Roebling’s Bridge of Water
LACKAWAXEN, PA.: The sight of a canalboat crossing a river was hardly remarkable in 1849 when, on April 26, a local crowd and engineers from all over the country gathered on the banks of the Delaware River in upper Pennsylvania. The boat in view was an ordinary barge. What was curious was how it got from shore to shore—floating inside a wooden flume suspended thirty feet above the water from two iron cables, which dipped across the river over stone piers.
The flume, or trunk, held the waters of the Delaware & Hudson Canal, which had been routed right through Lackawaxen into New York State on the opposite bank. The aqueduct’s designer, the forty-two-year-old John A. Roebling—who later designed the Brooklyn Bridge—couldn’t possibly have foreseen the uses to which this creation, his third suspension structure ever, would later be put.
The Delaware & Hudson Canal Company, which then provided the only means of getting anthracite coal out of the region, had launched a $1.1 million plan to expand and speed up service. The company saw that big traffic tie-ups at points where the canal crossed the river could be eliminated by aqueducts. In early 1847 management chose Roebling’s suspension scheme, which would take up less room in the river than a conventional truss bridge.
The first aqueduct to go up was the Delaware, at Lackawaxen. For the walls of its 535-foot-long trunk, Roebling laid planks of white pine side to side in two layers on opposite diagonals, for stiffening. The floor was also built up in two layers, with bottom planks running lengthwise and top ones across. Wood- en towpaths were set along the tops of the trunk walls and supported from beneath by diagonal struts. Finally, Roebling hung the whole thing from the cables with U-shaped wrought-iron rods, which were bolted to beams under the floor.
Roebling had ingeniously prepared the trunk for a formidable task. Hundreds of tons of water pressing against the inside walls, plus the weight of the towpaths and struts, would bear down on the ends of the transverse beams and actually counteract the stress imposed by the water at the center of the floor.
The cables, strung across the piers in four spans, each consisted of 2,150 individual wires bundled into seven strands and compacted together into an 8.5-inch diameter. To protect it all from the weather, Roebling wrapped each cable in a continuous helix of wrought-iron wire. This technique was Roebling’s own invention.
D&H was so pleased with the Delaware Aqueduct that it asked Roebling to build three more. When the canal finally succumbed to the rails, in 1898, all the aqueducts were dismantled or abandoned except for the one at Lackawaxen, and the subsequent history of that structure is enough to make a preservationist’s blood curdle.
In 1901 the now-dry aqueduct was sold to a lumber dealer who used it to cart logs. He charged a toll to other users. Eventually, the towpaths were chopped off and a handrail installed. In 1933 the trunk burned away completely—and once the floor was rebuilt, no one bothered with walls.
The bridge changed hands many times, and slender profits were made from tolls. Meantime, vehicles of all kinds continued to hurtle over the floorboards. In 1977 a truck fell through the deck, triggering a sticky lawsuit but closing the bridge only temporarily. It wasn’t until 1980, when the National Park Service stepped in and bought it, that any thought was given to restoring what bv then had become the oldest cable suspension bridge in the country and probably in the world.
The effect of years of abuse was immediately apparent to Henry Magaziner, regional historical architect for the Park Service. “All 1 had to do was hop up and down on one end,” he recalls, “and I could see a ripple running all the way from Pennsylvania to New York.”
Roebling had designed the aqueduct to support a dead, or unchanging, load of up to five hundred tons of water per span. On top of this tremendous weight, which kept the whole structure stable, the load of people and mules moving over the towpaths amounted to virtually nothing. And the boats simply displaced their weight in water.
But the weight of the floor alone was not enough to keep it from shifting under heavy, moving loads. As cars or trucks drove across, the cables were constantly pulled in the direction of the occupied span; they slid back and forth over the tops of the piers and began to fray. And the wooden floor had been deprived of anything that would give it rigidity. The obvious solution was to close the bridge to traffic. But Congress would not fund a restoration unless the aqueduct remained a vehicular bridge.
The Delaware Aqueduct’s story is enough to make a preservationist’s blood curdle.
