Hot Water Under Pressure. 37 



Two things can be said in favor of this system. It 

 permits of setting the boiler on the surface of the 

 ground. It can be used in the growth of such crops as 

 are benefited by overhead heat. 



CHAPTER VII. 

 HOT WATER UNDER PRESSURE. 



The third system is that of hot water under pressure, 

 and requires that both pipe and boiler be of wrought 

 iron, if the pressure is over twenty pounds.; 



Two inch gas pipes are commonly used for this and 

 they may be laid as already described, on the "down 

 hill " plan ; but for this no expansion tank is required. 



This system is practicable only when it can be con- 

 nected with water pressure, either public or private, 

 and should not be less than ten pounds as indicated by 

 a steam gauge. 



If you have city water works, carry an inch pipe from 

 the coldest part of your boiler — which is usually the 

 rear and near the bottom — connecting the same at some 

 convenient point with the hydrant. This will consti- 

 tute your expansion tank, and the amount of pressure 

 you can carry on your boiler without emptying it, will 

 be a fraction less than a gauge will indicate as being the 

 pressure of the hydrant water. 



If a higher degree of pressure is .maintained on. the 



