We will now leave the iron industry gathering momentum and return to the time when Three-times-Great Grandfather was cutting his first tooth to take up the subject of bridges and the role this new iron would play in their story and in the story of British transportation as a whole.
Let us begin by circling around the subject of bridges with the aim of giving you a view of it from every angle. It might be helpful to first put bridges in the broader context of the very many obstacles that there had always been to getting oneself and one's goods around the British Isles in a safe and timely manner. My objective is to enable you to better appreciate how the introduction of iron not only revolutionized the construction of bridges, but also how it affected every aspect of transportation.
When the Romans arrived in 43 AD, the British landscape consisted of dense forest relieved by the occasional heath, wetland, or moor. The native Britons avoided the predators, brigands, and other hazards of the ancient forest by confining their overland travel to the ancient tracks of the bare uplands such as the Icknield Way just under the ridge of the Chilterns or the track on the North Downs which is known to us as Chaucer's Pilgrim's Way to Canterbury. By far the most practical way to get around in ancient times was by water along the coast or up and down the many rivers.
The intrepid Romans began building the first British roads shortly after their arrival. Their purpose was military and their intention that of maximizing the effectiveness of the few Legions that they had brought across the Channel by having the ability to rapidly deploy them from one trouble spot to another on these new roads. It was a tactic they had found very successful elsewhere and it did not fail them in Britain.
The Romans built no more bridges than necessary, but these few dictated the location of their roads. Although a direct route from A to B might have been miles shorter, the distance traveled was not as important a consideration to the practical Romans as the certainty of being able to get across a river at the location of a bridge rather than risk finding that a more convenient ford was impassable. At key crossings like Cambridge, a number of roads from different directions converged there simply to take advantage of the bridge. Fully half of the north/south Roman roads converged on the bridge at London. (Trevelyan p. 19).
The Dark Ages were not kind to this transportation system after the Romans left in 410 AD. The stone causeways subsided or were used as quarries by the neighboring landowners. "From driving roads they declined into pack-horse tracks, finally disappearing for the most part to moor and plough-land" (Trevelyan p. 46) In the turmoil of the next 700 years during which successive indigenous and invading factions vied for control of Britain, travelers who valued their goods and their lives avoided overland travel whenever possible and soon fell back into the ancient habit of using the rivers and coast line as their highway.
With the arrival of the Middle Ages, Holy Church reappears in our story to again come to the rescue. By this time, and in support of their religious vocation, the monastic orders had become enormous land owners and businessmen involved in many enterprises as diverse as sheep raising and iron making. As an international organization with communication needs and widespread commercial enterprises, the Church needed a network of good roads and reliable bridges. In addition to taking on the usual Christian burden of caring for the sick and the poor, their concern extended to travelers many of whom would have been on a religious pilgrimage or even perhaps on Crusade. The Freres Pontifes were one of the orders that aided travelers and provided them lodging, ferry service, and even bridges, as their name implies. Saint Benezet, a self-taught but extraordinary engineer, was sanctified for a number of miracles including the building of the famous French bridge at Avignon. It was completed in 1185 and was so well designed and constructed that even after over 800 years of epic floods, four of the original 22 arches remain standing.
The English Cistercians included the building of roads and bridges among their charitable activities. Using a creative approach to providing for the public good, the Church encouraged the private financing of expensive projects of community benefit as an act of penance or piety. Wealthy merchants of 14th century England might heed the urging of Piers Plowman and would finance the building of a local bridge in the same spirit with which they might have charitably endowed an alms house. The Bishop of Durham, for example, offered 40 days of indulgence to those who would finance the repairing of Botyton Bridge (Engineering in History, Kirby et al, p. 109)
Royalty was not exempt. King John, who had a lot on his conscience if he indeed had one at all, was involved in building a stone reconstruction of the ancient wooden London Bridge. In 1176, Peter of Colchurch, "priest and engineer" began work on the stone replacement at the King's request. After Peter's death, King John appointed the French engineer, Brother Isembert, as his successor. This London Bridge, completed in 1209, survived the next 600 years (despite several fires and reconfigurations of its super-structure) until it was again replaced in 1831. At that time, the original black oak pilings were discovered, perfectly preserved by their submersion, and with their original wrought iron "shoe" of a type only the Romans could forge.
Still, as late as Tudor times there were very few bridges. In fact, whole regions of Britain were virtually without roads of any kind, such as Scotland, Wales and Cornwall. As always, the preference was to travel by water and the inland cities were dependent on barge traffic up and down the rivers.
These bargemen deplored bridges since they considered the mid-stream piers to be a threat to their difficult-to-steer towed craft. The ferrymen also strongly opposed any new bridges since they posed a threat to their very livelihood. Since no point in England is more than seventy miles from the coast (Trevelyan p. 340), this transportation situation was fairly satisfactory until the very last part of the 17th century: about the time that Three-times-Great Grandfather was born.
