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.
The present shaft is the third which has been manufactured; the two first having proved notorious failures: thus 200 tons of iron have been wasted; which we think is sufficient proof either of the unfitness of the material, or of imperfection in the method of construction. Moreover, I fear that when the vessel encounters a rolling sea, the sudden check and strain produced by the total immersion of one paddle-wheel and the freedom of the other, will subject the present shaft to a strain which will affect its duration; and a vessel costing nearly a million of money may thus be left to reach her port with crippled powers of propulsion.
Where, it may be asked, is the skill in devising engines more powerful than the ingenuity of man can beneficially work out? This has indeed been done in the case of the Leviathan; a monster vessel has been built, but all the engineering skill expended upon it has as yet been insufficient to bring it to perfection.
The skill hitherto displayed in welding large forgings of wrought iron into shafts, or other large masses, has been of a very low order; much more may be done than has yet been accomplished, if men will only set about it in a scientific manner. The present mode of proceeding is to build a structure of iron much as a builder would raise a structure of bricks; large and small pieces being mixed together until the requisite mass is obtained.
Now, a much simpler method, and one which we have tried on several occasions, is first to construct several segments of iron of the requisite length, and of dimensions equivalent to the intended object; each segment being fitted to fill its place amongst a given number of other segments (whether twenty, forty, or fifty segments be required,) so as to form a complete cylinder; as the [wood-cut] will fully explain:—