PHILIPPE Petit is not small as his name suggests, but he is wiry, as a high-wire artist probably should be. His tread is catlike, his handshake firm, without the overinsistance that would mean he is trying to prove something.

After all, he has walked a wire between the spires of Notre Dame in Paris, his home town, and between the towers of the World Trade Center in New York, his adopted home town, among other aerial exploits. His major ambition, at the moment, is to walk a mile-long wire across Niagara Falls -- an activity that the state of New York has made illegal.

Which explains why he's in Washington watching a rehearsal in the Folger Shakespeare Library.

"I knew about him before, of course," says the Folger Theatre Group's producer, Louis W. Scheeder, "but he was called to my attention when I went up to Niagara Falls to do some research on the French tightrope walker Blondin. A man at the library told me that Philippe had been sent up there doing some kind of research. I was planning to produce a play about Blondin, and Philippe was planning to walk across the Falls. So and here he is."

Petit, 31, is serving as a technical consultant for the Folger's American premiere production of "Crossing Niagara," by Alonso Alegria, a two-character play about Blondin's 22nd crossing of Niagara Falls, 120 years ago when it was legal, if not advisable. That was the time he carried another man on his back -- having previously made the trip blindfolded, on stilts, pushing a wheelbarrow, or stopping in the middle to sit down and cook and eat an omelette. "If I could stretch a rope to the moon and the sun," Blondin says in the play, "I'd go across it without a second thought."

Petit know those feelings firsthand, and "I try to give all this to the actors, and something of the physical feeling -- what it is like to be out there in the middle, with the wind rushing past and the wire twisting under you."

Out on the wire, he says, "sometimes you have crazy thoughts and you have to chase them away.You must think very intensely about what you are doing. Sometimes, at difficult points, you think of moments in your past life, and you hang onto them like a rope."

Petit has served as a consultant before for films, plays and books, but this time, he says, is different: "It is the first time I have worked with a writer and a director, the first time I have really been able to communicate the experience."

He agreed to work with production after "I read the script and liked it very much. It shows what I do as an art, not a stunt, treating it with a kind of respect that does not happen much. He wrote it without talking to anyone, and I find that amazing.

"I do not usually like fictional or dramatic treatments of my art -- 'Barnum,' [the broadway musical] for example. I saw it and I hated it very much. It is an example of exploiting circus atmosphere, not of taking it seriously as a subject. There is a danger in the theater of using other arts for spice and background without really understanding them."

At the Folger, he is teaching the actors to walk a tightrope of the mind. In the climatic scene, where Blondin is carrying his young associate, Carlo across the gorge, actor Michael Tolaydo's feet remain firmly on the stage, although his body movements are those of a man on a swaying rope 160 feet in the air. The scenery at this point is simple but effective: a sky-blue background with flecks of cloudand the diagonal lines of guy wires coming in at angles. Under the actors' feet, the stage is covered with an enormous mirror, reflecting the blue-sky background, so that the actors seen to be susupended in a deep space. Blondin bounces and sways and sometimes for long, horrible moments, he stands frozen, while Carlo runs through the remarkable variety of positions one man can take while he rides on another man's back. And in the background, a cnstant, thematic undertone, is the rushing, pounding noise of Niagara.

"It is a special challenge," says Petit, "to pepare and action in this than in the actual act of walking."

He did take that actors up on a low wire once in Tobias Haller, who plays Carlo, up on my back," he says, " and I walked the wire with Michael behind me for a moment, holding my shoulders, to see what it is like." Playwright Alegria wanted to take the same walk -- but, he says, "my plane was fogged in and I got to New York to late."

Alegria has never been on a tightrope and never talked to a tightrope walker until he was introduced to Petit during the preparations for the Folger production. It was not necessary, he thought, because the play is not really about tightrope walking but human relations. The high wire came into the plot because Alegria became interested in Blondin and because he finds universal symbolism in that narrow, swaying journey across great heights.

"It's all here, you see," Alegria says, tapping his head. "You sit in the audience and you look up at a high-wire artist and you wonder what is going through his head. He is inscrutable, like ballet dancers who deliberately adopt a deadpan expression, and you wonder what is he thinking? Is he singing inside his head, or is he simply counting?"

Alegria, who was born in Peru and studied drama at Yale under Joseph Papp, was the producer/director of the Peruvian National Theater for seven years.Now a teacher of drama at Waynesburg College in Pennsylvania, he has written two other plays and a number of adaptations besides "Crossing Niagara."

"Actually, I got the idea for 'Crossing Niagara' when I first saw Jaws," he says. "That story found and used a universal symbol: the dark unknown in the water, under the boat, ready to snap. I like to think that some images have to do with everyone's unconscious, and fear of falling is one of the strongest, fear that is felt by a newborn child. The beautiful thing about Blondin's art and Petit's is the way a tightrope walker uses and masters that fear."

In "Crossing Niagara," Alegria's theme is somewhat more complicated than the simple fear of falling. The play and the crossing involve two different temperaments: Blondin, a combination of artist and showman; and Carlo, an 18-year-old portrayed as a prime product of the Age of Optimism, a devotee of technology who believes that you can do anything if you master the proper technique.

Ultimately Carlo wants to cross Niagara to prove his philosophical point, while Blondin's deepest motivation is the experience -- the exhilaration and danger. Why do you do it? Carlo asks at the end of their piggy-back trip, and Blondin's answer is: "For this -- for being up here." In a sense, they are two incomplete personalities, the poet of body language and the technician in embryo, and the story of the play is that of their fusion (at least temporarily) into a single unit.

"What interested me when I discovered this story was the idea of two people conquering their fear together," says Alegria, "the whole relationship, starting from scratch. How do you develop the kind of trust and mutual condfidence to do such a thing? In the end, they both learn that anything is possible with the right relations and attitudes and coordination of minds

"The play is about achieving through trust things that would otherwise be impossible. It is not a theme much used in our time. There seems to be a slant today toward pessimism and defeat. Moments of trust, confidence and achievement go unrecorded, and people in the theater believe that a play with a happy ending cannot be good. Perhaps 'Crossing Niagara' enough to have a life in which these values did amount to something. I have never been fundamentally betrayed."