426 



A RECTANGULAR TUBE DETERMINED ON. CHAP. XIX. 



passage across the Straits." l Mr. Fairbairn then pro- 

 ceeded to construct a number of experimental models 

 for the purpose of testing the strength of tubes of 

 different forms. The short period which elapsed, how- 

 ever, before the bill was in committee, did not admit of 

 much progress being made with those experiments ; but 

 from the evidence in chief given by Mr. Stephenson on 

 the subject, on the 5th of May following, it appears that 

 the idea which prevailed in his mind was that of a 

 bridge with openings of 450 feet (afterwards increased 

 to 460 feet) ; with a roadway formed of a hollow 

 wrought-iron beam, about 25 feet in diameter, pre- 

 senting a rigid platform, suspended by chains. At the 

 same time, he expressed the confident opinion that a 

 tube of wrought iron would possess sufficient strength 

 and rigidity to support a railway train running inside 

 of it without the help of the chains. 



While the bill was still in progress, Mr. Fairbairn 

 proceeded with his experiments. He first tested tubes 

 of a cylindrical form, in consequence of the favourable 

 opinion entertained by Mr. Stephenson of tubes in that 

 shape, extending them subsequently to those of an ellip- 

 tical form. 2 He found tubes thus shaped more or less 

 defective, and proceeded to test those of a rectangular 

 kind. After the bill had received the royal assent on 

 the 30th of June, 1845, the directors of the company, 

 with great liberality, voted a sum for the purpose of 

 enabling the experiments to be prosecuted, and upwards 

 of 6000/. were thus expended to make the assurance 

 of their engineer doubly sure. Mr. Fairbairn' s tests 



1 * Account of the Construction of 

 the Britannia and Conway Tubular 

 Bridges.' By W. ^airbairn, C.E. Lon- 

 don, 1849. 



2 Mr. Stephenson continued to hold 

 that the elliptical tube was the right 

 idea, and that sufficient justice had 

 not been done to it. A year or two 



before his death Mr. Stephenson re- 

 marked to the author, that had the 

 same arrangement for stiffening been 

 adopted to which the oblong rectan- 

 gular tubes owe a great part of their 

 strength, a very different result would 

 have been obtained. 



