184 ON SUPERHEATED STEAM. 



are curious in such matters, I must refer to ele- 

 mentary works on tlie subject; and tlieir name 

 is legion. My essay, first of all, is rather upon 

 the practical, theoretical, and dangerous operation 

 of boiling the usual water under high pressure, 

 and also the effects produced upon the material of 

 the boilers. 



Water, then, is the first body with which we 

 have to deal. And indeed a wonderful and mighty 

 agent it is ! Who can tell of all its wonderful, its 

 beneficent, its mighty agency ? — no one but the 

 all- wise Creator of the universe. We have here 

 to do with only one of its manifold elements of 

 power. This water, then, in the boiler, is a liquid 

 which expands with heat into seventeen hundred 

 times its volume, and forms steam, the lightest of 

 all vapours. Up to a certain point, the increase 

 and pressure are nearly uniform, and very safe 

 and controllable : this point of safety is at about 

 50 lbs., or rather more than three additional 

 atmospheres — 15 lbs., or one atmosphere, always 

 being upon it in its first condition. Now beyond 

 this point of pressure, I venture to call it pre- 

 sumption to carry it, except as a matter of 

 necessity, as the laws which regulate its pace 

 upwards are unknown. A steam-boiler is like a 

 horse which is well under control and manage- 

 ment up to ten miles an hour ; but once let it go 

 faster, or run away, and no one can tell at what 

 moment an accident may happen. The Table of 



