208 ELEMENTARY SCIENCE 



driven into rooms by a ventilating system is warmed first, 

 but usually there is a heating system which is independent 

 of the ventilating system, as well as co-operating with it. 

 A thermostat is a device for regulating temperature. It 

 is a great convenience. You can set the hand of a thermo- 

 stat at the desired temperature, and the heat-supply will 

 be so regulated that this temperature will be attained and 

 maintained as long as there is an adequate fire in the fur- 

 nace. In the usual type of thermostat a small metal rod 

 is attached to a curved strip of metal. The end of this 

 rod lies between two electric terminals. If the temperature 

 rises above what is desired, the rod, due to the expansion 

 of the curved metal strip, at once comes in contact with 

 one of the electric terminals, completing the circuit, and 

 the heat-supply is thereby checked. If the temperature 

 falls, the other circuit is completed, and the heat-supply is 

 increased (see Fig. 79). 



QUESTIONS 



1. What is meant by ventilation? 



2. How should a room be aired? 



3. What are the advantages and disadvantages of a fireplace? 



4. Why are stoves better than fireplaces ? 



5. What are the advantages and disadvantages of a hot-air furnace ? 



6. Why is the hot-water heating system better than the hot-air 



system ? 



7. What advantages have steam-heating plants? 



8. What is the use of a thermostat? 





