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POPULAR SCIEiVCE MONTHLY. 



Rodman Fifteen-inch Gun. 



the eiglity-fif til round, the hollow-cast at the two hundred and fifty- 

 first round. Its endurance was thus three times that of the other. 

 Rodman's process was of fundamental importance, because it 

 established experimentally the principle of initial exterior exten- 

 sion and interior com- 

 pression. This princi- 

 ple is applied in all gun 

 construction to-day, al- 

 though the use of cast 

 iron has been wholly dis- 

 carded. Like many other 

 ideas of great importance 

 in the history of inven- 

 tion, it seems to have 

 been evolved independ- 

 ently by several claimants. The names of Blakely, Whitworth, 

 Armstrong, Longridge, Brooke, Treadwell, and Parrott are at 

 once called to mind. To describe their inventions and discuss 

 conflicting claims would require a volume. The discovery of such 

 an important principle, followed by the outbreak of the American 

 civil war, gave an impetus to the improvement of ordnance which 

 was felt over the entire world. 



Hitherto the materials used in gun construction were cast iron, 

 wrought iron, and bronze, this last being an alloy of copper with 

 ten per cent of tin. In tenacity bronze is superior to cast iron, 

 but it is softer, more fusible, and more expensive. Cast iron is 

 moderately fusible, but not fixed in composition, having a vari- 

 able amount of carbon, silica, and other impurities diffused 

 through its mass. Its properties are correspondingly variable, 

 but it is in general hard, brittle, and more or less crystalline. 

 Wrought iron is the result of oxidizing out all of the carbon by 

 puddling, then squeezing out the silica, and rolling so as to de- 

 velop a fibrous in place of crystalline structure. It is much more 

 tenacious than cast iron, almost infusible, but capable of ready 

 welding and forging. The admixture of carbon seems to confer 

 the property of fusibility. 



Steel is the product of the recombination of pure wrought iron 

 with a very small percentage of carbon and sometimes of man- 

 ganese or nickel. Like cast iron, it is fusible ; like wrought iron, 

 it can be readily forged ; and it is superior to each in elasticity 

 and tenacity. The idea long ago suggested itself that steel ought 

 to be the best material for the construction of cannon. But the 

 practical obstacle was the great difficulty of securing large enough 

 forgings of steel, and this of sufficiently good quality. Only since 

 1860 have the methods of steel manufacture been so improved as 

 to make this metal available on a large scale. 



