Covered bridges are a favorite among New England visitors and photographers, from kids to senior citizens

The affection that many visitors acquire for New England’s covered bridges often begins behind a camera. These small gems of 19th-century ingenuity are fun to photograph as weather and the changing seasons costume them with icicles, snow caps, dustings of pale green pollen, dramatic summer cloudbanks, and brilliant fall foliage.

But as people look closer at the structure of the bridges – especially the lace-like, triangulated truss systems that form their skeletons – the simple shutterbug itch begins to turn to curiosity and admiration.

Covered bridges were built by Europeans in the Middle Ages, but the 19th century brought a new and distinctly American twist to this engineering form. The first covered bridge in the United States was built in Philadelphia in 1805; by the 1870s, more than 10,000 covered bridges were built in almost every state in America.

Today only about 750 covered bridges remain, concentrated mostly in Pennsylvania, Ohio, Vermont, Indiana, New Hampshire, and Oregon. The National Society for the Preservation of Covered Bridges, founded in1950, has chapters in many states, and works to preserve and study and enjoy the bridges.

“The subject is immense,” said Benjamin D. Evans, co-author of “New England’s Covered Bridges: A Complete Guide” (2004, University Press of New England). He isn’t kidding. Talk to a covered bridge enthusiast, and the data on history, design, preservation, restoration, and aesthetics just roll on and on.

By definition, a covered bridge must have two trusses (rigid frameworks), one on each side of the bridge. Trusses come in many designs – often named after the man who created them -- and they are always based on a triangle shape because it will not distort under stress. A covered bridge must have a deck and a roof. To be “authentic,” a covered bridge must use one or more of the 21 specific trusses that are used throughout the United States. If a bridge looks like a covered bridge, but does not have trusses, receiving its support only from horizontal beams (stringers) anchored on opposite sides of a stream, it is called a “stringer” or a “romantic” bridge, not an authentic covered bridge.

Trusses are the single most distinctive and defining characteristic of covered bridges, and every guide to these bridges invariable includes the name of their truss system. Most take the names of their designers: Theodore Burr of Connecticut; Ithiel Town of Connecticut; Col. Stephen H. Long of New Hampshire (a U.S. Army engineer in the Civil War); William Howe of Massachusetts; Peter Paddleford of New Hampshire; and Thomas Pratt of Boston.

Each state has its distinctive bridges. Pittburg, New Hampshire, has the Happy Corner Bridge. According to a description published by the state Department of Historical Resources, “Happy Corner was a bustling neighborhood in the late 1800s.... an elderly gentleman who lived in a house on the northeast corner of the crossroads liked to sing and dance, and he owned a Victrola which he played frequently. People at his house had a happy time singing and dancing.” Another odd name: the town of Franklin has Sulphite Bridge, so named because of the large amounts of sulfur transported over the rail lines for use by the pulp and paper mills nearby. It is also known as the Upside Down Covered Bridge because the railroad track crosses over the top of the structure rather than running through its center.

New Hampshire also shares with Vermont the Cornish-Windsor Bridge. Built in 1866, it is the longest wooden covered bridge in the United States and the longest two-span covered bridge in the world. At 450 feet long, the bridge consists of two spans supported by Town lattice trusses.

A final note: Why were the bridges covered? A handful of myths attempt to answer the question, but the only correct answer is simple: to protect them from weather. Here’s one weather-related disadvantage to covering the bridges: during the winter, bridges had to be “snowed.” That is, snow had to be carted into the bridge and spread over the dry deck to allow horse-drawn sleighs to pass through.