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An account of the construction of the Britannia and Conway tubular bridges : with a complete history of their progress, from the conception of the original idea, to the conclusion of the elaborate experiments which determined the exact form and mode of construction ultimately adopted / by William Fairbairn
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10

CORRESPONDENCE.

of the experiments, and the letter of August Gth, addressed asbefore to Mr. Stephenson. This letter, it will be observed,strongly enforces the striking and unmistakeable evidence, affordedby the experiments, of the necessity of a close adherence to theprinciple of the simple beam or girder.

Up to this period, my object had been to test the principle,originally suggested by Mr .Stephenson, of a structure, every partof which, although rigid, should be brought into a state of ten-sion, and whose strength should consist, not as that of a beam orgirder does, in its resistance to extension on the one side, and tocompression on the other, but in a resistance to extension onboth sides. For the adoption of such a form, if it could havebeen found, there was this plausible argument,—that the tenacityof wrought-iron being much greater than its resistance to com-pression, there would obviously have been an economy of thematerial, in so shaping the tube as to call into action its tenacityonly. All my attempts to find such a shape as this were howeverfruitless. Every experiment gave the most certain evidence ofa compression on the one side of the tube and extension on theother, and it yielded alike when the one resistance or the otherwas overcome. From this time the question presented itselftherefore under a simpler form. I looked upon the tube as ahollow girder, whose strength was dependent upon the samecauses as that of any other, and I saw plainly the direction myexperiments should take and the principle by which I was to beguided. That determined opposition, which I shall be foundin this correspondence to have given to the use of chains, orany other flexible auxiliary for the support of the tube, datesfrom this period.

My dear Sir, Millwall , August 6th, 1845.

For the last eight days, I have been constantly employed on the ex-periments, and although some of them have not always indicated theresults expected, they are nevertheless not only useful, as regards theobject of our research, but highly satisfactory.

 
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