"I suppose they splay out into a circle?"
"Right," he exulted. "And if they impeded from splaying out? If, instead of separate ribs, we have a hollow, bottomless cone of metal? Where does the force go?"
I thought it over, then said, with deliberation, "In all directions, Artie. One part shoving up-to-the-right, one part up-to-the-left, like that."
"Sure," he said, his face failing to fight a mischievous grin. "And since none of them move, where does the resultant force go?"
I shrugged, "Straight up, I guess—" Then my ears tuned in belatedly on what I'd said, and a moment later I squeaked, "Artie! Straight up!"
He nodded eagerly. "Or, of course, straight east, straight west, or whichever way the ferrule of this here theoretical umbrella was pointed at the time the twirling began. The point is, we can generate pure force in any direction. What do you think? Can you build it?"
"It'd be child's play. In fact, Artie, it's too damned simple to be believed! What's the hitch? Why hasn't anyone tried it before now?"
"Who knows?" he said, his blue eyes dancing. "Maybe no one ever thought of it before. You could sit down and twist a paper clip out of a hunk of soft wire, couldn't you? Easy as pie. But someone had to invent the thing, first. All the great inventions have been simple. Look at the wheel."
"Okay, okay," I said, since I'd been sold on his gadget the moment I pictured that umbrella moving ferruleward like a whirling arrow. "Still, it looks like you're getting something for nothing. A kind of by-your-own-bootstraps maneuver...."