Before carrying the story of bridges forward, as you expected, I am first going to make a new circle around the subject, as I have given you fair notice was likely, so that we can view bridges from another perspective: that of how to build one. We will probably not give the subject the full treatment that the engineers among you hope for, but we will have a nice overview and then end triumphantly with the building of the first bridge to be made of iron.
When in 1779 the first iron bridge was built across the River Severn Gorge, it was flaunting 10,000 years of bridge building history. It had a single span of 120 feet stretching from bank to bank with no central support. To fully appreciate how audacious such a bridge appeared in its day, it helps to know how bridges had always hitherto been constructed.
Rivers have never taken kindly to being bridged. Sooner or later, like a half-broken horse, they manage to shed their bridge by washing it away with floods or by eroding the river bed beneath its piers and abutments or by pounding it to pieces with ice floes or debris. Over thousands of years, man developed ways of building bridges that were able to defy their waterway's attempts to destroy them and such proven methods had been followed ever since.
Placing large flat stones underwater to provide secure footing at a shallow reach or ford was the earliest method of improving a river crossing. This concept was expanded to become the clapper bridge where monolithic boulders were laid in line across the river with spaces between them for the water to pass. Large flat stones were laid on top of the boulders above the water level to create a weighted mass heavy enough to survive in place during most floods. To prevent the current from eddying around the boulders and undermining them, pointed shear waters were built facing both upstream and down. They served as well to protect the bridge from potentially damaging flood debris in times of high water by deflecting it up and safely over the bridge. There are clapper bridges dating back to Neolithic times still standing in various parts of Britain. This basic concept with minor variations was used to build stone bridges for the next umpteen thousand years.
Wood was the material of choice for most bridges and they were occasionally very ambitious structures. The Rhine was bridged in wood first by Caesar and then 800 years later by Charlemagne. The Romans built the original London Bridge in wood. It was destroyed and rebuilt many times until King John replaced it, as noted, with a stone bridge in the 13th century.
When traffic at a particular river crossing was heavy enough to justify such a considerable investment of time and money, an arched stone bridge was built following the Roman prototype. A testament to the permanence of stone bridges is the antiquity of those still solidly spanning rivers and canals all over Britain.
Both stone and wood, therefore, had seemingly been perfectly satisfactory materials for bridge construction for several millennia. So, it was not for lack of a suitable material that iron was so happily embraced when it became available in the late 18th century. Iron provided the solution to an engineering problem that bridge builders had until now been unable to solve for lack of a suitable material.
It is always preferable to eliminate supports in mid-stream. Pier foundations are difficult to lay down under moving water and are later vulnerable to being undermined by the swirls and eddies created by their interruption of the current. In addition, the types of soil underlying the river are not always capable of providing stable support for the enormous weight of a stone bridge that will be concentrated at the location of the piers. These ways for the river to rid itself of a bridge are avoided by eliminating piers from the design.
Boatmen hate piers since they pose serious hazards to shipping. Large, immoveable objects directly in the path of one's vessel which is already coping with the steering difficulties presented by the wind, the current, and perhaps the tow rope can easily lead to a fatal collision with the pier. There is also the danger of being capsized by the violent turbulence of the water flowing around the piers, especially in tidal creeks and rivers. The Thames was especially notorious at old London Bridge when the tidal flow was at its height against the current.
To minimize the number of mid-stream bridge piers, one must maximize the span of the bridge between its supports. The limiting factor to accomplishing that is the tensile strength of the material being used to build the bridge. In other words, given the weight of a bridge's horizontal road bed under the load of a passing vehicle, what is the distance that it can it remain unsupported before it begins to sag or fail ?
Over the ages, man devised several methods of construction which would enable him to increase the distance between supports - beyond that which the relatively poor tensile strength of stone or even of wood would normally allow - by transferring some of the load from the center of the span onto the supporting piers. The Romans and the Chinese invented similar versions of the stone arch and several other cultures devised the cantilever. The truss is based on the fact that the sides of a triangle are mutually self-supporting. This method of wood bridge construction was advocated by Leonardo Da Vinci (1452-1519) who noted under his sketch of a truss bridge "This bridge is unbreakable". Leonardo was well ahead of his time in this concept as in so many others.
The "new" iron provided various ways of greatly maximizing bridge spans in order to eliminate midstream piers. Its versatility and tensile strength meant that a bridge builder could take advantage of either the arch or the truss or the cantilever principles. It was especially adaptable to the rediscovered principle of the suspension bridge. Iron bridges were able to span breath-taking distances and their cost was a fraction of the expense of a stone bridge while enjoying almost as long a life expectancy. Compared to a wooden bridge, an iron bridge had the further advantage of not sharing a wood bridge's susceptibility to rot and fire.
All these advantages were to be dramatically demonstrated in 1779 by the first bridge to be built entirely of iron.