WHEN THEY were first built to span the Niagara River, the bridges into Canada rivaled Niagara Falls, just upstream, as a tourist attraction.

The manmade wonders in steel challenging the gorge and the thundering backdrop of the falls have inspired a double fascination -- and the newest exhibit at the Museum of American History, "Spanning Niagara: The International Bridges."

Thirteen bridges have linked the two countries at Niagara Falls at one time or another since 1848. This show thoroughly details the bridges' construction, and, at times, destruction.

John Roebling, designer of the Brooklyn Bridge, made his reputation building one of the first such bridges. Actually, he would've built the first, but Charles Ellet Jr., a pioneer in suspension bridges, won the commission.

Ellet's fragile 1848 suspension bridge was temporary, to be used in building a railroad bridge. It had no roadway. Instead, men crossed it in a basket ferry suspended on a cable. The very shaky-looking iron cradle on display here carried them across the 700-foot- wide gorge, and they were probably scared out of their wits.

Ellet abandoned the project, as he disagreed with the bridge director over ferry tolls: He felt he should be allowed to keep them. When Ellet resigned, Roebling was awarded the contract for the railroad bridge.

Opening to fanfare in 1855, the Roebling bridge was hailed as the greatest architectural curiosity in America. Then the only major suspension bridge in the world, it measured nearly double any existing railroad span.

It was much photographed, and illustrated in prints, on banknotes and on stamps. And it was studied, by the commission on the feasibility of the Brooklyn Bridge. In an 1869 photograph of the study group crossing the bridge, John Roebling and son Washington can be spotted amog the consulting engineers.

High winds, ice jams in winter, the tumultuous current and the depth of the river would take their toll on the Niagara bridges. (There are now six international ones.) Back in Roebling's day, there were still more concerns.

In his hand-written specifications, Roebling justifies his proposal to build a double-deck bridge, with the roadway below the railroad bridge: "Horses meeting Rail Roadtrains with their puffing and panting Locomotives, will always scare . . . It becomes absolutely necessary to place the roadway below the Railroad track, and secure it so, that frantic animals cannot jump off." SPANNING NIAGARA: THE INTERNATIONAL BRIDGES, 1848-1962 -- At the Museum of American History through May 14.