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A remarkable single span masonry bridge constructed by the self-taught William Edwards and completed in 1755 or 1756.
William Edwards had contracted to build a bridge over the River Taff at what is now Pontypridd. His first attempt had three arches, and was destroyed as a result of flooding, which brought down large trees and debris and effectively dammed the river.
The second bridge failed due to the weight of the haunches pushing the arch upwards at the crown. Edwards perceived that he needed to reduce the weight here, so the final bridge had lightening holes.
Some sources say that the present bridge represents the fourth attempt. An authoritative source says that the second (of four) collapsed when a flood carried away the wooden centering during construction.
With its single span of 140 ft., it is believed to have held the record, until the end of the 18th century, as the longest span for a masonry bridge in Europe, exceeding that of the Rialto Bridge in Venice by 40 ft.
The height of the arch, which adds to its impressive appearance, resulted in a steepness of ascent and descent which were serious barriers to heavy horse-drawn traffic. The bridge was also narrow, at 11 ft. As built, the approaches were lower, adding to the gradient problem.
Visitors will note that the parapet walls become thicker towards the crown of the bridge. A writer in 1893 recorded: 'It is known that the weight of the haunches was the cause of previous failure, that the cylindrical holes were introduced for the lightening of that part; but increasing the weight upon the centre of the arch [by increasing the wall thickness] to add to stability is certainly the outcome of a genius. It may be that, while building, the tendency to lift was noticed, and. being too late to adopt any other structural arrangement, it was a necessity to do so; but, be that as it may, the increased thickness gave a reason for believing there was a fourth bridge.' The writer added that 'besides the cylindrical holes, the spaces between them were left hollow and filled up with charcoal'.. [The charcoal claim has since been questioned].