A curtain parts in the dressing area behind the Cincinnati arena and a half-dressed clown wanders out, wearing undershots and size 58 tennis shoe. He shuffles, rubbing his nose. The air smells like stale popcorn. "Fifty minutes!" someone cries.

Soon it's showtime and the unicycles burst through backstage with the elephants close behind them and a long blast of the ringmaster's whistle echoing on their heels. Glitter, feathers, hoops and stilts. Even backstage, the light glints off the start of the circus parade.

Cincinnati is the last stop for the Ringling Bros. and Barnum & Bailey Circus before it moves by rail to Washington, where it will open Wednesday night at the D.C. Armory. As the circus' last performance in Cincinnati unfolds, the silver train waits outside in the rain.

The polar bear is restless backstage and dances an impatient samba in his cage: right paw forward, back. Right paw foreward, back. When the bears are paired one stays awake, alert for danger. They must have triple sifted white ponderosa pine in their cages. They are not polite, these bears.

There are camels also, and Ilamas and brown bears and tigers and horses. The death-defying acrobats come from circus families - England, Bulgaria, Poland, Hungary. The trapeez artists are from Cuba and Mexico. The clowns are mostly young Americans who started out in theatre or dance and are crazy in love with circuses.

It has always been a life that sucked people up. Jeanette Williams, who trains horses and is third-generation circus, was sent off as a teen-ager to boarding school in Germany and sat in class making lists of the acts she was missing : 3:41 to 3:48, horses. 4:03 to 4:15, trapeze.

That is still true, in its ways.Ibolya Axt has a baby, also named Ibolya, who sleeps in a playpen at one corner of the dressing room. "I was born in a circus wagon," says the older Ibolya Axt. "My father was on the parade, and when he got home from the parade I was already born." She is from Hungary, and very small. The whole family is circus.

Twice a day, and three times on Saturdays, Ibolya Axt climbs onto her brother-in-law's back, shinnies up a pole on his shoulders, climbs her husband's back, shinnies up a pole on his shoulders, and then at the top of these poles and men executes a slow, graceful handstand.

She did this until she was 4 1/2 months pregnant. Now she has to find someone to hold the child while she difies gravity. "Do you mind," she says, and hands her baby to the reporter in the Cincinnati dressing room.

P. T. Barnum started it all, of course. Back in 1870, when he was managing Tom Thumb, Jenny Lind, and a New York City museum full of oddities to amaze the curious, two young Wisconsin showmen suggested that he turn circus sponsor. That appealed to Barnum, and a year later the huge tents of the P. T. Barnum Circus Museum and Menagerie spread out over three acres in Brooklyn.

Now put in on a train, said Barnum's young partners. So the circus took to the rails and rolled out into America. On the road Barnum encountered a hot young competitor named James Anthony Bailey, proprieter of the Cooper and Bailey Show, Howe's Great London Circus, and Sanger's Royal British Menagerie.

Bailey, who had changed his name from McGuiness as a young man to take on the name of a circus man he knew, ahd been orphaned at 6 and had run away at 11. He was hardcore circus.

In 1881, Barnum and Bailey signed up together, merging their assorted miraculous entertainments into a three ring extravaganza that somebody, probably Barnum, named The Greatest Show on Earth. Which in the years to come was not exactly accurate, because in Baraboo, Wis., five enterprising young sons of a harness maker named August Ringling were turning a homey little juggling and dance act into something that would vie for the title.

The Ringling Brothers, so the story goes, had gone down to the Misssissippi River one day when their family still lived in Iowa, and there they watched a circus unload from a stern-wheeler. Apparently they never recovered. Five of the seven brothers made a tent out of some borrowed rugs and began singing and dancing for the locals. When their father (who had a hard time staying in one place) moved the family to Baraboo, the brothers moved their little circus, too. They began hitting small town opera houses, and by 1890 they had an enormous tent and a 31 car railroad train that chugged around trying to outdo The Greatest Show on Earth.

For 18 years the two circuses competed frantically with each other, plastering the same towns with posters, scrambling for the most amazing and stupendous acts of the civilized world.

They shared ownership of a third, smaller circus for a while, and then in 1919, after four of the Ringling Brothers had died, the separate shows closed down and merged into the Ringling Bros. and Barnum & Bailey Combined Shows.