That left the architectural firm of Beyer Blinder Belle and engineers from Ammann & Whitney to come up with another answer, and the one they found neatly reconciles the twentieth-century demand with Roebling’s original specifications. A roadway of precast concrete units will effectively replace the dead load of water; on top of that, even a truck will be a flyweight.
The whole structure underwent extensive testing in 1983, and the results were encouraging. Samples extracted from the cable cores withstood stresses of up to 91,000 pounds per square inch, a figure beyond what even Roebling had wanted and a credit to his protective wire wrappings.
Most of the original supporting structure still exists, and was restored in 1985. But the supported structure needed total reconstruction, and after hitting on the initial solution of the roadway, the architects weren’t sure how to proceed. They considered simply laying the roadway and stiffening the bridge as necessary, leaving it to look as it had since the thirties, when the trunk walls were removed. Alternatively they could rebuild parts of the original aqueduct, putting up short sections of the trunk wall at both ends to show what once was.
They finally decided to restore everything—trunk, towpaths, and all. When the work is completed in the fall of 1986, visitors will see an elegant structure closely resembling the one that opened in 1849.
TAKING THE STAND: Last year a House Committee task force on science policy invited Alex Roland, a historian of technology at Duke University, to talk about his specialty—the history of federal science and technology policy. Roland went along, but he was frankly dubious. After all, how much would a roomful of politicians really care about what he, an obscure history professor, had to say?
As it turned out, the task force was thrilled to hear something other than the usual legislative droning, and Roland was asked provocative, well-informed questions. “Some were specific and historical,” he says, “but others were broad in scope, such as, ‘What has the relation been between applied and general research?’ I couldn’t answer them all.”
Roland had made the happy discovery that, for at least some of the nation’s lawmakers, the historical record is worth scrutiny. Every year for at least the past fifteen years, historians of technology have testified before one congressional committee or another on everything from how single inventions developed to where federal funding for R&D should go.
Melvin Kranzberg has long been trying to get more historians involved in the policy-making process. Kranzberg, a historian of technology at Georgia Tech, probably has appeared at more hearings than anyone else in the field. Between 1967 and 1972 he spoke frequently before the House Committee on Science and Technology in favor of creating an Office of Technology Assessment. At the time, he says, “People weren’t conscious of the fact that technology has social implications. I was trying to show that there is an interaction, and that by looking back in the past, we can see what the interaction might be in the future.”
But how effective can this sort of testimony be? “Historians add perspective,” says one who has attended these hearings, “but they do not turn policy around.” And Alex Roland, whose experience was very positive, believes that merely improving the perspective of those who make policy decisions is ultimately “the best we can hope for.”
On the other hand, the historical perspective may be at least as crucial as the purely scientific or technological one. A. Hunter Dupree, author of Science in the Federal Government, insists that “when it comes to judging where science policy should go, the scientists themselves are out of their specialties. We need the historian’s advice.”
The historian Carroll Pursell agrees. Federal agencies, he points out, now hire historians for special projects. Pursell himself was once asked to look into possible effects of new copyright laws on video and computer technology, by studying how radio developed under patent laws of the 1920s and 1930s.
In spite of these forward-looking activities, Pursell stresses—as do many historians—that history is not predictive. But where technology is concerned, historians are often valued on the Hill less for their foresight than for simply being able to define their subject in human terms.
DEARBORN, MICH.: Talk was of air conditioning, medieval vehicles, Arthur D. Little, and women in the coal industry at the Society for the History of Technology’s twenty-eighth annual meeting, held last October. One special session was devoted to the work of Hugh G. J. Aitken, author of Taylorism at Watertown Arsenal and the definitive histories of radio technology, Syntony and Spark and The Continuous Wave. At the awards luncheon, Thomas P. Hughes received SHOT’S highest honors, the Leonardo da Vinci Medal and the Dexter Prize—the latter for Hughes’s groundbreaking study of technological change, Networks of Power. And Melvin Kranzberg gave a keynote address entitled “Kranzberg’s Laws,” in which the former SHOT president enumerated his most beloved tenets—among them the memorable “Technology is neither good nor bad, nor is it neutral.”