CONSTRUCTION OF THE TUBE.

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sisting strain, the strong plates in every case being further from thecentre or vertical axis of the beam.

I am, my dear Sir, yours faithfully,

Robert Stephenson, Esq. William Fairbairn.

P.S. If the 7" iron for the vertical sides could berolled in this form, I should prefer it.

Fig. 31.

The distribution of the material, the size of the plates, andthe methods to be pursued for putting them together, becamematter of considerable importance ; and much time and thoughtwere devoted to the consideration of this part of the subject.The stiffness of the top, the strength of the bottom, and the re-tention of form in the sides, required the utmost attention ; andno skill, however perfect, could have saved it from failure, unlessgoverned by the facts and appearances presented in the expe-riments, which were on every occasion the unerring guide.

The great object to be attained was, that the resistance ofevery part of the tube should be so duly proportioned as to con-tribute its full share of resistance to strain when supportingitself, and the load which might be placed upon it. The sides,which appeared to have less work to perform than the top andbottom, were nevertheless an important part of the structure;and, owing to their great height, required some vertical supportto keep them straight and free from “ buckling.” This was ac-complished by the introduction of frames of T iron, which coveredthe joints of the vertical plates on both sides, and thus formeda hue of pillars supporting the cellular top of the tube. Theseframes were also of great value in giving union and strength toevery part of the structure, and, by their connexion with thetop and bottom platforms, the requisite degree of solidity andstiffness was attained. We are indebted to Mr. Stephenson’s

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