involved in the Construction of Artillery. 253 



depends upon the direction of the crystalline axes in relation to the strain, and that 

 the elasticity is a maximum, in the direction of the principal axis of the crystals, or 

 line of fibre; and the important deduction arises, that for artillery purposes, the 

 ultimate strength of a gun, In which the explosive strains are all resisted by wrought- 

 iron acting in the line of fibre, is to that of one acting transversely to the same, as 

 234-80 to 3047, or about 1\ to 1. This ratio expresses, in fact, the relative 

 strength of a " twist barrel," and of a common " skelp welded" or longitu- 

 dinally welded one ; and more than the whole advantage of this difference is 

 sacrificed and lost in massive forgings. 



25. — Effects of Forging into large Masses, on the useful qualities of Wrought-Iron. 



213. We have found that the effect of large increase in the mass, of wrought- 

 iron, in connexion with its necessary or existing modes of manufacture, is to 

 prevent by the process any regular or uniform arrangement of its integral crys- 

 tals ; — that as such masses are necessarily continued long heated while forging, 

 occupy long in cooling, and contract considerably in all their dimensions in 

 cooling, so the crystals are developed to a large size, and become arranged, to 

 a greater or less extent, in directions transverse to the siu-faces of external 

 contour of the mass. 



The results are irregular " planes of weakness ;" reduction of ultimate 

 strength, to resist a quietly and steadily applied tensile force of from 20 to 17, 

 or in very large masses of from 5 to 4 in round numbers, and reduction of re- 

 sisting power to such impulsive forces as are concerned with artillery, in the ratio 

 of from 7 J to 1, or probably even more ; for a train of difficulties are introduced 

 in the manufacture, and of injuries done to the chemical qualities of the material, 

 in proportion as we continue to increase the magnitude of the mass to be forged. 



214. When the mass exceeds a very moderate bulk (in breadth and thick- 

 ness), the processes of rolling, &c., are at an end, — those of forging by the tilt or 

 steam-hammer alone are available. Skilled labour, and all the mishaps to which 

 the results of the most adroit workmanship are exposed, in dealing with the heat- 

 ing and hammering of vast and scarce manageable masses, are inevitable. The 

 mass must be gradually built up and aggrandized in size, by continual welding 

 on to it, of small pieces, involving reiterated heating and partial cooling ; expo- 



VOL. XXIII. 2 L 



