deduced must exist because of the chemical behavior of
the compound NH3, but which Denham alone had managed to procure.
Tommy deduced that it was an allotropic modification of the substance
which forms an amalgam with mercury, as metallic tin is an allotrope
of the amorphous gray powder which is tin in its normal, stable state.

He set to work with feverish excitement. For one hour, for two he
worked. At the end of that time he was explaining the matter curtly to
Smithers, so intent on his work that he wholly failed to hear a motor
car outside or to realize that it had also grown dark in this world of
ours.

"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