involved in the Construction of Artillery. 191 



remainder of the muzzle. But while the extreme rigidity and high elasticity of 

 cast-iron guns, are thus valuable and important, these properties, coupled with 

 the crj'stalline structure and low coefficient of rupture of cast-iron, cari-y with 

 them a train of disadvantages. 



17. — Effects of Rigidity and repeated Discharge in Iron Guns — Limit to the 



Number of Rounds. 



108. The rigidity of the iron gun, greater in proportion as the metal is whiter 

 and harder, is such that partial distorting forces transfer themselves, to a great 

 extent, to the whole mass. The expansion of the interior of the gun, acting 

 tangentially, exercises against its rigidly resisting exterior, a powerful splitting 

 strain. The elongation of the interior of the chase, from the same cause, drags 

 or forces the exterior, to elongate along with it. The condensation by repeated 

 rounds, straining the metal of the interior beyond its elastic limit, is rapidly 

 propagated at every pulse to the exterior "couches" of material, and hence 

 gradually diminished resistance. The crystals forming the mass are at each 

 blow shaken more and more from perfect contiguity or contact, and from their 

 respective positions of molecular equilibrium, the particles of the whole mass 

 are loosened, and after a number of rounds greater or less, the gun finally fails 

 with a charge perhaps far below that of the proof, which it has many times 

 before withstood. 



Such a result is unknown, or rather impossible, with gun-metal guns, unless 

 unequally overheated, or overloaded, and simply because of their long range of 

 elastic yielding, the high value of T^, that is, of the " work done" to stretch 

 the material to any given extent. To use a popular illustration, — the molecular 

 properties of gun-metal in resisting active forces are of the same character as 

 those which are exhibited (in their extreme limits) by the flexibility and elas- 

 ticity of caoutchouc, combined with the plasticity of tempered clay ; while those 

 of cast-iron are represented in their extreme limits, by the almost perfect 

 elasticity, rigidity, and cohesion of glass, or of various amorphous or crystallized 

 minerals — quartz, for example. 



109. In the experiments made by Mr. Hodgkinson, for the Koyal Commis- 

 sion, on iron structures, it was ascertained that no cast-iron bar would sustain, 



2 c 2 



