184 GREENHOUSES 



slight pressure of the column of water in the 

 system may permit the water in the boiler to 

 reach a temperature slightly above 212 de- 

 grees. If water can be kept under pressure it 

 may be raised to almost any desired tempera- 

 ture, and in a heating system this would mean 

 less necessary radiating surface. The boil- 

 ing point of water under various pressures 

 above normal or atmospheric pressures is 

 shown in the following table: 



Pounds pressure Boiling point 



Normal 212.0 Fahr. 



y 2 pound 213.7 



1 " 215.3 



2 " 218.5 



3 " 221.5 



4 " '....224.4 



5 " 227.1 



6 " 229.7 



10 " 240.0 



Several systems have been evolved to pro- 

 duce pressure in a heating system. One of 

 the earliest was the closed tank system in 

 which the expansion tank was made air-tight 

 and fitted with a safety valve set so as to let 

 the air in the tank escape at a certain pres- 

 sure. By this means the water in the coils 

 may be made to reach a temperature con- 

 siderably above the boiling point. 



