"You see, Smithers, if a two-dimensioned creature wanted to adjust two right angles at right angles to each other, he'd have them laid flat, of course. And if he put a spring at the far ends of those right angles--they'd look like a T, put together--so that the cross-bar of that T was under tension, he'd have the equivalent of what I'm doing. To make a three-dimensioned figure, that imaginary man would have to bend one side of the cross-bar up. As if the two ends of it were under tension by a spring, and the spring would only be relieved of tension when that cross-bar was bent. But the vertical would be his time dimension, so he'd have to have something thin, or it couldn't be bent. He'd need something 'thin in time.' "We have the same problem. But metallic ammonium is 'thin in time.' It's so fugitive a substance that Denham is the only man ever to secure it. So we use these rings and adjust these springs to them so they're under tension which will only be released when they're all at right angles to each other. In our three dimensions that's impossible, but we have a metal that can revolve in a fourth, and we reinforce their tendency to adjust themselves by starting them off with a jerk. We've got 'em flat. They'll make a good stiff jerk when they try to adjust themselves. And the solenoid's a bit eccentric--" "Shut up!" snapped Smithers suddenly. * * * * * He was facing the door, bristling. Von Holtz was in the act of coming in, with a beefy, broad-shouldered man with blue jowls. Tommy straightened up, thought swiftly, and then smiled grimly. "Hullo, Von Holtz," he said pleasantly. "We've just completed a model catapult. We're all set to try it out. Watch!" He set a little tin can beneath the peculiar device of copper-tubing rings. The can was wholly ordinary, made of thin sheet-iron plated with tin as are all the tin cans of commerce. "You have the catapult remade?" gasped Von Holtz. "Wait! Wait! Let me look at it!" For one instant, and one instant only, Tommy let him see. The massed set of concentric rings, each one of them parallel to all the others. It looked rather like a flat coil of tubing; certainly like no particularly obscure form of projector. But as Von Holtz's weak eyes fastened avidly upon it, Tommy pressed the improvised electric switch. At once that would energize the solenoid and release all the tensed springs from their greater tension, for an attempt to reach a permanent equilibrium.