Miguel Vazquez, at 18, is a circus performer who has done the impossible: the quadruple somersault on the flying trapeze. He did it for the first time before an audience last July 10, in Tucson. He had done it for the first time in practice on Aug. 19, 1981, in Long Beach, Calif., after 270 attempts.

And he did it Thursday night, at the D.C. Armory, before a crowd of 5,549.

Slim, introspective, with the detached air of a young bullfighter, Miguel Vazquez now finds the quadruple both his achievement and his burden. Now that he has done it, he must try to do it every night--as advertised by Ringling Bros. and Barnum & Bailey.

He has succeeded 41 times since Tucson. But lately he has been finding the quad very hard to do. Before Thursday, he had done it on Feb. 10--and that was his only quad this year.

In North Carolina a few weeks ago, the Flying Vazquez troupe was practicing at night, after the crowds of the Charlotte Coliseum had gone home. Miguel had tried the quad in both performances that day, and missed both times.

Felipe Vazquez, a smiling young man whose workout leotard has funny bright red feet, easily performed a triple somersault--the legendary trick by which trapeze artists once gained fame. He was caught by Juan, 34, the senior Vazquez brother, who tossed him back to the other trapeze with hardly a break in their conversation. The only other sound in the arena was the bouncing of a single basketball, far away. Juan called a greeting to his wife, Patricia, watching from a bench below. She is a flier too, but pregnant now, her place taken by her sister Rosita Segrera.

Miguel stepped upon the pedestal, swung out, pumped very high--higher than the rigging itself, so that at his apex the trapeze jerked him back. On the third swing, he released and tucked, exploding into a backward-somersaulting ball. The ball rose, seemed to hover and, still spinning, fell, revolving at 75 miles an hour. Juan's arms thrust out, but the ball went by him, straight down, accelerating, untouched, twisting like a cat as it augered into the net 40 feet below.

"See, he came out of the tuck too early," said Patricia Vazquez from her bench.

Twice more, Miguel tried the quadruple, and twice more he plunged past Juan, brushing his arms. All week, in Charlotte, audiences come hoping to see the amazing new star Miguel Vazquez do the quadruple. But all week, when the time came, Miguel spun by his brother to crash into the net.

The quad goes by very fast. To blink is to miss it. It is the only act in the circus that the ringmaster hyperbolically celebrates afterward: "Ladies and gentlemen, you have just witnessed the most spectacular feat in the history of . . ."

But the quad, when it succeeds, is always perfect. It has to be, because Miguel spins so fast that there is no chance for last-minute corrections. It is a risky trick, too, in which Juan, hanging upside down, is often battered hard; he and Miguel prepare for the shocks by sparring daily with boxing gloves. Even so, much skin of their arms is lost as the flier's and catcher's hands clutch and slip. The greatest danger is that, as Juan attempts to grab the spinning mass, he might deflect his brother's fall beyond the narrow protection of the net.

For all that, the quad has not been much recognized outside the circus. Miguel has not been celebrated like a winning prizefigher, or a World Series pitcher, or even a golfer who has sunk a hole in one in a network-televised tournament. You have to go to the circus to see him, and because the circus is determined not to be overexposed, Miguel is not paraded around to talk shows or shopping center openings, or entered in televised junk-sport celebrity refrigerator-carrying contests paired against some Charlie's Angel from prime time.

That is not part of the tradition. The tradition of the trapeze is more rigorous, and less lucrative. The Vazquez contract, negotiated before the quadruple was added to the act, is for somewhat less than $100,000--to be divided among the four-person troupe according to need.

"It's a shame that Miguel has done something which has never been done before and yet it is not really appreciated," Juan said. "In boxing, everybody knows the champion of the world. Or in football, they know who won the Super Bowl. Miguel is the only one in the world who has ever done the quadruple, and it just passes. Nobody knows. The problem is that a boxing match builds up, round by round. But the quad is so fast people don't even realize it happened. The first time he did it, they hardly even knew to applaud."

The trapeze is as much an art form as it is an athletic display. It shares with bullfighting a circular ring, though the trapeze has risen above it. Like matadors and picadors, the trapeze artists emerge formally into their arena, padding in slippers to the accompaniment of brassy music. The best aerial acts today are from Mexico; the best fliers are slight of build, like bullfighters, and Spanish bravado still results in similar postures of nonchalance. Bullfighters traditionally spit on the sand to demonstrate that the mouth is not dry with fear. Aerialists merely ascend their ladders as if the ladders were unnecessary.

Miguel is polite, but his air is remote. He is innocent, but it is the kind of innocence that in a Hollywood script would sooner or later draw Ava Gardner with two glasses of champagne. Miguel, however, has no patience with such extraneous matters. Increasingly noticed by members of the opposite sex, he ignores them: "You just have to forget about girls. The work comes first."

