64 



HEAT 



of distributing heat, but depends on the other two ways. 

 The air near a stove becomes heated by conduction and 

 radiation. As a result it expands, and becomes lighter. 

 While a cubic foot of air at 32 F. (0 C.) weighs about 1.2 

 oz., at 80 F. (27 C.) it weighs 1.1 oz. The lighter air 

 then rises to make room for the cooler 

 air, which flows in to take its place 

 (Fig. 222, 274) . Thus ' ' convection 

 currents" are set up, and the air 

 rises while it is warm and descends 

 again when it has become cooled. 



' ' Hot air " furnaces heat our houses 

 (Fig. 56) because the warmed air 

 flows upward through the "registers " 

 to make room for the cold air taken 

 in at the bottom of the furnace (' ' cold 

 air intake" ;c/- 248). 



FIG. 56. 



Convection Currents from 

 a Furnace. 



When air is cooled, convection currents are also set in motion. 

 Because of its greater density the cold air falls, and warm air flows in 

 to take its place. Hence in ice-boxes (Fig. 57) the waste water is not 

 allowed to drop directly into the air, but is controlled by a "trap" 

 which permits only the waste water, and not 

 the cold air, to flow out at the bottom of the 

 refrigerator. 



Convection currents can also be seen 

 when a liquid is heated and cooled, if bits 

 of paper or sawdust are put into the 

 liquid. Hot water heating of houses de- 

 pends upon such currents (cf. 243). 

 Convection currents, and the fact that water 

 is most dense at 4 C., prevent the freezing 

 of lakes and rivers except near the surface 

 (cf. 87). 



Fio. 57. 



Trap in an Ice-Box, to Pre- 

 vent Cold Air from 

 Falling Out. 



