involved in the Construction of Artillery. 153 



r. Upon what circumstances the more or less complete homogeneity 

 of the crystalline alloy we call cast-iron depends, when cast into 

 guns ; and, therefore, how with the best, or with any, external furm, 

 we may most avoid the formation of " planes of weakness," so far 

 as the moulding and casting are concerned. 

 2°. The effects due to the contraction of the metal in process of cooling, 

 and of sudden changes of mass or of dimension and form upon 

 this. 

 3°. The effects of rapid and of slow cooling, and of unequal cooling, 

 4^ The effects of casting under the fluid pressure due to increased "head" 

 of molten metal. 

 And to add a few remarks upon the presumed relative advantages, so much and 

 so loosely talked of latterly, of cold-blast and hot-blast iron, and of foreign 

 and British iron, as materials for ordnance. 



24. It is known to every practical iron-founder upon a large scale, that, gene- 

 rally, the larger the mass of the casting he makes with any given quality of cast-iron, 

 the " coarser is the grain," that is, the larger are the crystals that develop them- 

 selves in the mass. The same metal that shall produce a fracture, bright gray, 

 matted, and without a crystal visible even to a single lens, in a bar, cast, say, two 

 inches diameter, shall, if cast into a cylinder of two feet in diameter, produce a 

 dark, confusedly crystalline surface of fracture, as coarse as granite rock. 



To meet this, the practice is to prescribe for material for large castings a 

 certain large proportion or mixture of " small, close-grained scrap metal," with the 

 pig-iron, of whatever best quality may be denoted. The remedy fails — as fail that 

 always must which is founded upon a misconception of the laws of the pheno- 

 mena. As well might small seeds be sown to produce small trees. The small 

 scrap is no sooner recast into the large mass than it resumes the large crystalline 

 grain. 



25. The experiments of Mr. Fairbairn (Trans. Brit. Ass., 1853) on the re- 

 peated melting of the same cast-iron, by casting into inch-square bars, are con- 

 cluded by him to prove that the grain of the metal and the physical qualities 

 of the casting improve by some function of the number of meltings; and he 

 fixes on the thirteenth melting as that of greatest strength. 



