Iron was the miracle material of the nineteenth century—abundant, cheap, and extremely useful. It was perfect for jobs requiring great strength in proportion to weight: cylinders for pumps and steam engines; boats and barges for canals; beams and columns for mill buildings; and, eventually, bridges. Several thousand iron bridges were built in America between 1840 (when iron began to replace wood and stone) and 1880 (when it was in turn being superseded by steel); some six dozen survive.

These bridges used cast iron for compression members and wrought iron for tension members. Steel at the time was prohibitively expensive; wrought iron cost twice as much as cast iron but resisted tension (stretching) so much better that it was worth the expense for certain parts of a bridge. Wood was still often the cheapest material for much of the century, but it grew increasingly obsolete. Iron was the modern wonder—strong, affordable, mass-producible, portable, fire-resistant, and capable of being shaped into the loveliest designs.

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