involved in the Construction of Artillery. 295 



wrought-iron certainly undergoes a sudden change, and the rapidity of increase 

 of the rigidity of approaching coldness, is such, that the mechanical strain 

 brought upon the metal, is likely to produce rupture, for the heat is sufficient 

 to diminish the tenacity of the material, though not to much increase its 

 ductility. Rings shrunk-on upon each other, therefore, at temperatures under 

 1100° Fahr. should be placed in an annealing oven, to cool with greater slow- 

 ness. The best practice, however, will be, to shrink-on every ring upon the 

 preceding (with the necessary allowance for external and internal diameter), at 

 a full red heat, say from 1200° to 1300° Fahr., in which case, the metal may be 

 safely permitted to cool at the ordinary rate in air, or may be even suddenly 

 cooled by plunging into water, without danger of rupture. 



294. It does not admit of question, that this general method of construction, 

 for cylinders exposed to great internal pressm'e, admits, from its practical faci- 

 lity of execution, of numerous other valuable applications, as well as to guns ; 

 for example, to the cylinders of hydraulic presses, for which wrought-iron, thus 

 applied, would afford a valuable, trustworthy, and economical substitute for 

 cast-iron, which the history of the Britannia and Conway Bridges, and many 

 other instances, have proved so impossible to rely upon. 



295. It has been stated that wrought-iron rings, thus shrunk-on at a suffi- 

 ciently elevated temperature, may be cooled suddenly with impunity. Such is, 

 in fact, the general practice with mechanical engineers in shrinking-on the tyres 

 of the wheels of locomotive engines, and of other railway wheels ; and for these 

 purposes, provided a safe amount of tenacity remain in the tyre to provide against 

 the effects of centrifugal force, and of accidental blows and strains, the harder 

 and less extensible the tyre the better (although some lamentable accidents in 

 the flying off of driving-wheel tyres prove that this is not always insured), — but 

 for application to the construction of artillery, it is never to be commended. 

 The value of a long range of extensibility, in the material for ordnance, has been 

 already fully proved. Wrought-iron will be brought into use for this purpose 

 to the most advantage, when we preserve this the greatest — when, in fact, it is in a 

 state, presenting the yielding extensibility of gun-metal, in combination with 

 the resilience and higher tenacity, which are its own. These constitute the real 

 merits of wrought-iron, as a material for ordnance, and it shares them in no 

 respect with steel, or with any other known material. 



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