84 ANNUAL OF SCIENTIFIC DISCOVERY. 



tube as they approached or" receded from different points of support, 

 was most accurately ascertained. In these, as well as - in all the pre- 

 vious experiments, the trial tubes were loaded till they gave way. 

 From the fibrous nature of wrought-iron, as compared with the crys- 

 talline composition of the cast metal, the tendency to rupture in most 

 of these experiments was slow and progressive. Destruction was 

 never instantaneous, as in cast-iron, but it advanced gradually ; the 

 material, for some time before absolute rupture took place, emitting an 

 unmistakable warning noise. 



" Although it can mathematically be shown that the two sides of a 

 thin hollow tube are of but little use except to keep the tops and bot- 

 toms at their duty, the power of resistance of the latter being, how- 

 ever, enormously increased by the distance that separates them, it 

 was nevertheless necessary to ascertain the precise amount of lateral 

 strength necessary to prevent the aerial gallery writhing from storms 

 of wind. The riveting process was likewise subjected to severe trial, 

 as also the best form and application of the slender ribs, termed 

 ' angle-irons,' by which not only the plates were to be firmly connect- 

 ed, but the tube itself materially strengthened, in fact, the angle- 

 irons were to be its bones, the thin plate-iron covering being merely 

 its skin. 



" Mr. Stephenson had two main objects in instituting the investiga- 

 tions we have detailed. First, to determine by actual experiment 

 what amount of strength could be given to his proposed galleries ; 

 and secondly, of that maximum how much it would be proper for him 

 to exert. And as his decisions on these subjects will probably be in- 

 teresting to our readers, we will endeavour very briefly to explain the 

 calculations on which they appear to have been based. 



" As a common railway train weighs upon an average less than a 

 ton per foot, as the greatest distances between the towers of the 

 Britannia Bridge amount each to 460 feet, and as it is a \vell-known 

 mathematical axiom among builders and engineers, that any description 

 of weight spread equally along a beam produces the same strain upon 

 it as would be caused by half the said weight imposed on the centre, 

 it follows that the maximum weight which a monster train of 460 

 feet (an ordinary train averages about half that length) could at one 

 time inflict on any portion of the unsupported tube would amount to 

 460 tons over the whole surface, or to 230 tons at tiie centre. Now, 

 to insure security to the public, Mr. Stephenson, after much delibera- 

 tion, determined that the size and adjustment of the iron to be used 

 should, according to the experiments made and recorded, be such as 

 to enable the aforesaid unsupported portions of the tube (each 460 feet 

 in length) to bear no less than 4,000 tons over its whole surface, or 

 2,000 tons in the centre, being nine times greater than the amount of 

 strength necessarily required^; and as the results of the searching in- 

 vestigation which had been instituted incontestably proved that this 

 Herculean strength could be imparted to the galleries without the aid of 

 the chains, which even as an auxiliary, had been declared unnecessary, 

 and as Mr. E. Clark had very cleverly ascertained that it would 

 be cheaper to construct the tubes on the ground than on the aerial 



