Ifandbut was one of several readers to point out the arrival in Oshkosh of the first practical jetpack. It was invented by a New Zealander Glenn Martin, who has been working on the idea for 27 years. He plans to sell the gizmos for somewhere in the neighborhood of $100K. While previous attempts at jetpacks have flown for at most a couple of minutes, Mr. Martin's invention can stay aloft for half an hour. Both "practical" and "jetpack" may need quotation marks, however: The device is huge and it's incredibly noisy. And, "It is also not, to put it bluntly, a jet. 'If you're very pedantic,' Mr. Martin acknowledged, a gasoline-powered piston engine runs the large rotors. Jet Skis, he pointed out, are not jets, and the atmospheric jet stream is not created by engines. 'This thing flies on a jet of air,' he said. Or, more simply, it flies."