CHAP. XII. TELFORD'S PROPOSED MEN AT BRIDGE. 



445 



over the Strait : one at the S willy Bock, consisting of 

 three cast iron arches of 260 feet span, with a stone 

 arch of 100 feet span between each two iron ones, to 

 resist their lateral thrust ; and another at Ynys-y-moch, 

 to which he himself attached the preference, consisting 

 of a single cast iron arch of 500 feet span, the crown 

 of the arch to be 100 feet above high water of spring 

 tides, and the breadth of the roadway to be 40 feet. 



TELFORD'S PROPOSED CAST IRON BRIDGE. 



The principal objection taken to this plan by engineers 

 generally, was the supposed difficulty of erecting a proper 

 centering to support the arch during construction ; and 

 the mode by which Mr. Telford proposed to overcome 

 this may be cited in illustration of his ready ingenuity 

 under such circumstances. He proposed to suspend the 

 centering from above instead of supporting it from below 

 in the usual manner a contrivance afterwards revived 

 by another very skilful engineer, the late Mr. Brunei. 

 Frames, fifty feet high, were to be erected on the top of 

 the abutments, and on these strong blocks, or rollers and 

 chains, were to be fixed, by means of which, and by the 

 aid of windlasses and other mechanical powers, each 

 separate piece of centering was to be raised into, and 

 suspended in, its proper place. Mr. Telford regarded 



PROPOSED PLAN OF SUSPENDED CENTERING. 



