involved in the Construction of Artillery. 161 



t 



inches in diameter, consisting of a spongy mass of scarcely coherent crystals of 

 cast-iron, usually in arborescent masses, made up of octohedral crystals ; the whole 

 so loose that upon a newly-cut section dark cavities can be seen by the naked 

 eye in all directions, out of which often, single or grouped crystals can be picked 

 with the hand, and so soft that a sharp-pointed chisel of steel may be easily 

 driven into the mass some inches, as if into lead or soft stone. It fortunately 

 happens that in pieces of artillery a large portion of this defective core of spongy 

 metal is removed in the process of boring out ; but where the hollow taken out 

 thus, does not extend very close back to the exterior of the breech — in other 

 words, where the thickness of the breech in the line of the axis is considerable, 

 u portion of the spongy uncompact metal is left remaining, and forms the part 

 of the gun at the bottom of the bore or chamber. This is most remarkably 

 the fact in large mortars. An absurd adherence to a false analogy with long 

 guns, or to antiquated routine, compels all mortars in our service still to be 

 cast solid, and then bored out. The diameter of the mass is great in a 13-inch 

 sea mortar — about 3 feet 4 inches. The mass of metal left below the chamber 

 when bored out, is immense, and quite useless. The effect of both is, that every 

 mortar has got a " soft spot," just at the bottom of the chamber, and extending 

 downwards from it in the line of the axis ; and that every part of the chase of 

 mortar is in a state of violent molecular strain, from the consolidation of its 

 external walls, long before the interior portions ; and hence weakness in the 

 whole piece. 



45. Fig. 1, Plate in., is a section in line of axis, and plane of trunnions, of a 

 13-in. sea mortar, with the head of metal remaining attached, and the whole in 

 the position in which the mortar is usually cast, with the parts to be cut off and to 

 be bored out marked by and above a black line, exterior to which is the finished 

 mortar. The shaded central portions represent the weak and porous parts of the 

 metal about the axis, extending down, it will be observed, below the bottom of 

 the chamber, where it leaves a soft spot, easily hammered and burnt away, by 

 the shock and blaze of the powder. From the conditions of internal strain already 

 explained, the exterior of the cylinder is in a state of compression, and the 

 interior in a state of tension, a state (as we shall show hereafter) precisely the 

 reverse of that calculated to give the metal its greatest power of resistance to in- 

 ternal strains in the direction of the radius. 



