involved in the Constrtiction of Artillery. 259 



directions that the principal axes of the crystals assume at the moment that these 

 pressures disturb their previous equilibrium, that is to say, at the moment of 

 makinar the nick. 



O 



Fig. 3 shows the further change in crystalline arrangement after that 

 ' assumed by the bar, as above described, due to the " nick," and subsequently 

 produced by the bending, prior to final fracture. 



Supposing the bar bent by pressure, or blows, towards the side remote from 

 the nick, as soon as the fracture is complete, it presents a surface, as in Fig. 5, 

 consisting of facetted crystals, piercing the bar transversely from the angle of the 

 nick to a certain depth ; a central portion where the original longitudinal fibre 

 of the bar has remained unchanged, but is broken across ; and again, a narrow 

 strip of flat facetted crystals transverse to the line of fibre, at the side furthest 

 from the nick. 



Now these latter, were produced by the bendmg of the bar, after the nick 

 had produced the former. The side furthest from the nick is the compressed 

 side of the bar, the neutral axis being somewhere between. The direction of least 

 pressure at this compressed side is, therefore, transverse to the bar, and hence 

 the new direction taken up by the crystals at this point, in accordance Avith the 

 general law. Had the bar been bent towards the side nicked, in place of the 

 opposite way, the nicked side, would have been the compressed side, where the 

 transverse crystals were already formed, and the fracture, when broken across, 

 would have been fibrous out to the very edge, remote from the nick (provided 

 the whole bar had been uniformly fibrous beforehand, and nicked only at one 

 side), as shown in Fig. 4 ; and for this reason the bar would have been harder 

 to break in this direction, as every blacksmith knows to be the fact. 



222. Thus, then, this change of crystalline arrangement at ordinary tempera- 

 tures is another case of obedience to our general crystalline law, that the principal 

 axes are found in the directions of least pressure within the mass, and that the 

 change of direction is possible to be produced in " cold iron" is due to the fact, 

 of its having more or less ductility at all temperatures, which means in fact 

 that more or less permanent displacement of molecules is competent to the 

 material at any temperature. There is, therefore, no ground for anticipating 

 that wrought-iron artillery would rapidly or at all (if originally properly pro- 

 portioned) deteriorate in tenacity, and so, gradually, and yet unascertainably, 

 become unsafe in service. 



