Page:Cassier's Magazine Volume XV.djvu/19

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which the additional new main girders required were run out and lowered.

These arrangements applied only so far as portions of the old structure were used in the new one. In the centre gap of the old bridge from which the main girders fell, and were consequently destroyed, another arrangement had to be adopted. This consisted in floating the girders out from land, and placing them on the new piers at a low level from which they were afterwards raised, span by span, by hydraulic power to their proper position. Temporary columns were used for carrying the weight while they were being raised, and the system adopted was to raise them by hydraulic jacks, resting on girders secured by pins to the columns, step by step, as if one were rising on a ladder. As these girders were raised into position, the iron piers on which they were supported were built underneath, so that when the girders were ultimately at their final level, the weight was transferred from the temporary columns to the main piers. This bridge was begun early in 1882 and completed in June, 1887, and after being most carefully and thoroughly tested by the British Board of Trade, was opened for traffic immediately thereafter.

Not long after the Tay Bridge was begun, Mr. Arrol secured the contract for the Forth Bridge. In this great work the design adopted was that technically known as the cantilever-and-central-span. The cantilevers are supported from main steel piers founded on each bank of the river, with a third, resting on the island of Inchgarvie, equidistant from those on the banks. The cantilever and central girder span is not claimed by the designer, Sir Benjamin Baker, as new in principle, though it is well within the facts of the case to state that no structure approaching the importance of the Forth bridge had been previously constructed on this principle.

The novelty consisted both in the vastness of the structure itself, and in the design of the many and various portions of which it is composed.