The president and producer now is Irvin Feld, a Hagerstown boy who sold snake oil through the Depression and grew up into an impresario. His musicians - Chubby Checker, Paul Anka, Nat King Cole - played arenas, and that was where Feld wanted the circus.

Arenas were warmer than tents in winter and cooler in summer and you didn't have to carry around all those tent rigging men. On July 16, 1956, in Pittsburgh, the Greatest Show on Earth played its last tent.

Feld has a special seat near the front in each arena, which the guards keep empty until 15 minutes before showtime. When he comes he watches closely, smoking a cigar, rubbing his hands thoughfully as the acts unfold. During the glittery spectacular in the middle of the show, children are brought in from the audience to hold the performers hands. That delights Feld, whose idea it was. "Isn't that lovely," he cries, pointing.

Between cities the performers live in a train 39 cars long and 10 feet wide. There are two of these trains, one for each of the two shows that travel simultaneously in different parts of the country. The performers have refrigerators, television, stereo record palyers, and for the little people the furniture is scaled down.

The neighborhood, wherever they are is a trainyard. In the evening the Eastern Europeans climb down on the gravel and the train tracks to build bonfires. They sing something, in Bulgarian or Polish or Hungarian, and the smell of cabbage and onion and beef barbecue comes up through the cars of the circus train.

A bird in the arena is bad luck, and some performers will not go inside until the bird is removed. People knitting in the audience also bring bad luck. You do not open umbrellas indoors, or go back into the dressing room when you are called to perform, or wish anybody good luck before going onstage.

The little clown with orange hair and Raggedy Ann sort of bloomers is Terry Frantom La Porte, of Annapolis. She is 20 and just married, "to the boss clown," she says, with pride, "Steve."

She was a theater major at Towson State in Baltimore, doing mime, when the circus people came to town. Terry La Porte tried out, and the subtle mystery of mime gave way to the huge, loose gestures of clowning.

She went to a Clown College, the Circus, institution in Venice, Fla., and fell in love. "He was teaching stiltwalking and how to make rubber noses. I was learning how to walk on stilts, I fell out and he caught me."

The week's shopping list for the circus 11 tons of hay, 1,200 pounds of crimped oats, 1,500 pound molasses-mostitened bran, 4,400 pounds of beef, 400 pounds carrots, 40 pounds oranges, 20 loaves unsliced brad (elephant snacks), 5 1/2 tons bedding straw, 4 tons triplesifted ponderosa pine sawdust, coconut oil soap (for the tiger shampoo, keeps fur soft and fluffy) to fill a 55 gallon drum, 10,000 rolls toilet paper, 25,000 baloons. A trash collection company is hired to carry off 20 cubic yards of animal waste per day.

The brown-haired child hunched over a paper and crayons is Carmelita Faria, 7. Now she is making a bunny rabit, white cotton on wood, with a green-topped carrot and two Easter eggs. Later she will hurtle off a trapeze and sail into her father's arms. Her bunny rabit is named Sam. "How do you spell Sam?" Carmelita asks.

Michu, the 33" tall Scotch-drinking Hungarian who gets "married" every show is a recreation of Tom Thumb's wedding, kicks a soccer ball down the concrete backstage.

Tito Goana, the Mexican trapeze artist who finished his flying triple somersault with a piroutte that looks as though some invisible hand is suspending him briefly in the air, is carrying on simultaneous conversations in Spanish, Hungarian and Polish. He says he is just learning Polish.

Leg first, hip first, head first, grunting: the clowns fold themselves one after another into their orange clown car. "Watch it, says a muffled voice, somewhere deep inside. They have foam rubber ukeleles and trombones with boxing gloves jammed on the ends and frantic orange and yellow hair.

One car always listens for the "Twelfth Street Rag," the sign that something has gone wrong and the clowns must distract the crowd. "It's the best role I've ever done," says a clown who once was in the theater.

Toward the end of the show it is quiet backstage, and a man in blue coveralls walks slowly, hands in the pockets, singing in Bulgarian. A clown without makeup sits on a crate and smokes a cigarette. "You've got a real responsibility," he muses, speaking of clowns and the audience whose laughter they crave. "You never want to break the magic. If you haven't got it, you can just take off that rubber nose and go home."