PISCATAWAY, N.J.—When the guy swung at Mark Donnelly, his only means of defense was a black umbrella—and a foppish one at that. But, ducking under a roundhouse punch, he jabbed the pointy end of the umbrella into the attacker's gut, stopping him cold.

Mr. Donnelly, who is 43 years old and several inches short of 6 feet tall, then straightened his waistcoat, and the two men shook hands.

The skirmish was a rare demonstration of Bartitsu, an obscure Victorian system of gentlemanly self-defense practiced by Sherlock Holmes, Sir Arthur Conan Doyle's legendary detective.

Mr. Donnelly's assailant was 42-year-old Robert Earhart, a professional stuntman in protective gear.

"Is this a good weapon?" Mr. Donnelly, clutching the umbrella, asked a crowd of several dozen here last weekend at the Steampunk World's Fair, an annual steam-age science-fiction festival. "It is if it's in my hands at the time of attack."