“You’d change your mind,” said Wilson, with irritated bluntness, “if you knew the entire scheme instead of only part of it.”

“Well,” said the constable, pensively, “I had the idea that it wouldn’t work, and up to now I’m right anyway.”

“Very well, then, let it stand at that, and give it a further show.  It has worked at least as well as your own methods, you perceive.”

The constable hadn’t anything handy to hit back with, so he discharged a discontented sniff, and said nothing.

After the night that Wilson had partly revealed his scheme at his house, Tom had tried for several days to guess out the secret of the rest of it, but had failed.  Then it occurred to him to give Roxana’s smarter head a chance at it.  He made up a supposititious case, and laid it before her.  She thought it over, and delivered her verdict upon it.  Tom said to himself, “She’s hit it, sure!” He thought he would test that verdict now, and watch Wilson’s face; so he said reflectively: 

“Wilson, you’re not a fool—­a fact of recent discovery.  Whatever your scheme was, it had sense in it, Blake’s opinion to the contrary notwithstanding.  I don’t ask you to reveal it, but I will suppose a case—­a case which you will answer as a starting point for the real thing I am going to come at, and that’s all I want.  You offered five hundred dollars for the knife, and five hundred for the thief.  We will suppose, for argument’s sake, that the first reward is advertised and the second offered by private letter to pawnbrokers and—­”

Blake slapped his thigh, and cried out: 

“By Jackson, he’s got you, Pudd’nhead!  Now why couldn’t I or any fool have thought of that?”

Wilson said to himself, “Anybody with a reasonably good head would have thought of it.  I am not surprised that Blake didn’t detect it; I am only surprised that Tom did.  There is more to him than I supposed.”  He said nothing aloud, and Tom went on: 

“Very well.  The thief would not suspect that there was a trap, and he would bring or send the knife, and say he bought it for a song, or found it in the road, or something like that, and try to collect the reward, and be arrested—­wouldn’t he?”

“Yes,” said Wilson.

“I think so,” said Tom.  “There can’t be any doubt of it.  Have you ever seen that knife?”

“No.”

“Has any friend of yours?”

“Not that I know of.”

“Well, I begin to think I understand why your scheme failed.”

“What do you mean, Tom?  What are you driving at?” asked Wilson, with a dawning sense of discomfort.

“Why, that there isn’t any such knife.”

“Look here, Wilson,” said Blake, “Tom Driscoll’s right, for a thousand dollars—­if I had it.”

Wilson’s blood warmed a little, and he wondered if he had been played upon by those strangers; it certainly had something of that look.  But what could they gain by it?  He threw out that suggestion.  Tom replied: