"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 after he had altered two important units of the device, a new note came to his ears through the "watch-case" receivers that were clamped to his head.