The Brooklyn Bridge is beautiful, so are racing bikes and sailboats. The airy, shining sculptures of artist Kenneth Snelson, which go on view today at the Hirshhorn Museum and Sculpture Garden, are beautiful as well. But their beauty somehow chills.

It is the beauty of pure structure, of principle made manifest. Snelson's tube-and-cable structures, displayed on the plaza of that round museum, appear poised for flight. They seem as taut as bowstrings, as spare and as efficient as proofs in mathematics. But there is in their precision more elegance than soul.

Once upon a time, the scientist, the alchemist, the engineer, the artist, were one and the same. Because Snelson is convinced that he has discovered the rings that rule the atom, because he patents his inventions and fills his notes with numbers, many in the art world regard him as a scientist. Physicists, however, grin at his hypotheses. Snelson does not mind. He smiles as he quotes a friend: "Hardening of the categories leads to art disease."

He had used that good line often. Many artists mumble, but Kenneth Snelson doesn't. His intelligence is high, his confidence is great. He has the patience of a pedagogue. His eyes are blue, his hands are strong, his body lithe and compact. He seems, at 54, perhaps 10 years younger. He built model airplanes as a boy, and loved their balsa tautness. His father, a photographer, ran a camera shop in Oregon, and the artist is a fond as his father was of the well-machined. Kenneth Snelson also has a sailor's love of knots, and was once in the Navy, stationed in this city. While here he studied painting at the Corcoran School of Art. "I was assigned to an LMD," says Snelson. "You remember the LMD? The Large Mahogany Desk." His speech is free of mystery. In humor and in sculpture, he likes the lightweight and the strong.

Almost all his structures draw surprising strength, as well as their cool beauty, from the perfect balance of tension and compression. The principle is not new. When you tense your muscles, your bones resist the tension. A ship's mast and the stony piers of the Brooklyn Bridge are materials in compression; the taut stays that hold the mast upright, and the cables that support the roadway of the bridge, are, instead, in tension. "The carousel is a precedent.The animals ride round on a platform that's suspended. There are many others," Snelson says: "bicycle wheels, kites, the bamboo bridges made with vines that we all used to see in the old Tarzan movies." Almost every object in this retrospective is made of metal tubes compressed by metal cords in tension.

Snelson first encountered tension and-compression while studying with Buckminster Fuller at Black Mountain College in 1948. Snelson speaks of Fuller now as "a charismatic, a magician," and something of a plagiarist. In late 1948, tinkering at home, Snelson made his "Early X Piece," a little work of wooden struts and nylon threads that he sent to Fuller. Fuller praised the "Early X Piece," in a letter to its maker, as an "original demonstration . . . a prototype structure," but later, Snelson says, Fuller used the principle as if it were his own. Fuller kept the sculpture. A later reconstruction is included in this show.

For 10 years in New York, while working as a cameraman, Snelson put the principle aside. When at last he returned to it, he explored it unrelentingly. He discovered how to make star forms, towers, tripods, in which the metal tubes somehow never touch. His structures seem to draw in space not just lines of force, but triangles and spirals, tetrahedra, empty cubes. One amazing object here, "Cantilever," made in 1967, sticks out from the wall and, 30 feet away from its sole support, suddenly turns upward long after one expects its weight to pull it down.

The forms that Snelson wrings from his stainless steel pipes, his tubes of dull aluminum, his cords and threads and cables are astonishingly varied. Some appear severe, other seem eccentric. One, called "Easy K," which Snelson placed in Holland, hovers in the air over a Dutch pond. It has all the awkward grace of a sea bird taking off. Though it is 100 feet long, it does not touch the water. The Greeks who watched while Euclid drew lines in the sand must have been astonished at the fresh geometries he conjured Viewers in our own day have been similarly amazed at the clean or sullen beauties that such modern masters as Sol Le Witt and the late Tony Smith have wrong out of the cube. Snelson's Hirshhorn show -- though, for spice, it does include his models of the atom and his panoramic photographs -- repeats itself a bit too much. But his finest pieces outside make the Cor-Ten works of others, installed all about them, look clunky, earth-bound, stale. Whether Snelson is an artist or a scientist or an engineer -- or, as he prefers, "a speculative philosopher" -- there is no doubt at all that he's a true believer. His show was organized by the Albright-Knox Gallery, Buffalo, N.Y. Its catalog contains a fine essay written by the Hirshhorn's Howard Fox. Snelson will speak to the public on his work tonight at the museum, where his exhibition closes Aug. 9.