involved in the Construction of Artillery. 249 



in continually changing directions, and on various portions of the mass ; the 

 directions of maximum pressure within it, as constantly change, as do the inten- 

 sities of these pressures, not only in depth, but as transferred from point to 

 point struck. The elasticity of the metal (thougli, no doubt, of a different sort, 

 from its elasticity of rigidity when cold) still exists, but in diiferent parts of the 

 mass, is kept during the hammering, and perhaps for long after, in a state of vari- 

 able instability. 



The lines of least pressure, therefore, are constantly changing under all these 

 varying causes, and with them the directions of the principal axes of the crys- 

 tals, become changed and changed again, perturbed, broken, and confused ; and 

 if the mass be sufficiently large when cold, and its forging completed, its fracture, 

 however fine and good the wrought-iron, presents nothing but a confused mass 

 of small crystalline facets, differing scarcely at all, except in brightness, lirom 

 the appearance of that of bright-gray cast-iron, in moderately large castings. 



205. Yet no change, other than that of molecular arrangement, has necessarily 

 occurred in the large mass, for it is a fact, that such a confusedly crystallized 

 mass may be built and " faggotted up" from small rolled bars, each of which is 

 previously perfectly and uniformly fibrous ; that they lose their fibrous struc- 

 ture, and assume the confusedly crystalline one in the process of being united 

 by forging into one large mass, and that a portion broken or cut off from the 

 mass may be again rolled down into small bars, which shall be as fibrous in 

 structure as at first. 



206. The difference of ultimate tenacity, however, due to this mere charge 

 of molecular arrangement, is formidable. If the original bars of the " faggot" 

 have a tenacity represented by 46, that of an equal section cut from the " faggot" 

 will be only 38 ; and it will mount again up to 52 in the small bars rolled or 

 forged down out of the faggot. Such were the results of actual experiments 

 in America (Note Q). 



207. The development in size of crystal varies with the particular sort of 

 iron: it appears to be largest and most lamellar (in large masses) in the most 

 highly refined iron, and which contains an unusual dose of silicium ; but the rela- 

 tions of size of crystal to chemical constitution require much further examination. 

 Wohler, in a most interesting paper, " Sur la Crystallization du Fer" (Ann. de 

 them. t. li. p. 206), describes cubic crystals with perfect faces as large as an 



