ON THE FORM OF THE CELLS, ETC.
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deflection of only 2^ inches in the middle, was a sufficientguarantee for the lateral strength of the large tubes, and theconsequent safety of the bridge*.
Another security, opposed to the strength of the wind, is theenormous weight of the tube itself. The Conway tube (whichweighs about 1300 tons), when tested upon temporary piersand left unsupported, was exposed to one of the severest galesof last winter; and the oscillations produced on that occasion,although quite perceptible, did not exceed 1 inch.
During the progress of the experiments and investigation, ithad often been stated, that the attempt to erect tubes of 460 feetspan across the Menai Straits was chimerical, and that their ownweight would prove the principal element of their destruction.To this doctrine I did not subscribe; indeed, from later theore-tical investigations, it appears that instead of 460 feet being thelimit of the span, 1700 feet are nearer the mark at which a tubewould break with its owm w r eight. This span is however greatlybeyond practical limits, either as regards the power of obtainingthe material, or the cost of construction.
Mr. Ilodgkinson’s reported experiments on the comparativestrengths of square and circular tubes to resist a crushing force,in the shape of pillars, again opened the question as to theadmission of the latter into the top of the tubes.
The circular cells are unquestionably superior in strength,but for practical reasons hereafter adduced, it was finally deter-mined to adhere to those of the rectangular form.
My dear Sir, Manchester, December 17th, 1846.
Mr. Clarke would put you in possession of all the facts, connected withour last experiment on the model tube. After subjecting it for 18 hoursto a strain of 58 tons on the middle, it was relieved of the load and turnedover on its side, when the deflection with its own weight was found tobe 0-85 inch. In this position it was loaded with weights of about one