involved in the Construction of Artillery. 193 



of these coefficients, above the maximum stram that it is exposed 

 to in discharge ; that is to say, upon the small amount of extension 

 of its material at every round. 

 3°. Upon the uniformity of its molecular arrangement, precluding ex- 

 cessive local strains and extensions (which determine the final 

 destruction of the gun at this, as the weakest point), and upon the 

 absence of " planes of weakness." 

 4°. Upon the coefficient of velocity for force transmission, for the ma- 

 terial. 

 In every case assuming no injury done by overstraining in any discharge, 

 or by local overheating. 



Hence all absolute comparisons which neglect to take into account these 

 several conditions are fallacious, and founded on an incomplete conception of 

 the question. 



113. And from these properties it is perhaps chiefly that such capricious 

 uncertainty exists as to the number of rounds that a cast-iron gun will bear before 

 being disabled. Two guns, cast from the same metal, by the same founder, 

 apparently equal in all dimensions, of the same age, both unused, except as to 

 having stood proof, and taken from the same tier in the arsenal, when brought 

 into service with ordinary charges, the one shall stand perhaps 2000 rounds 

 without observable injury, and the other burst after one-tenth, or less, of the 

 number. How do they differ ? Not in mere ultimate cohesion ; a piece cut 

 from either may sustain almost exactly the same passive load before fracture : 

 but the value of T^ largely differs in one and in the other. The one has its 

 crystalline particles so arranged, and their constitution such, that a long range 

 of change of form must be passed through, before rupture is possible. The other 

 has its crystalline arrangement such, that it is rigid, harsh, and unyielding, 

 though not less tenacious, than the former, because it will require as great 

 a force to break it ; but it suffers more by every shock, and is sooner, so to say, 

 shaken to pieces. And hence it is, that in our arsenals and gun-foundries, the 

 attention of those in authority, has been and is, so much misdirected, in seeking 

 only for materials for ordnance of the greatest ultimate cohesion, and apparently 

 remaining ignorant of this other equally important, though less obvious or easily 

 grasped condition. It is not a little remarkable that of the three foundries 



