ON SUPERHEATED STEAM. 



187 



wearing away that part, and also because there 

 are two degrees of temperature there, where the 

 water and steam divide, and which is due to the 

 different heat-conducting properties of steam and 

 water ; the fire in the flue acting partly on steam 

 and partly on water, which weakens the metal 

 rapidly. In passing, I would observe, another 

 element of danger is the pumping-in of cold water : 

 this might be avoided. Another cause of explosion 

 may be found in the earthy deposit alone, which, 

 from being a bad conductor of heat, is not pre- 

 vented by the water from getting red hot, which 

 thus repels the water ; but steam will pass to it 

 and be the medium to enable it to act upon the 

 water at a moment when a sudden alteration of 

 pressure takes place, and which is assisted also by 

 some of the impurities of the water, to send it 

 into steam Avithout regard to apparent pressure. 

 Boilers are found to burst in a variety of ways : 

 I once saw a boiler that burst from its place, and 

 passed through the side of the building, falling 

 outside, eighty yards off. 



I incidentally mentioned that water mechanically 

 wore away the iron of the boilers, and this is a 

 point not oflen alluded to ; but that it does so, a 

 very Httle reflection will convince any one of the 

 fact ; and be it remembered also, that the friction 

 ofthe constant boiling of water on the iron is 

 greatly more, in consequence of the earthy impuri- 

 ties which it contains. 



