2 4 



HEAT 



The principle of hot-water heating is shown by the following 

 simple experiment. Two flasks and two tubes are arranged as 

 in Figure 13, the upper flask containing a colored liquid and the 

 lower flask, clear water. If heat is applied to B, one can see at the 



end of a few seconds the 

 downward movement of 

 the colored liquid and the 

 upward movement of the 

 clear water. If we repre- 

 sent a boiler by B, a 

 radiator by the coiled 

 tube, and a safety tank by 

 C, we shall have a very 

 fair illustration of the 

 principle of a hot-water 

 heating system. The hot 

 water in the radiators 

 cools and, in cooling, 

 gives up its heat to the 

 rooms and warms them. 

 In hot-water heating sys- 

 tems, fresh air is not 

 brought to the rooms, for 

 the radiators are closed 

 pipes containing hot 



FIG. 13. The principle of hot-water heating. 



water (Fig. 14). For this reason it is necessary to raise win- 

 dows at intervals. 



Some systems of hot-water heating secure ventilation by con- 

 fining the radiators to the basement, to which cold air from 

 outside is constantly admitted in such a way that it circulates 

 over the radiators and becomes strongly heated. This warm 

 fresh air then passes through ordinary flues to the rooms above. 

 In Figure 15, a radiator is shown in a boxlike structure in 



