HOT WATER 



21 



warm for a while. So long as the water in the radiators is 

 warmer than the room they give off heat to the room, while 

 in hot-air heating the extinction of the central source of heat 

 causes the immediate removal of the hot-air supply, and hence 

 the immediate loss of heat. 



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

 ing simple experiment. Two flasks and two tubes are 

 arranged as in Figure 15, 

 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 circulation of 

 the colored liquid and the 

 upward circulation of the 

 clear water. If we rep- 

 resent a boiler by B, radi- 

 ators by the coiled tube, 

 and by C, we shall have 

 a very fair illustration of 

 the principle of a hot- 

 water heating system. 

 The hot water in the radi- 

 ators cools and, in cool- 

 ing, gives up its heat to 



FlG. 15. The principle of .hot-water heating. 



the rooms and thus warms 

 them. 



This system does not ventilate the rooms, since the radiators 

 are closed pipes containing hot water. It is largely for this 

 reason that thoughtful people are careful to raise windows at 

 intervals. Some systems of heating secure ventilation by 



