326 



THE POPULAR SCIENCE MONTHLY. 



at A ; and in the engines using coal, tbe heat of which can not be so 

 perfectly controlled as that of gas, a plug of fusible metal is set into 

 the crown-sheet, which melts when the temperature rises above a cer- 

 tain point, and allows the water to flow out and extinguish the fire. 



Fig. 5. 



The design of the boiler of the power-engine is somewhat different 

 from that of the pumping one, but the means of automatically regulat- 

 ing both the steam-pressure and the water-supply are substantially the 

 same. This engine has in addition a very ingenious governor which acts 



