The extent to which this improvement may be carried has never yet been ascertained; every fresh manipulation improves its quality. The tenacity of wrought iron is best displayed in a wire, drawn out until it is not thicker than a human hair. Large masses of wrought iron are weak and spongy in geometrical progression with the mass, and the crystalline or molecular form increases with the mass. If large forgings are carefully examined, crystals will be found whose facets would produce inches of surface; as was clearly demonstrated by the bursting of a 10-inch gun at Woolwich: made, if we mistake not, by Mr. Nasmyth. Another very important cause which renders large masses of wrought iron unsound (and which was fatal in Mr. Nasmyth’s gun) is the impossibility of condensing tons of wrought iron equally all through the mass. No one has yet been able to overcome this difficulty. When the force of a blow, however great, is exerted on the surface of a mass of metal, its effect is neutralized within a few inches of the surface; condensation takes place in inverse ratio from the point of impact, and thus the effect is limited. The force which produces this condensation tends also to elongate the fibres of the metal. This elongation is greatest in the immediate vicinity of the force; the fibres in the interior of the mass are less elongated therefore than on the exterior; and the fibres in the interior of the mass being less ductile (from the cause already explained) than those on the exterior, the interior of the mass elongates, by disintegration of its fibres or crystals, and a porous open mass is thus produced, surrounded by a fibrous case. Instances of this are to be seen in broken engine-shafts and anchors; and, indeed, in all large masses of wrought iron, whether fractured by design or accident. Another cause of this defect in large masses of wrought iron, is the long continued heat to which it is necessary to expose such large forgings. The iron expands as it is heated, but it does not expand equally all through the mass; and the result of this is that the interior becomes porous and spongy: an appearance which must have been observed by every one who has operated upon large masses. The shaft of the _Leviathan_ weighs 26 tons; but, instead of resisting twenty-six times the pressure of a shaft one ton in weight, it will, from the causes already mentioned, be found unequal to half that amount. We have watched with much interest the forging of these immense shafts; and the difficulties attending the forging of this structure prove the accuracy of our reasoning on the strength of large masses of wrought iron. The weight of the shaft when finished is 26 tons, and the waste during the process of welding amounts to 74 or 75 tons.