We have satisfied ourselves we are not whistling in the dark," says Terry Miller, 47, of Crestline, Kans. Nor, he might add, is he full of hot air, though his invention, a pollution-free automobile, is.

Air-powered vehicles are nothing new. One of the earliest patents was issued in 1885 to Charles E. Buell of Springfield, Mass. But Miller, who has put $15,000 and most of the last three years in the project, has built a soon-to-be-patented sequential arrangement in which compressed air is passed from cylinder to cylinder, something like a bucket brigade, to turn an axle. The compressed air's power is thus exhausted through four cylinders, rather than escaping after only one as it did in previous inventions.

Miller's prototype, a three-wheeled, 1,400-pound vehicle, can travel up to 32 mph and has a range of some 17 miles (the consumer version will go 45 miles). He can refuel the compressed-air containers in four minutes, using a windmill, at an overall cost of less than a cent a mile.

Miller, fascinated with engines since childhood, graduated from the Spartan School of Aeronautics in Tulsa. He's licensed as a pilot and an aircraft mechanic instructor. While he was developing his air-powered car, Miller and his second wife, Sharon, made their living by customizing campers.

Currently he is demonstrating the car full-time at state fairs and energy exhibits, although he has not entered it in the $25,000 Los Angeles-to-Rochester, N.Y. rally in September for new alternative-fuel cars. Alcohol cars and salad-oil cars will be among those competing for that prize.

Mass-produced, Miller's car would sell for about $4,000, he estimates, including his modest royalty of about $10 per vehicle. Meanwhile he sells design-and-building instructions for $2. After five Buffalo, Okla. high school students used them to build their own air car, they applied to drive it at Ronald Reagan's inauguration. Their rejection letter arrived the day after the parade.

But the air car is not a toy, Miller insists. "It's a weapon that can be used," he says. "It allows us to think about wind energy as a viable alternative to petroleum products."