422 



PLAN OF SUSPENDED CENTERING. CHAP. XIX. 



constructed on the principle of suspension, being con- 

 sidered indispensable conditions of the proposed structure. 



Mr. Stephenson next considered the expediency of 

 erecting a bridge by means of suspended centering, after 

 the ingenious method proposed by Telford in 1810 ; x by 

 which the arching was to be carried out by placing 

 equal and corresponding voussoirs on opposite sides of 

 the pier at the same time, tying them together by 

 horizontal tie-bolts. The arching thus extended outwards 

 from each pier and held in equilibrium, would have 

 been connected at the crown with the extremity of the 

 arch advanced in like manner from the adjoining pier. 

 It was, however, found that this method of construc- 

 tion was not applicable at the Conway ; and it' was 

 eventually abandoned. Yarious other plans were sug- 

 gested ; but the whole question remained unsettled even 

 down to the time when the Company went before Par- 

 liament, in 1844, for power to construct the proposed 

 bridges. No existing kind of structure seemed to be 

 capable of bearing the fearful extension to which rigid 

 bridges of the necessary spans would be subjected ; and 

 some new expedient of engineering therefore became 

 necessary. 



Mr. Stephenson was then led to reconsider a design 

 which he had made in 1841 for a road bridge over the 

 river Lea at Ware, with a span of 50 feet, the 

 conditions only admitting of a platform 18 or 20 inches 

 thick. For this purpose a wrought-iron platform was 

 designed, consisting of a series of simple cells, formed 

 of boiler-plates riveted together with angle-iron. The 

 bridge was not, however, carried out after this design, 

 but was made of separate wrought-iron girders composed 

 of riveted wrought-iron plates. 2 Eecurring to his first 



1 See ' Lives of the Engineers,' vol. 

 ii. p. 445. It appears that Mr. Fair- 

 bairn suggested this idea in his letter 

 to Mr. Stephenson, dated the 3rd 

 June, 1845, accompanied by a draw- 



ing. See his ' Account of the Con- 

 struction of the Britannia and Conway 

 Tubular Bridges, &c.' London, 1849. 

 2 Eobert Stephenson's narrative of 

 the early history of the design, in 



