148 GENERAL SCIENCE 



of the house. After losing some of its heat the air 

 returns to the furnace through the cold-air duct, where 

 it is mixed with fresh air from the outside and re- 

 heated. Many furnaces have no fresh-air inlet. The 

 air in houses having this sort of furnace is almost 

 certain to have many impurities in it, since 

 the only sources of fresh air are the cracks 

 around the doors and windows. The air 

 which feeds the fire does not reach the 

 rooms, but passes out the chimney, as 

 indicated by the arrows. After the fire is 

 well started the damper should be closed, 

 since a great deal of heat is lost by way 

 of the chimney. 



Hot- water Heating. Figure 128 shows 

 an arrangement which illustrates the prin- 

 ciple of hot-water heating. The whole 

 FiG.i28~ Con- apparatus is filled with water, the water 

 vection Currents in ^ ^ U pp er ves sel being colored. When 



L JulQUlQ* 



heat is applied to the lower vessel, con- 

 vection currents will be produced, as indicated by the 

 arrows. 



There are several different ways of arranging hot-water 

 systems. One arrangement is shown in Figure 129. The 

 water is heated in the jacket around the furnace A and 

 rises to the reservoir R, returning through pipes pp' by 

 way of the radiators BB f . The circulation in this system 

 is maintained in the same way as in the apparatus shown 

 in Figure 128. 



In modern buildings a system known as the " direct- 

 indirect " is now much used. In this system fresh-air 

 ducts lead to coils heated by steam or hot water. These 

 coils heat the air, which by convection is carried to the 



