HEAT 



149 



the pipes a&, &c., and the coils C, C', are nearly full of cold 

 water, and that the fire below the boiler is lighted. Heat pusses 

 through the bottom of the boiler by the process of conduction, 

 and heats the layer of water near it, which, expanding, rises and 

 passes up the tube ab to the top of the building, where it gives 

 out its heat to the rooms. The place of this water which has 



FIG. 66. Heating a large building by hot water pipes. 



thus risen is taken by cold water from the other pipe terminating 

 just past d. This in its turn gets warmed and rises, and its 

 place is taken by the water which has become cold by its passage 

 through the pipes in the various rooms. There is thus no dif- 

 ference in this case from what we have seen to be true in the 

 flask of water being heated from below. 



