A balloon shaped like a French cha teau containing two businessmen wafted up over the Mall yesterday, and 350 well-dressed Washingtonians applauded elegantly, and 16 early-morning commuters drove across the dividing strip.

The two businessmen were Malcolm S. Forbes of Forbes magazine and Postmaster General William F. Bolger, and the business they transacted there 80 feet high, with half the press in the Western Hemisphere gawking up slack-jawed, was the sale of $10 worth of the new 20-cent stamps commemorating the 200th anniversary of ballooning.

Actually, they were a bit ahead of themselves. It was June 5, 1783, when Jacques-Etienne and Joseph-Michel Montgolfier took off from the town square of Annonay, France, in a 30-foot linen-and-paper "globe aerostatique" held together with buttons and powered by burning chopped wool and straw in an iron grate. The reason for the chopped wool was that it is very smoky, and the brothers believed it was the smoke that made things go up because of something they called "levity." Anyway, they were lifted a mile high, to their delight and the utter flabbergastation of the local peasants, and came down a couple miles east. They were so delighted, in fact, that three months later they sent up another balloon from Versailles crewed by a duck, a rooster and a sheep, who were only moderately delighted.

Levity was the word yesterday too, even though it was 8 a.m. It had to be early because balloons don't like wind, especially ones shaped like cha teaux with fac,ades 105 feet long and 65 feet high. Forbes, a master balloonist who in 1973 pioneered the first coast-to-coast flight in a hot-air balloon, wore a red jacket with "Capitalist Tools" on the back and attributed his success to "sheer ability and the death of my father."

There was some excitement getting the cha teau aloft. The crew had been working since 6, anxiously watching the sky from which everybody had expected rain or snow or both. A spare gondola, looking like a giant society picnic basket with its varnished wicker and chocolate velvet trim, swung from a crane cable in case it did snow and they had to fake the pictures.

But they didn't. Forbes turned on the hot air which went "Haaaaaaah" and the cha teau dutifully rose. Then he was supposed to say something into a mike, but to be heard he had to turn off the hot air, upon which the cha teau sank to the ground again. Some TV announcer, safe and warm in his studio, was trying to interview him in midair.

"Can you give me a two-minute warning?" the 63-year-old Forbes called to the TV crew. He sounded calm and in command. He won the Bronze Star and Purple Heart in World War II. Cameramen clustered around.

"He wants a two-minute warning," shouted the relay woman.

"You got a two-minute warning," yelled the technician.

The hot air went "Haaaaaaah."

"We can't stay up very long," said Forbes. "We're low on fuel. Oh well, if we run out of hot air, we've got the Capitol building behind us." It was only the second hot-air joke of the day, which was not bad considering. Finally, five assistants stood under the gondola and held it up during the interview.

When you look up inside a balloon you see a strange serene indoor world with soft light like inside a greenhouse.

Forbes owns the cream and brown balloon, by the way, which is a scale model of the Cha teau de Balleroy in Normandy. Forbes owns the cha teau too. Today he is off to Pakistan to fly the balloon up the Khyber Pass. That's nothing. A while back he flew it in China, which forbid him to let it off its tether, but somehow it got loose and drifted over the landscape, causing an incident when he landed.

Balloons will do that. J.A.C. Charles, following the Montgolfiers on Aug. 27, 1783, in a hydrogen balloon that rose from the Champ de Mars in Paris, drifted 15 miles and came down in a hayfield where peasants pitchforked it to death and scared Charles out of his pants.

The Chinese ambassador was here yesterday, as were hundreds of diplomats, media executives and people from Big Business and Big Government, including Maryland Sen. Charles McC. Mathias, looking more like George Washington than ever. He lives in Chevy Chase, and that is as far away as he gets to go when Congress recesses, so he has offseason things like this all to himself.

And the breakfast. There is always a breakfast with a balloon launching. It was served in a warmed red-and-white-striped tent with balloon centerpieces and included Moet & Chandon at each table, scores of white-gloved waiters, and truffles, lox, blinis, croissants, kiwi fruit, scrambled eggs in pastry shells, lamb chops, sausage, Canadian bacon, raspberry souffle' glace, man-killing chocolate mousse cake and two bars, all paid for by Forbes. Some people didn't get away till nearly lunch time.