ing on the idea for several days, and the
sketch was ready in his mind to be transferred to paper.

"What you goin' to do?"

"Something crazy," said Tommy. "A mirror isn't the only thing that
changes angles to right ones."

"You're the doctor," said the imperturbable Smithers.

He set to work. He puzzled Tommy sometimes, Smithers did. So far he
hadn't asked how much his pay was going to be. He'd worked
unintermittantly. He had displayed a colossal, a tremendous calmness.
But no man could work as hard as Smithers did without some powerful
driving-force. It was on the fourth day that Tommy learned what it
was.

The five coils had been made, and Tommy was assembling them with an
extraordinary painstaking care behind a screen, to hide what he was
doing. He'd discovered a peep-hole bored through the brick wall from
the lean-to where Von Holtz worked. He was no longer locked in there.
Tommy abandoned the pretense of imprisonment after finding an
automatic pistol and a duplicate key to the lock in Von Holtz's
possession. He'd had neither when he was theoretically locked up, and
Tommy laughed.

"It's a farce, Von Holtz," he said dryly, "this pretending you'll run
away. You're here spying now, for Jacaro. Of course. And you don't
dare harm either of us until you find out from me what you can't work
out for yourself, and know I have done. How much is Jacaro going to
pay you for the secret of the catapult, Von Holtz?"

Von Holtz snarled. Smithers moved toward him, his hands closing and
unclosing. Von Holtz went gray with terror.

"Talk!" said Smithers.

"A--a million dollars," said Von Holtz, cringing away from the brawny
red-headed man.

"It would be interesting to know what use it would be to him," said
Tommy dryly. "But to earn that million you have to learn what we know.
And to learn that, you have to help us do it again, on the scale we
want. You won't run away. So I shan't bother to lock you up hereafter.
Jacaro's men come and talk to you at night, don't they?"

* * * * *

Von Holtz cringed again. It was an admission.

"I don't want to have to kill any of them," said Tommy pleasantly,
"and we'll all be classed as mad if this thing gets out. So you go and
talk to them in the lane when you want to, Von Holtz. But if any of
them come near the laboratory, Smithers and I will kill them, and if
Smithers is hurt I'll kill you; and I don't imagine Jacaro wants that,
because he expects