ONE JANUARY EVENING IN 1786, after a round of cards, Paine invited a group of visitors up to the Philadelphia garret housing Hall’s workshop. “The ladies were handed over [the bridge], one by one.” The structure, made from twelve-inch cherrywood spars, assembled into a series of parallel supporting ribs, rose from the floor a little less than six inches. As the ladies traversed its gentle span, the small wooden bridge appeared to support their weight with integrity and ease. Paine was thrilled by this demonstration and the following day invited Philadelphia’s greatest mechanic, the scientific instrument maker David Rittenhouse, to view his model.
Although Rittenhouse was impressed, he did observe a potential problem. While bearing visitors’ weight, the bridge sagged slightly, causing its feet to slide outward. This lateral thrust, Rittenhouse suggested, caused by the downward pressure of weight on the arch, would slowly weaken the bridge. Rittenhouse’s fear was that structural deterioration, combined with the invariably damaging effects of the American climate, would lead to catastrophic failure. The solution, he advised, were abutments whose mass was sufficient to counter the compression at the arch’s center. The point was important, though hardly damning. For Paine, however, it was a very hard knock.
Among the virtues of his design, he believed, was its elegance and apparent lightness of structure. The idea that ever-larger spans would need to be countered by ever-larger abutments, fashioned from timber or stone, would add to the cost of the bridge, perhaps outweighing the benefits of a single arch.
Paine and Hall had chosen to build their first model from wood, America’s cheapest, most abundant building material. So prevalent was wood construction, in fact, that some worried about the sustainability of American building practices. Peter Kalm, a Swedish naturalist who had traveled in the American colonies from 1748 to 1751, feared for the fate of the white cedar tree, whose wood colonists used to shingle their roofs. In New Jersey, where he observed the harvest of the trees, colonists seemed “bent only upon their own present advantage, utterly regardless of posterity. By this means many cedar swamps are already quite destitute of cedars, having only young shoots left.”1
Wood’s structural strength lies in its flexibility or its capacity to absorb tension caused by twisting and pulling. It is limited, however, in its capacity to absorb a compressive force produced by dead weight. To compensate for this limitation, Rittenhouse’s observation suggested that any wooden arch would require abutments sufficient to counter whatever downward force its load imposed. Stone, the other principal bridge-building material, has the opposite qualities. Its capacity to resist compression is far greater than its capacity to endure the twisting and pulling associated with tension. Built from stone, then, a long, low-slung arch of the sort Paine envisioned would be just as vulnerable to collapse as a wooden arch, but for the opposite reason. Any twisting or pulling forces could dislodge stones and cause failure. So it was that, in their vulnerability to the contrary forces of compression and tension, wood or stone bridges built from a small segment of a large circle remained an architectural rarity.2
In January 1781, British builders unveiled an ingenious alternative to stone and timber construction in the form of a stunning iron bridge erected over the Severn River Gorge in the English West Midlands. Earlier in the eighteenth century, Sir Christopher Wren had used iron columns to support the galleries in a refurbished House of Commons, and architects in the iron-producing regions of England had begun using the same technique to support church galleries. But never before had an entire structure been built from iron. The unique bridge was simply called the Iron Bridge, the name also of the village that arose at its northern gate.
Travelers from around the British Isles and Europe came to marvel at the extraordinary structure. For painters and engravers, the one-hundred-foot iron span would be profitable subject matter for decades to come. Its image would emblazon dishware, trade tokens, bank bills, business cards, and other industrial-age ephemera. While serving as American minister to France, Thomas Jefferson purchased a print depicting the bridge; the image would later hang in his dining room at Monticello next to another depicting Virginia’s Natural Bridge.3
The appearance of this architectural marvel near the Shropshire town of Coalbrookdale was the result of the area’s unique industrial history. Since the reign of Elizabeth I, brick makers, potters, lead smelters, and other small manufacturers had been coming to the Severn River Gorge to exploit its abundant coal reserves. In 1709, the Quaker Abraham Darby, a Coalbrookdale ironmaster, discovered a means of using that same resource for transforming raw iron ore into iron. For centuries, the smelting process, which extracted impurities from iron ore to make pig iron, used charcoal for fuel. But the high cost of the wood needed to make the charcoal meant that iron was too costly for use in anything so extensive as a building or a bridge. Darby discovered that after slowly cooking coal, the by-product (known as “coke”) could replace charcoal in the iron-making process and could do so at lower cost while producing greater purifying heat. The discovery transformed Shropshire into England’s leading iron producing region. By the end of the eighteenth century, the booming iron industry had left its hillsides scarred by coal and iron mines.
As the region’s economy grew, the Severn, which had been the crucial link between Shropshire and the rest of England, had become something of a barrier. Local officials, small merchants and farmers, Quaker worshippers, and others whose affairs took them across the river, faced a grave challenge. The river’s banks lay at the bottom of a deep gorge, parts of which rose nearly vertically. Making the journey across the river by foot or on horseback was difficult enough, but for carriages and carts hauling coal or iron ore, it was often impossible. By the 1760s, the difficulty of crossing the Severn had become a drag on the local economy.
