新罕布什尔州的县桥 County Bridge in New Hampshire (© Denis Tangney Jr/E+/Getty Images)

新罕布什尔州的县桥 County Bridge in New Hampshire (© Denis Tangney Jr/E+/Getty Images)

Bridge of Hillsborough County

The County Bridge has offered a picturesque passage over the Contoocook River in southern New Hampshire since 1937, when it replaced its covered predecessor that had been destroyed in a flood. But why even build a covered bridge? In a word, longevity. The roof and walls help protect the timber supports from rot. For comparison, an uncovered wooden bridge lasts an average of 20 years, while the covered variety can reach 100 years or more.

But even their durable reputation couldn't withstand our drive toward faster travel—the railroad, the automobile, and the heavier loads that came with both. As iron and then steel replaced timber as the bridge-building material of choice, covered bridges gradually went the way of the horse and buggy that used to traverse them. Out of about 12,000 covered bridges that were built in the US, only 750 remain today, with a quarter of those in Pennsylvania.