a navigable channel which was wide. It was manually operated for the first eight months, and then powered by a steam engine, reverting to manual operation in 1913, when the steam engine failed. The bridge was last opened in 1953, and the traverser section was demolished in 1974, but public outcry at the action resulted in the bridge being listed as a Scheduled Ancient Monument, and the rest of the bridge was kept. It was later used as a road crossing, until the construction of the Chandos road bridge alongside it, and is now only used by pedestrians. Parts of the steam engine were moved to Westonzoyland Pumping Station Museum in 1977. The bridge is now a Grade II* listed building. The main line crossed the River Parrett just south of Bridgwater by the Somerset Bridge, with a span but a rise of , half that of the Maidenhead bridge. Work started in 1838 and was completed in 1841. Brunel left the centring in place as the foundations were still settling, but in 1843 complaints that navigation was being interfered with had to be responded to. He said that "although the Arch itself is still perfect, the movement of the foundations has continued ... and the centres have, in consequence, been kept in place. [As instructed] by the Directors, measures are being adopted to enable us to remove these centres immediately, at the sacrifice of the present arch." Brunel demolished the brick arch and replaced it with a timber arch, which was in turn replaced in 1904 by a steel girder bridge. At Taunton the River Tone was straightened to avoid the need for two bridges close together. West of Taunton gradients of 1 in 80 were needed to cross the Blackdown Hills and at the summit on the Somerset-Devon border the Whiteball Tunnel was constructed. William Froude, resident engineer for the western section of the main line, developed an empirical method of setting out track transition curves and introduced an alternative design to the helicoidal skew arch bridge at Rewe and Cowley Bridge Junction, near Exeter. Topography After leaving Bristol, the main line was laid on easy curves and gradients as far as Taunton; there is a slight summit at Flax Bourton cutting the line is generally level, in part traversing the Somerset Levels, running north-west of the Mendip Hills and south-east of the Quantock Hills. From Taunton the