Innumerable "blood-and-sand" movies taught Spanish youths the glory of bullfighting, and the trapeze has had its movie, too. "Trapeze," first released in 1956, was a love triangle with Burt Lancaster, Tony Curtis and Gina Lollobrigida. Curtis was the flier, Lancaster was the catcher, and their goal was the "impossible" triple somersault. Although the triple had actually been performed first in the 19th century, "Trapeze" was a boon to aerial acts everywhere and remains an inspiration for new generations of fliers.

The Vazquez brothers had seen it many times but by 1977 were still performing on horizontal bars 35 feet in the air.

"We finally decided to get into the trapeze because everybody in the circus knows it's the most beautiful act," Juan said. "Miguel wasn't even with us at the time; he was still in school in Mexico. We sent for him later." It took Felipe and Vinicio--a brother who now has his own act--only three months to put together an act. Felipe, Juan and Miguel all learned to do the triple somersault, and by 1981 they had been hired by circus owner Irvin Feld for The Greatest Show on Earth.

"When the family first sat down to discuss it," Juan said, "Felipe and my other brother immediately announced that they would be fliers. Naturally, nobody wanted to be the catcher. They left me no choice, because I was the oldest."

The movie, Juan said, is for the most part accurate. "When they first try for the triple and Burt Lancaster gets a bloody nose, that's very realistic. Happens to me all the time up there. The unrealistic part is when they decide to take the net out. Nobody ever took away the net from the trapeze. Nobody.

"You know," Juan went on, "we had heard that when the first person to actually do a quad came along, Burt Lancaster was going to give this person a $25,000 bonus, and buy the rights to do the movie of it. So when we were in California, we got in touch with the Lancaster people. Because that was what he was supposed to have said, a $25,000 bonus, and then a movie.

"But it wasn't true, after all. There wasn't any bonus."

Miguel's quadruple has taken aerialists beyond a tradition that, before July 10, 1982, was already extremely demanding.

The flying trapeze act is believed to have been developed by Jacques Leotard, from whom the costume takes its name. That was in 1859, in Paris. The first triple somersault is credited to Lena Jordan, a young Russian who performed it in Australia in 1897. She later joined the Barnum & Bailey Circus, and did it 28 times. The great Alfredo Codona of the Flying Codonas performed the triple consisently early in the century, and by the 1920s was considered the greatest aerialist of all time.

He was married to another circus star, Lillian Leitzel, whose whirling aerial act had made her even more famous than he. Envy of her fame, it is said, caused Codona to have an affair with a bareback rider named Vera Bruce. Leitzel then went on tour to Europe without him. In Copenhagen her rigging broke, and she fell to her death. Cordona married Bruce, but his triple somersault was never the same. He was injured several times and, unable to fly, left the circus in 1933. For a time thereafter, he worked in a gas station in California. In 1937 Vera Bruce sued him for divorce. In the lawyer's office, after asking to be left alone with his wife, he shot her five times with a .45-caliber pistol, and then turned the gun on himself.

The great trapeze artist of recent days was Tito Gaona. As a boy of 14, he had seen the movie "Trapeze" and been inspired. By the time he was 18, he was performing the triple for Ringling Bros. and Barnum & Bailey as consistently as had Alfredo Codona. He did it for 13 years. In 1973, in 600 attempts, he missed the triple only three times.

In 1974, Gaona began to talk about the quadruple. "I will certainly do a quadruple before I retire," he told Sports Illustrated. The quadruple, he said, would be like "winning seven gold medals in the Olympics or being the first man on the moon . . . I will succeed, and I will be always remembered."

By 1978 he was practicing in earnest, with videotape equipment. On the eve of an 11-week engagement at Madison Square Garden, a New York magazine trumpeted his quest with a long feature article. In it he was asked what would happen to the Flying Gaonas if he failed to achieve the quad. "We'll still have a great act," he said.

Tito Gaona attempted the quadruple every performance that engagement, and never did it once. He has still not done it. He left The Greatest Show on Earth the next year. His latest engagement was with the Knee Circus of Switzerland.

Barring injury, trapeze artists can have a long career. Juan Vazquez believes he has at least another 20 years, "and many fliers keep going right into their forties." The triple is no longer a rarity. Vazquez said there were now 25 or 30 performers doing it, six or seven of them consistently.

Injury is a probability, serious injury a possibility. Juan's sister, Margarite Michele, was badly hurt last year.

"In the act she does, she hangs from her hair," Juan explained. "When she does that, her hair is braided with a cotton cord. But in the last part of her routine, when she was spinning, somehow the cord came off and she went to the ground." Margarite broke her back and injured her neck. "But she's coming along well," Juan said. "She's going to be back to work soon."

The problem of the quad, however, goes beyond danger. It has made the Flying Vazquez troupe headliners, but in every town the circus posters now say, "Will he make it? Be There! See Miguel Vazquez Attempt the Quadruple Somersault!" In the end, it is not enough that he attempt it. To prove the science of his fame, he must be able to repeat the experiment on demand. And he knows it.

"The difficulty is in doing it every time," Miguel says solemnly.

Other young aerialists are in pursuit, for now they know it can be done.

He is ready for the first of them who succeeds.

"I will ask," Miguel says coolly, "how it feels to be the second."