vention. If
I can perfect it, it may be valuable."

"Yes, but what is it? What's it for?" persisted Thurman.

Jack explained what he hoped to accomplish with the instrument, and an
instant later was sorry he had done so, for he noticed an expression of
cupidity creep into Thurman's eyes. The youth persisted in asking a host
of questions, and Jack, having started to explain, could not very well
refuse to answer. Besides, inventors are notoriously garrulous about
their brain children, and Jack, even though he did not like Thurman,
soon found himself talking away at a great rate.

"Huh, I don't think the idea's worth a cent," sniffed Thurman
contemptuously, when Jack had finished.

"I guess that's where you and I differ," said Jack, controlling his
temper with some difficulty, for the sneer in Thurman's voice had been
marked. "I'm going to make it a success, and then we shall see."

He left the wireless room, and the instant he was gone Thurman, with a
crafty look on his flabby face, eagerly began examining the detector. As
he was doing so Jack, who had forgotten his cap, suddenly reentered the
wireless room. Thurman had been so intent on his scrutiny of the
detector that he did not hear him.

"You appear to be taking great interest in that useless invention," said
Jack in a quiet voice.

Thurman started and spun round. His face turned red and he had an almost
guilty look.

"I didn't think you were coming creeping back like that," he exclaimed,
"a fellow would almost think you were spying on him."

"Have you any reason to fear being spied upon?" asked Jack.

"Me? No, not the least. That's a funny question."

"I want to tell you, Thurman, that my invention is not yet completed and
therefore, of course, is not patented. I was pretty free with you in
describing it, and I shall trust to your honor not to talk about it to
anyone."

"Certainly not," blustered Thurman. "I'm not that sort of a chap."

But, after Jack had gone out, he resumed his study of the detector a
second time, desisting every time he heard a step outside.

"So it's not patented, eh?" he muttered to himself. "That will help.
It's an idea there that ought to be worth a pot of money."

CHAPTER XXIII.

JACK'S BIG SECRET.

The next day Jack found an opportunity to sandwich in some work on his
invention between his regular work. The thing fascinated him, and he
tried and tested it in a hundred different combinations. Suddenly, just
aft