The Smithsonian is trying to sell us the Brooklyn Bridge. And listen, they have made it such a deal, who could resist?

The new exhibit at the Museum of American History, marking the centennial of "The Eighth Wonder of the World," is packed full of the sort of stuff museums used to -- and ought to -- showcase: big photos and drawings, and captions that tell at least as much as you want to know.

The building of the bridge, by far the longest suspension span attempted in the 19th century, was "a triumph born of tragedy," as the newspaper guys loved to say: German immigrant John Reobling, the designer, died of injuries received while overseeing the early work; his son Washington Roebling was crippled by "caisson disease" (the bends) while supervising the setting of the mighty piers; Washington Roebling's wife, Emily Warren Roebling, devoted a decade of her life to seeing that her husband's orders were carried out and defending him from the jackals who tried to snatch control of the project, which he followed by telescope from his sickroom window.

Smithsonian curator Robert M. Vogel gives us the family saga in full measure, but has had the good sense to concentrate on the nuts and bolts of the technological marvel that signaled the opening of what we were pleased to call "the American century."

Suspension bridges, which erased geographic barriers that had hindered transportation throughout history, were perhaps the first major engineering advance that was purely American. Judge James Finley (1757-1828), a Pennsylvanian, invented the suspension bridge in one stroke of genius (he built Washington's original Chain Bridge in 1807). By 1847 the first free span of more than 1,000 feet was carrying the National Road across the Ohio River at Wheeling, Virginia. (That bridge still stands, but now it's in West Virginia.)

John Roebling designed an ever better Ohio River bridge at Cincinnati and bridged the Niagara. The great virtues of suspension bridges are that they can span rivers and gorges that are too swift or deep to allow pier construction, and, as on the East River between Brooklyn and Manhattan, they don't interfere with shipping.

Roebling's designs were superb and his construction methods even better. Once he perfected the technique of spinning the great cables in place, which meant any size cable could be used, the only limitation on the length of span was the tensile strength of the individual wire strands.

Roebling was acknowledged as preeminent in his field, and the Brooklyn project fell to him almost as a matter of course. But his design was so big and bold -- it was also, by the rule that "form follows function," beautiful -- that committees of consultants almost haggled it to death.

Oddly enough, the vastness of the project comes through best on the scale models, which are splendid. And, while cable-spinning and caissons are fairly simple concepts, this visitor came away understanding them for the first time. On the other hand, having walked across the Brooklyn Bridge, I still don't believe it.

There also is a Brooklyn Bridge exhibit on the sixth floor of the Madison Building of the Library of Congress; which is worth going to see only because the walls of the corridors leading to it are lined with hundreds of huge plates from John James Audubon's Birds of North America. The bridge exhibit is small and unimaginative (why show us stereopticon photos without mounting at least one in a viewer to demonstrate the three-dimensional effect?) and careless (a text with handwritten changes by Walt Whitman is labeled as a typed copy when it plainly is a newspaper clipping). BUILDING BROOKLYN BRIDGE -- Through the summer at the Museum of American History. Open 10 to 5:30 (to 7:30, May 28 through Labor Day). ART, POETRY AND A WAY TO BROOKLYN -- At the Madison Building, Library of Congress, through October. Open 8:30 to 6.