These crabs were subsequently used at the floating of the Saltash trusses, and at the launch of the ‘Great Eastern.’
A similar arrangement was applied in the paying-out machinery of the Atlantic cable, and is still used for the picking-up gear in the ‘Great Eastern.’
[103] See Encyclopædia Britannica, ‘Iron Bridges,’ vol. xii. p. 601; Molinos et Pronnier, Construction des Ponts Métalliques, p. 320, pl. 23, 24, 25.
[104] These would have been magnificent specimens of timber-work, and a design for somewhat similar trusses had at one time been prepared for the Chepstow Bridge. It is worthy of mention that Sir Isambard Brunel at one time designed a timber arched bridge of 800 feet span to cross the Neva at St. Petersburg.
[105] Before the work was begun Mr. Brunel made calculations to determine whether or not it might be desirable to cross the river with one span of 850 feet, in order to avoid the great depth at the centre pier.
A few extracts from letters relating to bridges of large span will be interesting:
‘January 31, 1852.
‘I have revised my calculation as to a span of 1,000 feet, and find that even with the loads and limitations of strains which I adopt—namely a proper thickness of ballast, and a possible load of a train of engines without tenders, and a limitation under such a load of 5 tons’ strain per square inch, that a span of 1,000 feet may be made in England of the very best workmanship, and sent out and erected for I should say safely 250,000l., of course a single way—another 250,000l. ought, I should think, to cover the rest of the bridge.
I should like to explain to you the mode I should propose for raising such a bridge, weighing 7,000 tons.
‘December 1, 1852.