424 STRENGTH OF THE WROUGHT-IRON BEAM. CHAP. XIX. 



essential for the protection of the navigation. " I 

 stood," says Mr. Stephenson, "on the verge of a 

 responsibility from which, I confess, I had nearly 

 shrunk. The construction of a tubular beam of such 

 gigantic dimensions, on a platform elevated and sup- 

 ported by chains at such a height, did at first present 

 itself as a difficulty of a very formidable nature. 

 Eeflection, however, satisfied me that the principles 

 upon which the idea was founded were nothing more 

 than an extension of those daily in use in the profession 

 of the engineer. The method, moreover, of calculating 

 the strength of the structure which I had adopted, was 

 of the simplest and most elementary character ; and 

 whatever might be the form of the tube, the principle on 

 which the calculations were founded was equally ap- 

 plicable, and could not fail to lead to equally accurate 

 results." l Mr. Stephenson accordingly announced to 

 the directors of the railway that he was prepared to 

 carry out a bridge of this general description, and they 

 adopted his views, though not without considerable 

 misgivings. 



While the engineer's mind was still occupied with 

 the subject, an accident occurred to the Prince of Wales 

 iron steamship, at Blackwall, which singularly corro- 

 borated his views as to the strength of wrought-iron 

 beams of large dimensions. When this vessel was being 

 launched, the cleet on the bow gave way, in consequence 

 of the bolts breaking, and let the vessel down so that the 

 bilge came in contact with the wharf, and she remained 

 suspended between the water and the wharf for a length 

 of about 110 feet, but without any injury to the plates 

 of the ship ; satisfactorily proving the great strength of 

 this form of construction. Thus, Mr. Stephenson be- 

 came gradually confirmed in his opinion that the most 



1 Robert Stephenson's narrative in Clark's ' Britannia and Coriway Tubular 

 Bridges,' vol. i. p. 27. 



