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trains came upon it. Nevertheless, lie thought that rigidity suf- 
ficient to prevent this might be introduced in the platform, whilst 
the bridge still derived its main support from the chains. “ If 
the platform be made rigid, then I think the suspension prin- 
ciple may be applied.” So he stated before the Parliamentary 
Committee in May, 1845. This stiffening of the platform had 
to some extent been already accomphshed in' America, and by Mr. 
Rendel, at Montrose, by means of wooden trussing. Their plans 
were considered, but rejected, and Mr. Stephenson’s great merit 
consisted in choosing a better material and selecting a better form. 
The rigidity of tubes had long been known ; he seized on the 
fact, and applied it to solve his problem , — “ there was no way” 
he found, “ so simple, so cheap, or so rigid, as throwing iron- 
work into the form of a tube.” 
The idea of a rigid suspension-bridge was both original and 
valuable. Probably it was rightly abandoned in the case of the 
Britannia Bridge, because neither the experience nor the expe- 
rimental knowledge necessary for carrying it out were then 
attainable, yet attention is now again directed to it, and one of 
the most remarkable railway bridges in existence is that which 
Mr. Roebling has erected over the Falls of Niagara, of a span 
of 821 feet, and combining rigid iron girders and suspension- 
chains. Indeed, there are only two ways in which we can much 
exceed our present spans, — either by the introduction of a 
stronger material than wrought iron,* or by the adoption of the 
suspension principle under some modified form. 
But even whilst giving his evidence in May, 1845, Mr. Ste- 
phenson was wavering in his adherence to this principle. Still 
believing the chains to be necessary in the erection of the 
bridges and advisable as a permanent precautionary measure, 
he had begun to think of throwing the chief part of the 
strength of the bridge into the tube itself. Having abandoned 
the principle of the arch, he was faltering in his attachment to 
that of suspension, and was progressing towards the adoption 
of that of the simple beam or girder. This change in his views 
he attributes three times in the course of his parliamentary 
evidence to information supplied by Mr. Fairbairn, whom he 
had called to his aid about a month before. 
Mr. Fairbairn had been engaged in mon-ship building, and 
had known cases in which iron vessels of 250 feet in length 
had been stranded at the extremities and left suspended, pre- 
cisely in the condition of a bridge, with 1,000 to 1,200 tons of 
machinery in the middle, yet without injury. Relying on these 
* Attempts have already been made to introduce milled steel for this pur- 
pose ; and there is great probability of its being ultimately adopted when, by 
the new processes of manufacture, its price is sufficiently reduced. 
