82 ANNUAL OF SCIENTIFIC DISCOVERY. 



tained in imposing weights upon cast-iron girders of all shapes and 

 sizes, it had long been considered almost a mechanical axiom, that iron 

 possessed greater power to resist compression than extension ; whereas 

 Mr. Fairbairn's experiments, to his surprise, as well as to that of all who 

 witnessed them, most clearly demonstrated that, after bearing a certain 

 amount of weight, the resisting properties of cast and of wrought iron 

 are diametrically opposite ; in short, the results, in figures, prove to be 

 nearly as follows : 



" Cast-iron can resist, per square inch, 



Compression of from 35 to 49 tons. 



Extension of ' ; 3 to 7 " 

 " Wrought-iron can resist, per square inch, 



Compression of frpm 12 to 13 tons. 



Extension of " 16 to 18 " 



" The unexpected results thus obtained were of incalculable prac- 

 tical value, for, if the preliminary experiments proposed by Mr. 

 Stephenson had not been made, all the eminent engineers and mathe- 

 maticians of the present day would, on the correct principle of every- 

 where adjusting the thickness of iron to the force it has to resist, have 

 erroneously concurred in recommending that the proposed wrought- 

 iron tubes for crossing the Conway and Menai Straits should be con- 

 structed stronger at bottom than at top, instead of, as it appears they 

 ought to be, stronger at top than at bottom, in consequence of which 

 error the aerial gallery would have been improperly weakened in one 

 part, by an amount of iron which would have unscientifically overloaded 

 it at another. 



" By continuing, with great patience and ability, the experiments 

 above referred to, it was finally ascertained that the relative strength of 

 wrought-iron in the top and bottom of the tubes should be in the pro- 

 portion of about 5 to 4 ; and whereas, had they been constructed of 

 cast-iron, these proportions would have been reversed in the higher pro- 

 portion of nearly 5 to 1, it may reasonably be asked why, if the latter 

 bears compression so much better than the former, it was not selected 

 for the top of the tube? In theory, this adjustment of the two metals 

 to the force which each was peculiarly competent to resist would have 

 been perfectly correct. It, however, could not practically be effected, 

 from the difficulty of casting as well as of connecting together plates 10 

 and 12 feet in length, of the very slight thickness required. Mr. 

 Stephenson, therefore, adhered to his determination to make the whole 

 of his aerial galleries of wrought-iron ; and we may here observe, that, 

 to ensure the public from accident, he further resolved, that the amount 

 of the force of extension upon them should be limited to only one 

 third of their power of resistance, that of compression to one half, 

 the reason of the difference being that, inasmuch as any little flaw in 

 the iron would infinitely more impair its power to resist extension than 

 compression, it wa.s evidently safer to approximate the limits of the latter 

 than of the former. 



" As the exact strength of a hollow wrought-iron tube, such as was 

 proposed, was unknown to engineers, it w T as deemed necessary by 

 Mr. Stephenson that its form, as well as the disposition of its mate- 



