Ah, da Vinci, you have understood:

The speech writer in the office of the secretary of Commerce who spent three years developing "The Twid," a tubular device that counts the times one thumb can twiddle around the other.

The 84-year-old former insurance salesman who left his wife of 56 years when forced to choose between her and the automatic envelope sealing device he'd invented:

The car salesman, caressing the Styrofoam sides of his floating swimming pool, who says next year will be even better when he perfects his spaceship. He jealously guards details of the project.

Granted, the U.S. Patent Office lobby in Crystal City isn't da Vinci's Florentine village. But the 80 inventors, dreamers and visionaries at the agency's ninth annual Inventors Day over the weekend were his kind of people.

Take Charles Gelman, 5 feet 2 inches, in blue pinstripes and Woody Allen-type glasses, who was busy sliding a metal disc-like device back and forth over an envelope flap, a technological successor to the tongue -- U.S. Patent No. 4,180,430.

"I was married, but you know what she said to me when I retired? 'I want you to take that junk out of my house. Either it goes, or I will.' Well I cleared everything out and got a room," Gelman, an 84-year-old New Yorker, says in pure Brooklynese.

"All my life I was an insurance salesman , and all my life I hated it. All I ever wanted to do was work with my hands and create things. But I had to support a family.

" i would have stuck it out. But, she gave the orders. Now, I'm on my own and doing what I like best," he says. "I only wish I was 50 years younger and $100,000 richer. Then I could market this thing myself."

The thing. Two and a half years of work and two and half inches in diameter, topped by a globe that serves as both handle and water dispenser. Slide the envelope flap between the moistened discs, and, presto, no more sticky tongues. Moistens stamps, too.

"Cheaper than a good office girl," says Gelman.

Nearby, John Limegrover, a Buick salesman from Pittsburgh, scanned the crowd before relealing what he really wants to do. This year is only the try-out fluff.

"I'm going to build a flying saucer that is different than any other that has ever been built," Limegrover says in hushed tones, adding only that it will supplant the helicopter. His first and only patented invented, a model floating swimming pool dubbed the "Island Oasis," seems to fade before him. "The design is ready, only the money id needed. . .

"I have the plans for an underwater breathing device that will allow you to stay underwater indefinitely. You will be able to breathe without any outside sources. It will draw oxygen from the water and release the carbon monoxide back into the sea . . . The armed forces could use it."

How? "I can't reveal that," the car salesman says, snapping back into the more mundane description of the benefits of an Olympic size swimming pool -- encased in wire mesh -- on Styrofoam pontoons. Tie it to the back of a boat, submerge it, and the chicken-like coop becomes a sea swimmer's delight -- no anonymous squiggles nibbling at the toes. "Protects against sharks, you know."

And then, around the corner, the something-for-everyone.

"You know the saying that Nero fiddled while Rome burned? Well, that's not really true; his aide had a bit of a speech impediment. Nero twiddled while Rome burnes," says Horace Knowles, a 69-year-old speech writer and holder of U.S. Patent No. 4,227,342. Nattily decked in herringbone and oxford cloth, he gives his thumbs a twirl in the tubular thumb counting device -- available in gingham, with bells, or in Lucite with colored pellets. d

"I believe in thumb twiddlilng and with this device it keeps your thumbs from bumping into each other," says the inventor.

Five S-shaped aluminum tennis rackets, their heads bent a quarter of an inch in front of the handle, stand awkwardly on a corner table. Remnants of a court-side squabble?

"They're tennis rackets for people who have weak games or suffer from tennis elbow," explains Leo Planakis of Vienna. Only played tennis once himself, he says, but the purpose of the bend has something to do with "ventor force, that sort of thing."

Planakis says he doesn't really care if it sells. Like a true inventor, "Even if it doesn't sell I got my thousand dollars [in patent fees] out of it. I've got my name on it, I invented it and I'll frame it."