nsist upon being told the equation!
I insist upon it! I will not make the metal if you do not tell me!"

Smithers was in the laboratory, of course. He had been surveying the
big solenoid-catapult and scratching his chin reflectively. Now he
turned.

* * * * *

But Tommy took Von Holtz by the shoulders. And Tommy's hands were the
firm and sinewy hands of a sportsman, if his brain did happen to be
the brain of a scientist. Von Holtz writhed in his grip.

"There is only one substance which could be the metal I need, Von Holtz,"
he said gently. "Only one substance is nearly three-dimensional.
Metallic ammonium! It's known to exist, because it makes a mercury
amalgam, but nobody has been able to isolate it because nobody has
been able to give it a fourth dimension--duration in time. Denham did
it. You can do it. And I need it, and you'd better set to work at the
job. You'll be very sorry if you don't, Von Holtz!"

Smithers said with a vast calmness.

"I got me a hunch. So if y'want his neck broke...."

Tommy released Von Holtz and the lean young man gasped and sputtered
and gesticulated wildly in a frenzy of rage.

"He'll make it," said Tommy coldly. "Because he doesn't dare not to!"

Von Holtz went out of the laboratory, his weak-looking eyes staring
and wild, and his mouth working.

"He'll be back," said Tommy briefly. "You've got to make a small model
of that big catapult, Smithers. Can you do it?"

"Sure," said Smithers. "The ring'll be copper tubing, with
pin-bearings. Wind a coil on the lathe. It'll be kinda rough, but
it'll do. But gears, now...."

"I'll attend to them. You know how to work that metallic ammonium?"

"If that's what it was," agreed Smithers. "I worked it for the
Professor."

Tommy leaned close and whispered:

"You never made any gears of that. But did you make some springs?"

"Uh-huh!"

Tommy grinned joyously.

"Then we're set and I'm right! Von Holtz wants a mathematical formula,
and no one on earth could write one, but we don't need it!"

* * * * *

Smithers rummaged around the laboratory with a casual air, acquired
this and that and the other thing, and set to work with an astounding
absence of waste motions. From time to time he inspected the great
catapult thoughtfully, verified some impression, and went about the
construction of another part.

And when Von Holtz did not return, Tommy hunted for him. He sudden