involved in the Construction of A rtillery. 247 



and its mass great in proportion to the pressure brought upon it by the rollers. 

 The operation of rolling is then less effective in the first instance, to induce, by 

 its pressure, a general and uniform parallel arrangement in length of the prin- 

 cipal axes of the crystals; some remain in other directions to the bar's length, 

 as they were developed in the previous heating or other process of manufac- 

 ture. The bar, however, is now let to cool ; fresh internal pressures now become 

 developed by contractions within its mass ; the cooling goes on much more 

 slowly, for the mass is much greater in proportion to its surface than in the 

 long slender bai", and hence there is time for the new play offerees to act in 

 re-arranging the crystals. The heat is carried off most rapidly from the greatest 

 surfaces of the solid, but these are the sides of the bar ; the contraction is 

 greatest in the direction of its length ; the maximum pressure due to contrac- 

 tion, therefore, coincides with the length of the bar, and more or less of the 

 crystals arrange themselves now transverse to the length of the bar, in the direc- 

 tions of least pressure. 



199. Whether the crystals of iron expand and contract, by change of tempera- 

 ture, alike in all axes, is not known as yet ; if not, and that the principal axes are 

 those of greatest expansion and contraction, then, as the longitudinal contraction 

 of the whole bar is proportionally greater than that in either of its other dimen- 

 sions, so the previous longitudinal arrangement of the crystals, in so far as 

 rolling has been operative in producing it, now increases the tendency to the 

 secondary re-arrangement of the crystals, transverse to their former position. 

 The small slender bar, which cooled almost instantly and at once, fixed the 

 crystals in the longitudinal position they had assumed under the pressure of the 

 rollers ; length of time in cooling admits of the re-arrangement in the heavy thick 

 bar, aided by the softened condition of the mass, as it passes gradually from a 

 yellow heat to coldness. 



200. Thus, then, as the mass, the relation of this to form, and hence to sur- 

 face, and of all, to the pressure transmitted to the iron in rolling, and to those 

 induced subsequently by contraction in cooling, are varied, so will the main 

 directions of crystalline arrangement be in each particular instance, which may 

 be either total and complete, as in the case of the slender bar, or partial and 

 imperfect, as in the grosser bar. 



201. But the evidences of aiiy arrangement, also depend upon the extent 

 to which the individual crystals in any particular " make" of wrought-iron are 



2 k2 