The scheme to confront this problem with a new iron bridge came from Thomas Farnolls Prichard, a prominent architect from Shrewsbury, just upriver from Coalbrookdale. As his architectural reputation grew, Prichard had become interested in bridges and in 1773 built a large masonry bridge at the convergence of the Severn and the newly completed Staffordshire & Worcestershire Canal. The same year, Prichard began designing a bridge for the Severn Gorge near Coalbrookdale. With the encouragement of Shropshire ironmasters John Wilkinson and Abraham Darby III (grandson of the first Abraham Darby), he began exploring the use of iron for this new bridge. John Wilkinson had become iron architecture’s strongest advocate in Britain, and was said to have requested that he be buried in an iron coffin. Darby was less well-known and would be less prosperous than Wilkinson, but his Coalbrookdale ironworks were perfectly situated to fashion the members of a new iron bridge. In October 1775, Darby was commissioned to cast the bridge and six years later, this industrial-age marvel was completed. Pritchard died in 1777, before construction began, so it was left to Darby to build the structure.
Although the cast components of the Iron Bridge weighed as much as six tons, the total weight of the bridge was much less than that of a comparable stone bridge. Its cost, although higher than expected, was also lower than a stone-arch bridge. Much of this was owing to the relative ease of construction. Darby used derricks anchored to the banks of the river to lift five partially assembled cast-iron ribs into place. Within just a few months he had erected the entire iron arch without any substantial interruption of river traffic. It was a miracle in construction, performed by a group of ironworkers with no formal architectural or engineering background.
NEITHER HALL nor Paine left any mention of the Iron Bridge, even though Hall, who hailed from a region not far from Shropshire, surely knew of the structure. But in a fashion typical of Philadelphia, Birmingham, and other provincial centers of eighteenth-century innovation, the two men arrived at a conclusion not all that different from innovators on the other side of the Atlantic. They determined that iron construction would redeem Paine’s design. Iron was less vulnerable to tension than stone but was more rigid than wood. With the structure itself absorbing the lateral thrust, massive embankment towers would be unnecessary.
In the spring of 1786, Paine and Hall began building a new cast-iron model. Upon its completion in June, they displayed it at Franklin’s house, an obvious place to attract Philadelphia’s investors. After seeing the bridge, Gouverneur Morris, the New York attorney who now lived in Philadelphia and would become one of Paine’s sworn enemies, encouraged Paine to consider building an iron bridge over the Harlem River near Morris’s family’s estate, Morrisania, in what is now the South Bronx. While nothing came of the Harlem River bridge, Morris and his half brother Lewis were sufficiently interested in Paine’s bridge to convince him that he might find in New York City the same interest he found in Philadelphia. Paine and Hall followed Morris’s counsel and, by carriage and cart, took their model to New York.
The trip allowed Paine to check on his New Rochelle property and Hall to visit his nephew, Jack Capnerhurst, an ironworker who had immigrated to Morristown, New Jersey. During the summer of 1786, Hall and Paine began to consult with Capnerhurst, who had worked for Boulton and Watt before coming to America. These conversations gave Paine and Hall a subtler understanding of the properties of iron.4
AFTER RETURNING from New York, they built a third, much stronger and more stable model. Capnerhurst had urged them to use new iron-casting methods perfected in England, but the architect and his model builder could find no American iron founders skilled in those methods. Instead, they used wrought iron, which had to be forged by hand to expel the impurities that could make iron brittle. The process meant that instead of fashioning the bridge from the kinds of large castings used in the Iron Bridge, they would build it from a lattice of small hand-wrought iron bars.
These bars, assembled into individual arched ribs and then tied together to achieve the majestic whole, Paine came to believe, constituted another of his contributions to bridge architecture. By building bridges from many small parts, rather than a few enormous ones, bridge construction became a more adaptable enterprise. Now, instead of the unique product of a provincial iron industry and its industrial produce, an iron bridge could be an exportable commodity and could be erected wherever the need for such a structure arose. “Among the advantages of this construction,” Paine wrote, “is that of rendering the construction of bridges into a portable manufacture, as the bars and parts of which it is composed need not be longer or larger than is convenient to be stowed in a vessel, boat or wagon.”5
For now, though, Paine’s design would make for a much more laborious model-building process. Hundreds of small metal bars would be assembled and connected tinkertoy-like, using nearly four thousand temporary wooden blocks. The bars would then be riveted together to form the structural ribs. The process of raising the ribs and tying them together demanded temporary scaffolding to hold the ribs in place, while a wooden floor and wrought-iron banister were added. It was a cumbersome and costly undertaking. In addition to Hall, Paine had to employ an iron forge to fashion the metal bars and a joiner to build the wood flooring for the bridge.
