260 Iron and Steel in the Construction of Ships and Bridges. 



Numerous other improvements of great importance have 

 been introduced into the iron manufacture, and .valuable 

 machinery connected with it has been invented. As an 

 example of the former may be mentioned the process of boiling, 

 and of the latter the steam hammer, without which it would be 

 impossible to mould the enormous masses of iron required for 

 ship-building and other purposes. But, of all the modifications 

 which the manufacture of iron has undergone, that of Bessemer 

 is probably the most valuable. It enables us to obtain excellent 

 steel at a cost little higher than that of iron ; and when the 

 great difference between the strength of iron and steel is con- 

 sidered, the advantage of this will easily be understood. We 

 may observe that, while the tensile strength of pure malleable 

 iron is only 18,000 lbs. per square inch, that of good steel is 

 130,000 lbs. As with the hot blast, so with the Bessemer 

 process, there are some difficulties yet to be overcome, since, 

 when steel for very important purposes is required, it must be 

 made by the old method. 



In considering the fitness of iron as a material for ships and 

 bridges, and particularly for the latter, one most dangerous 

 defect to which it is liable must be taken carefully into account, 

 namely, its tendency to crystallize, and thus to lose a very 

 large portion of its strength. It appears from Mr. Fairbairn's 

 experiments, that with constant changes of load, or with the 

 same load, but with constant disturbance of the metallic mole- 

 cules, from vibration, fracture will, in time, certainly ensue of 

 itself. And it has been ascertained that an iron girder will 

 break with about 400,000 changes of load, accompanied by 

 vibrations : a fact which but too clearly shows that the spon- 

 taneous destruction of our iron bridges is but a question of time, 

 and not even of a very long one. The explanations of this very 

 alarming fact, the danger from which is, unfortunately, not 

 even confined to bridges, but extends to the axles of locomo- 

 tives and carriages, and to the various portions of machinery, 

 which from time to time so unexpectedly give way, is found in 

 the restoration to their primitive form of the crystals which had 

 been elongated and mutually entangled by the hammering to 

 which the iron is subjected during its manufacture. 



It might be supposed that steel, from its tensile strength, 

 being so much greater than that of iron, would be, in every 

 instance, far preferable to iron. Such, however, is not the case. 

 If, for example, steel is used in the formation of a girder, its 

 thickness may be much less than if iron were used. But this 

 very circumstance, as it is almost equally liable to oxidation, 

 causes its strength to be impaired with much greater rapidity. 

 Thus, a plate of iron half an inch thick is much less weakened 

 by oxidation to the depth of the sixteenth of an inch than a 



