THERMOSTAT 



215 



Overflow Pipe- 



warm air into the house, because it sets up convection 



currents. Hot-water heating systems (Fig. 203) depend 



upon the convection currents set up in cold water 



when one part of the 



water is heated. The 



heating is done in a 



furnace, and the water 



heated, being lighter than 



the colder water in the 



pipes and radiators, rises 



to take its place. The 



cooler water flows back 



to the furnace, to be 



heated. A "standpipe" 



in the attic keeps the 



radiators full of water. 



In steam heating, a furnace 

 is used to convert water into 

 steam, and the heat given off 

 by the steam, as it cools and 

 condenses (cf. 89), warms 



the house. The hot water remaining after condensation returns to 

 the boiler to be reconverted into steam. 



244. Thermostat. We have already learned that a 

 thermometer tells us temperature by registering the 

 difference in expansion between mercury and glass (cf. 

 62). We could also make a thermometer out of two 

 metals that expand different amounts when heated 

 through the same degrees of temperature. Thus, we can 

 rivet together a small bar of iron and one of brass so that 

 the resulting compound bar is straight when cold ; but when 



Fig. 203. 

 Hot Water Heating System. 



