What did Sarah Guppy patent?
In 1811, Sarah Guppy patented ‘a new mode of constructing and erecting bridges and railroads without arches or sterlings whereby the danger of being washed away by floods is avoided'. The patent described a system whereby vertical rows of piles (e.g. tree trunks) were driven deep into the ground and fixed together by a frame so that they would be capable of resisting water erosion. On this secure base the bridge piers were to be built. Several chains would then be placed or secured on top and drawn tight to create a platform - and a deck built on top.
Guppy's design was almost certainly site specific to the River Avon.
Was Sarah Guppy's patent a new idea?
Bridges made from rope or chain and laid with planks were well known in South America, India and China, and a few had even been erected in Europe. In 1741 the Wynch Bridge across the River Tees in County Durham was completed. Sarah may have visited this bridge or read about it in William Hutchinson's 1794 book 'The History and Antiquities of the County Palatine of Durham'. Her bridge was of a very similar type.
Did Sarah Guppy enter a design into the competition to build a bridge across the Avon Gorge?
When Bristol wine merchant William Vick died in 1754 he left £1,000 in trust to the Society of Merchant Venturers, a guild which managed Bristol’s port. Once this investment grew to £10,000 it was to be used to pay for a new toll-free stone bridge across the Avon Gorge. Vick believed his new bridge would be “a great publick utility”, providing a broad crossing point high enough to allow tall ships to pass beneath.
Before this fund matured, several people suggested designs for the bridge, including Sarah Guppy who was 'assiduously employed in forming the model of a bridge to be erected across the Avon'. However, Guppy did not submit any of these designs when a competition to design an iron suspension bridge to span the Avon Gorge was first announced in October 1829. This may have been because the Royal Navy regulations meant that any bridge needed to be at least 30 metres (100 feet) above high water level, so the masts of tall ships could pass beneath.
How do modern suspension bridges differ from Guppy's design?
In 1801 American James Finley constructed the first modern chain suspension bridge (a flat deck suspended from overhead iron chains) – and in September 1811 his fourth suspension bridge, which crossed the Merrimack river in Newbury, Massachusetts was widely reported on in the British press. Finley is known to have constructed around 40 similar bridges.
By 1820 Captain Samuel Brown completed the Union Bridge in Tweed. It was the longest wrought iron suspension bridge for vehicles in the world and the first in the United Kingdom. Brown used long links made from iron bar rather than a chain - an idea also seen in Thomas Telford's Menai Bridge (1826). The suspension bridges which followed all used iron bar links, rather than the chains favoured by Finley and Guppy.
Did Thomas Telford use Sarah Guppy's patent?
In 1826 Thomas Telford completed the Menai Bridge. An opinion piece in the Bristol Mercury in 1839 refers back to letter written by Guppy in 1811 and states that the bridge was completed using Guppy’s patented invention without charge. It is perfectly possible that Guppy’s patent was still in force when Telford started work on Menai Bridge and the piers (or towers) of this bridge were built in the tidal straits and would have required protection from water erosion, which could have made part of Guppy’s invention useful. However, the contemporary account stated that all masonry piers were constructed directly onto bedrock, albeit with considerable difficulty against fierce tides and occasional storms.
In 1830 Thomas Telford submitted a proposal for a bridge spanning the Avon Gorge which was supported by two enormous piers decorated in the gothic style. Again, the foundations of these piers may have been intended to incorporate part of Guppy’s patent as they were to be constructed on the tidal river bank – but the scheme was rejected and Telford's design was never built.
Did Isambard Kingdom Brunel use Sarah Guppy's patent?
Isambard Kingdom Brunel’s winning design for a bridge across the Avon Gorge did not feature riverbed foundations – but was instead constructed using a 214 metre single span stretching across the gorge 75 metres above high tide. The piers were not at risk of damage from water erosion and the deck was suspended from flat wrought iron bar links, rather than resting on top of chains.
Should we dismiss Sarah Guppy?
Sarah Guppy was an extraordinarily talented, creative, highly intelligent and charismatic woman whose remarkable inventions rightly deserve our attention and recognition. Although her patent of 1811 however can in no way be applied to Clifton Suspension Bridge, Guppy was said to have made models for Brunel of his inventions and her son Thomas Guppy worked closely if informally with him on the construction of the ss Great Britain and the ss Great Western.
With thanks to Adrian Andrews and Julia Elton