CHAP. XIX. A TUBULAR BEAM PROPOSED. 423 



idea of this bridge, Mr. Stephenson thought that a stiff 

 platform might be constructed, with sides of strongly 

 trussed frame-work of wrought-iron, braced together 

 at top and bottom with plates of like material riveted 

 together with angle-iron, after a method adopted by 

 Mr. Eendel in stiffening the suspension bridge at Mon- 

 trose with wooden trellis-work a few years before ; 

 and that such platform might be suspended by strong 

 chains on either side to give it increased security. " It 

 was now," says Mr. Stephenson, " that I came to regard 

 the tubular platform as a beam, and that the chains 

 should be looked upon as auxiliaries." It appeared 

 to him, nevertheless, that without a system of dia- 

 gonal struts inside, which of course would have pre- 

 vented the passage of trains through it, this kind of 

 structure was ill-suited for maintaining its form, and 

 would be very liable to become lozenge-shaped. Be- 

 sides, the rectangular figure was deemed objection- 

 able, from the large surface which it presented to the 

 wind. 



It then occurred to him that circular or elliptical 

 tubes might better answer the intended purpose ; and 

 in March, 1845, he gave instructions to two of his 

 assistants to prepare drawings of such a structure, the 

 tubes being made with a double thickness of plate at 

 top and bottom. The results of the calculations made 

 as to the strength of such a tube, were considered so 

 satisfactory, that Mr. Stephenson says he determined 

 to fall back on a bridge of this description, on the 

 rejection of his design of the two cast-iron arches 

 by the Parliamentary Committee. Indeed, it became 

 evident that a tubular wrought-iron beam was the only 

 structure which combined the necessary strength and 

 stability for a railway, with the conditions deemed 



Edwin Clark's ' Britannia and Conway Tubular Bridges,' vol. i. p. 25. Lon- 

 don, 1850. 



