages of the hot-air furnace. 

 The hot-air furnace is less expensive to install than 

 either steam or hot water, is easily operated, and gives up 

 heat quickly. The disadvantages of a furnace in a large 

 residence are that the long horizontal pipes do not take 

 heat to the distant rooms leading against the force of the 

 wind. The hot-air furnace heats irregularly, and is un- 

 satisfactory on days when there is a strong wind, and after 

 it becomes old gives off much dust and some gas into the 

 house. 



Experiment. To show the principle of hot- water heating. 



Materials: Apparatus shown in diagram. 



Method: Fill the apparatus with water. This can be done by pouring 

 water into the tank at the top and opening the clamp at the top of 

 the bent tube until water fills both tubes. Close the clamp. Drop two 

 or three crystals of potassium permanganate through the rubber tube 



H.-WHIT. CIV. SCI. IN THE HOME 13 



