WHEN THE MONTGOLFIER brothers used burning straw to power the first hot air balloon flight on November 21, 1783, they really started something. Within two weeks a fellow Parisian piloted the first hydrogen balloon; within a year they were ballooning in Bladensburg.

Balloons have been used for all sorts of things. During the Civil War, Confederate observers used hot air balloons to see what the Yankees were up to; the Yanks used hydrogen models. All combatants used observation balloons during World War I. The Japanese launched incendiary-laden balloons against our West Coast in World War II, and as recently as the early 1960s the U.S. used them to spy on Soviet installations. Balloons still carry weather probes and scientific instruments, but satellites do the spying.

In 1979 two East German families built a propane-powered balloon from sheets and curtains and floated to freedom.

Hot air, usually 180o, is used in nearly all balloons. Helium, used mostly in long-distance balloon racing, costs $2,000 to $3,000 per flight, putting it out of the reach of most. The average cost of propane for a hot-air flight is $35 to $40.

But the sport's greatest expense is the balloon. The rig -- envelope, burners, and gondola -- costs from $10,000 to $50,000. The Balloon Federation of America estimates that there are 6,500 pilots in the country and about half that many balloons. Envelopes are made of ripstop nylon, the same type of material used for jackets and tents.

The Federal Aviation Administration requires that private pilots have at least 10 hours of training and commerical flyers no less than 35 hours.

Although ballooning may look dangerous from the ground, "I don't look at it as a death-defying act," says Pat Michaels, who operates Sky High Adventures in Poolesville. "I don't think it's as hazardous as driving around the Beltway. You just have to exercise a reasonable amount of caution."

"Nobody flies when it's too windy," says Frank Gallegos of Hunt Country Balloons. "That's too dangerous. If you fly early morning or late afternoon, it's not too windy. When it starts getting windy, it's time to get down. Otherwise, it's a very gentle sport and not at all dangerous."

National Transportation and Safety Board figures support that. Last year there were 34 reportable balloon accidents but no fatalities. In 1983, three persons were killed in 29 accidents. A total of seven died in two of 1982's 30 accidents.

No balloonist has yet circumnavigated the planet as in Jules Verne's "Around the World in 80 Days," but two attempts are planned. Joe Kittinger, who last September made the first solo Atlantic crossing in the helium Rosie O'Grady Balloon of Peace, has tentatively scheduled an around-the- world effort in November. Englishman Julian Knott, holder of the world hot-air balloon altitude record (above 60,000 feet), may launch his globe-girdler as soon as this summer. Smithsonian curator Tom Crouch, author of "The Eagle Aloft: Two Centuries of the Balloon in America," will run the communications center for Knott's journey from the lobby of the Air & Space Museum.