THE CONQUEST OF NATURE 



lively cheap to manufacture, it is frequently used in 

 the construction of safes and burglar-proof gratings. 

 For this purpose, however, it is sometimes combined in 

 alternate layers with soft wrought iron, the steel resist- 

 ing the point of the drill, while the iron furnishes the 

 necessary elasticity to resist the blows of the sledge. 

 The bars used in modern jails and prisons are often 

 made in a similar manner of alternate sheaths of iron 

 and chrome steel. Against the time-honored "hack- 

 saw/ 7 the bugbear of prison officials for generations, 

 such bars an inch and a quarter in diameter offer an 

 almost insurmountable obstacle ; and they are equally 

 effective against a heavy sledge hammer. 



At least one case is recorded in which the use of 

 these "composite" bars resulted in a disastrous fire 

 in a prison. A small blaze having started in the base- 

 ment of this prison, attempts to reach it with a stream 

 of water were defeated by the bars of the steel gratings 

 at the windows, which would not admit the nozzle of 

 the hose. A corps of men armed with hack-saws, 

 crow-bars, and sledges attacked this grating, which, 

 if made of ordinary steel, could have been readily 

 broken. But against these composite bars they pro- 

 duced no appreciable effect. Meanwhile the fire 

 gained rapidly, threatening the building and its eight 

 hundred inmates, and was only checked after holes 

 had been made through fire-proof floors and ceilings 

 for admitting the nozzle. 



Manganese steel is peculiar in becoming ductile 

 by sudden cooling, and brittle on cooling slowly 

 precisely the reverse of ordinary steel. It contains about 



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