Every bridge is different--engineered to carry a particular kind of traffic scrota a particular barrier and to withstand local extremes of weather. While moat are supported by towers, arches, or cantilevers, some float. Built of wood, stone, concrete, iron, or steel, their evolution comprises the history of humankind's understanding of engineering and materials. To introduce these concepts, Carter describes seven basic types of bridge, giving an example of each (the Pont d'Avignon; the Brooklyn Bridge). Various suspension schemes and types of trusses are shown; along with the structural descriptions, bits of bridge lore and history are also introduced. A minor cavil: the cantilever is mentioned but how it works is not made clear, either in the otherwise lucid text or in Doty's simple, amusing drawings, which admirably convey much of the information here. Overall, an entertaining book that covers its subject with grace and humor. Glossary.