"Cone-shaped flywheels?" I said. "Why, for pete's sake?" "Only," he said, with specious casualness, "in order to develop a centrifugal thrust that runs in a _straight line_!" "A centr--" I said, then sat back from the drawings, blinking. "That's impossible, Artie." "And why should it be?" he persisted. "Picture an umbrella, with the fabric removed. Now twirl the handle on its axis. What do the ribs do?" "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...." "An inventor," said Artie, quoting his favorite self-coined aphorism, "must never think like a scientist!" "But"--I said, more to stem the tide I expected than to really make a coherent objection. "An inventor," he went dreamily onward, "is essentially a dreamer; a scientist is an observer. An inventor tries to make a result he wants happen; a scientist tries to tell the inventor that the result cannot be achieved." "Please. Artie. Don't tell me about the bee again."