involved in the Construction of Artillery. 



363 



a plane passing through the axis, and through the centre of the vent. There were no signs 

 of unsoundness in the metal at any point, nor any defect or sign of injury, other than the 

 splitting up, save that at the centre of the bottom of each chamber, a small, irregular cavity 

 was formed, with jagged sides and bottom, as though slowly burrowed into by some cor- 

 roding agent. 



The fractured surfaces, where rusty, presented an uniform, very coarse-grained character 

 of metal; and where the latter was freshly exposed by a large fragment recently cut out, 

 close to the interior at the muzzle of each split mortar, it proved to be a mixed metal of the 

 very coarsest grain, consisting of nearly white cast-iron, filled with large grains of very 

 dark gray and highly graphitic iron, greatly wanting in homogeneity (fontc fortemcnt 

 truitee), a material ill suited to ordnance of ar^ sort. Its general appearance was somewhat 

 that of Fig. 5, Plate v., but much coarser. 



The following mortars remained together, but in what condition I was unable to judge. 



These mortars have split, I apprehend, from the conjoint effect of three separate 

 causes: — 



1°. The metal is of bad quality, — coarse, heterogeneous, and, most probably, of low 

 specific gravity and small elastic range. 



2°. A certain amount of that condensation of the material at the interior of the bore, 

 which takes place at every discharge, in every piece of ordnance, and gradu- 

 ally disables it, has, probably, taken place here ; but — 



?t^. The mortars very rapidly fired in a cold climate, have had their interiors power- 

 fully heated, and expanded thereby, while their exteriors have been kept 

 almost cold, by the heat carried off by " evection" of the surrounding currents 

 of air, in the way described in the te.xt (chaps. 9-17). The large diameter and 

 great thickness of these mortars (one caliber) has exaggerated this effect, and 

 the splitting has taken place in a plane passing through the vent, because 

 the splitting tension of the unequally expanded interior and exterior will be 

 greatest, where the difference in temperature between the interior and exterior 



