Tonight the pianos will fly. The audience will be on the second-floor balcony of the National Building Museum. The air-conditioning units will be turned off so it will be especially silent. The infrared sensors will be turned on. The audience will be invited to walk, to move. The electromagnets will be triggered and will swing and strike the strings. Music will be made.

Dutch multimedia artists Paul Panhuysen and Johan Goedhart have been hoisting pianos in great halls for about six years. The idea, they say, is to combine art with architecture. Their first "Long String Installation," as they call it, grew out of an exhibition in 1982.

They were supposed to make drawings, but they became enamored of the space instead. "It was partly brick and partly concrete," says Panhuysen, in his singsong Dutch accent. "A very rough place. With a beautiful ceiling."

He is sitting on one of the Victorian wrought-iron-and-wood benches next to the silent fountain. He stops to drag on his filterless Gitane, held by two short, yellow-stained fingers. His partner Goedhart is pacing a bit, also smoking a cigarette -- but he rolled his own, lickety-split.

So, Panhuysen continues, instead of doing the drawings, they decided they'd rather stretch string around the space to call attention to the ceiling. Artists get ideas like that.

"But," he says, "the problem was it wasn't sensible. You couldn't really see it. So we decided to connect sound to it. To make sound with the strings."

Naturally, to make sound, you need taut strings. They needed a weight. Since this exhibition was at a new music festival, they decided (for the imagery) on a piano. Why not?

This was in Hungary. "It was at a Stalinist cultural center, made of tiles -- very solid," says Panhuysen. "We were quite sure they wouldn't let us drill holes. But it had a balustrade. So we attached the strings to the balustrade and hung the piano in the center." Just a few feet off the ground.

That time they used a Russian-made piano. For this installation, they have Korean pianos. Samick pianos. Panhuysen and Goedhart don't have piano brand preferences. Shiny and black is all they need.

At the Building Museum, the pianos are hanging, nestled in a mesh of yellow nylon ropes. The effect is sort of a gargantuan cat's cradle.

There's a problem, though. The pianos, once hoisted, start swinging toward each other, perilously close to the fountain in the center of the space.

The crew is baffled.

"We didn't count on the shear says Washington Project for the Arts music curator Douglas Quin, explaining how the pianos keep pulling to the center.

The first solution -- to tie off the pianos from the side walls, and hold them back from the center fountain -- is rejected. After about 20 minutes, a second solution is adopted: Plywood pyramids are quickly built and wedged under one leg of each piano, the leg closest to the water. The designers are delighted.

"It will have this dancer's pirouette, arabesque feeling to it," says Quin.

The concept of eliminating the barriers between art and life, and even between the individual arts, was the basis of the 1960s Fluxus Movement.

"I never joined the Fluxus," says Panhuysen, "but I had similar ideas.

"There was a time when a sculptor only sculpted and a painter only painted," he says. "But I don't think that's true anymore. You need to express yourself any way you can. I always thought I would like to show off what architecture is. Deal with the space in a more creative way. So, everybody who comes to hear this, everybody who will see this, will never see this space in the same way again. That's what I really like -- how it changes the perception."

By Wednesday, the pianos are firmly in place. The strings are carefully stretched across the museum's 316-foot Great Hall and being tuned.

Originally the strings were going to be tuned on the pentatonic scale. At the last minute, the pair decided to incorporate more of the architecture by tuning the strings based on the law of doubling. This principle seems to be the basis for the building's design: two columns, then four columns, then eight arches, and so on. "It's a very military design," he says. In fact, says museum spokesman Donna Anderson, the building was designed by a general.

Oh, and the pianos are tilted, rather than being perfectly horizontal.

"Because the weight of the pianos is mostly in the front," says Panhuysen, "where the steel plates are. Every piano is different. We have to adapt to that."

Even these two pianos, which were made by the same company and are the same style, are different. One is five inches longer than the other.

In this uncertain world, you can't even count on a piano.

Tonight's performance begins at 8 at the National Building Museum, 440 G St. NW. Tickets are $5 for members and $7 for others. The exhibition continues until Aug. 19.