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 bridges were so built, for stone and brick arch bridges are very ancient. In the course of time it was discovered that the triangle was the ideal form of framework and the truss was developed. Bridge building became the work of a craft, like the building of cathedrals, and men went all over Europe erecting bridges, yet no real principles underlay their work, which consisted in a cut-andtry method of design. The art of building truss bridges developed through correction of errors of judgment, but methods for computing the strength of suspension bridges were known fairly well about 1780. When railways commenced to supplant navigable canals and bridges were required to carry something more than light wagons many strange patents were obtained for trusses combining the principles of the truss, the arch and the chain.

In the summer of 1846 a Yankee school teacher, Squire Whipple, sat on the bank of a stream fishing and idly watching some carpenters repairing a wooden highway bridge close by. The school teacher learned that the foreman was a noted bridge builder, so he stopped fishing to converse with him. It was with considerable surprise that he learned there was no certain method then known for calculating stresses in bridge trusses. Upon his return home Whipple made a model of the bridge with small pieces of wood, joined at the angles with pins, having strings for counterbraces.