Through November and much of December 1786, Hall worked furiously, all the while struggling with crippling night sweats, coughing fits, nausea, fever, and the effects of the opium with which he treated his tuberculosis.6
PAINE, MEANWHILE, had a new sense of urgency. While Hall worked to finish the model, the Pennsylvania assembly was debating a proposed charter for the Schuylkill Permanent Bridge Company. In an effort to persuade legislators of the feasibility of a new bridge, the Agricultural Society displayed another architect’s bridge model at Carpenter’s Hall, just a few blocks from the state capitol. The model, built by a tailor named Sellers, infuriated Paine, who thought it violated the fundamental premise of any worthy American bridge design: it used midriver piers. Fearing that the state assembly would cave to pressure from the Agricultural Society and endorse such a design, Paine wrote George Clymer, who had promised funds for the new bridge project, pleading for patience and urging him to exert whatever influence he could to delay any final decision. The bridge then being considered, Paine explained, was not only unlikely to stand, “it is also a matter of more hazard than [the Agricultural Society] are aware of.” The supporting piers would create “obstructions [in] the bed and channel of a river,” and since “the water must go somewhere—the force of the freshets and the ice is very great now but will be much greater” when constrained by the bridge. In other words, the piers would narrow the river, increasing its destructive velocity, potentially driving it over its banks where it would destroy all in its path. “I am finishing as fast as I can my new model of an iron bridge of one arch,” Paine added. The society owed the people of Philadelphia a look at Paine’s more inventive design.7
By late December, the new model was complete, and Paine and Hall had it brought to Franklin’s house, just down the street from Carpenter’s Hall. A few days after Christmas, Paine asked Hall to take the model to the Statehouse, where it would be displayed for the next few weeks. The state’s assembly and its speaker reviewed the bridge, as did a parade of interested Philadelphians, among them “philosophers Mechanicks and even Taylors. Their sentiments of it was as different as their features,” noted Hall. “The philosopher Said it wd Ad new lights to the world—The Mechanick it was Grand and Strong The Statesman with the merchant it wd be of great Utility—And the Taylor (for it is an absolute truth) remarked it Cut a pretty figure.” It seemed that the proposed bridge would soon be a reality. The state assembly would incorporate the bridge company and “our Lilliputian Handyworke that is now 13 feet long . . . will be added . . . to the worlds present WONDERS.”8
HALL’S OPTIMISM WAS MISPLACED. No bridge company would be incorporated for more than a decade. The politics were too contentious, the funds were still scarce, and the assembly was uncertain about the feasibility of so ambitious a structure. But Paine was not to be deterred. The praise his model had won convinced him that if he could only build a larger prototype, the virtues of the design would quiet any lingering doubt. Financing would follow. By March 1787, Paine had devised a plan.
Perhaps Benjamin Franklin had once again offered advice. The great man had returned from his diplomatic post in Paris in September 1785, just after Hall had arrived in the United States. During his near decade in the French capital, he had become one of the most celebrated figures in the city, admired in particular for his extraordinary electricity experiments—but also for his social appetite. No other American would have been better equipped to advise Paine on the prospects of finding support for a new invention in the capital of America’s principal European ally. If it was Franklin who urged Paine to take up his bridge building in Paris, once again his counsel was sound. Although England was at the forefront of iron-bridge architecture, France had long been the world’s most progressive country when it came to designing and building roads and bridges.
Early in the eighteenth century, King Louis XIV had founded the Corps des Ponts et Chaussées (Bridges and Roads Corps) and in 1747, this world’s first engineering corps gave rise to the École des Ponts et Chaussées, the world’s premier school of civil engineering and architecture. Given this expertise, Paine had concluded that were his newest model to receive the endorsement of the French and were they to afford him the opportunity to construct a larger prototype, the Americans would be persuaded, once and for all, that his was the soundest path across the Schuylkill. He was supported in this view by the Pennsylvania assembly, which appointed a committee of Robert Morris, George Clymer, assemblyman Thomas Fitzsimmons, and several others to review the French response. The assembly decided to delay any further decisions about the Schuylkill bridge until Paine returned to the United States the following winter.
ON APRIL 20, 1787, Paine and Hall met in Trenton to tend to some unfinished business. Paine still owed his model builder money, which he paid dutifully. Hall was happy to see his friend, who would be sailing for France in several days. The two spent the evening drinking and discussing affairs of state and the politics of bridge building. It was a fitting celebration of what had been, by any account, a fruitful partnership. Now, Paine told Hall, he would take the business of bridging the Schuylkill across the Atlantic. If the iron arch were to be sold to the people of Pennsylvania, it would need endorsement from the only country in the world with a corps of professional bridge builders. All that was left to do now was for the two to say their good-byes. “I then shook hands,” Hall recalled, “and wishd him a good voyage and parted.”9