216 



GREENHOUSES 



the boiler is fitted with an altitude gauge, 

 which shows the height of the water in the 

 system; also with a thermometer to show the 

 temperature of the water. A valve is pro- 

 vided for draining the boiler and, if desired, 

 an automatic damper regulating device may 

 be installed. 



When used for steam 

 heating the boiler is only 

 partially filled with water, 

 and a water column and 

 guage is necessary to indi- 

 cate the height of the 

 water. A steam guage is 

 also necessary to indicate 

 the pressure; and a safety 

 valve to automatically re- 

 lieve the pressure, if it be- 

 comes too great for safety. 

 Steam boilers are usually 



Fig. 114. Water col- .... / 



umn and guage for equipped with automatic 

 steam boilers damper regulators. They 

 are rather more efficient than the regulators 

 used on hot-water boilers. A drainage valve 

 is provided the same as for hot-water boil- 

 ers. Many states require that all steam boil- 

 ers be equipped with a fusible plug, which is 

 simply a brass plug with a tin core, which 



