"Only in the atmosphere of the planet," I said.
"Sure, I know. But by the time the outer limits of the air are reached, the machine, with the same mass-thrust, will have less gravity-drag to fight, being that much farther from the Earth. The effect will be cumulative. The higher it gets, the more outward thrust it'll generate. Then nothing'll stop it!"
"You could be right," I admitted, hammering out helix after helix on an electric anvil (another gadget of Artie's; the self-heating anvil—The Thermovil—had begun life as a small inspiration in Artie's mind for a portable toaster).
It was just after sunset when we figured the welds were cool enough so we could test it. Onto the scale it went again, I flicked the toggle, and we stood back to watch the needle as the cones picked up speed. Along with the original whistling sound made by the cones we began to detect a shriller noise, one which abruptly became a genuine pain in the ear. As Artie and I became somewhat busy with screaming (the only thing we could think of on the spur of the moment to counteract the terrible waves of noise assaulting our tympana), it was all at once much easier to see the needle of the scale dropping toward zero, as the glass disc facing the dial dissolved into gritty powder, along with the glass panes in every window in the lab, the house, the heliport, and the movie theatre. (Not to mention those of a few farmhouses a couple of miles down the highway, but we didn't find that out till their lawyers showed up with bills for damages.)
Sure enough, though, the thing lifted. Up it bobbed, like a metal dirigible with agonizing gas pains, shrieking louder by the second. When the plaster started to trickle and flake from the walls, and the fillings in my teeth rose to a temperature just short of incandescence, I decided it was time to cancel this phase of the experiment, and, with very little regret, I flung a blanket-like canvas tarpaulin up and over the ascending machine before it started using its helices to screw into the ceiling. The cones bit into the tarpaulin, tangled, jammed, and the machine—mercifully noiseless, now—crashed back onto the scale, and lost a lot of symmetry and a couple of rivets.
"What's Plan C?" I said to Artie.
"Quiet!" he said, either because I'd interrupted his thinking or because that was our next goal.
The next four days were spent in the arduous and quite tricky business of reaming acoustically spaced holes along the flanges. Artie's theory was that if we simply ("simply" was his word, not mine) fixed it so that the sound made by each flange (anything whirly with a hole or two in it is bound to make a calculated noise) was of the proper number of vibrations to intermesh with the compression/rarefaction phases of the sounds made by the other flanges, a veritable sphere of silence would be thereby created, since there'd be no room for any sound waves to pass through the already crowded atmosphere about the machine.
"It'll make less noise than a mouse in sneakers drooling on a blotter!" enthused Artie, when I had it rigged again, and ready to go.