Troublesome problems seemed to be multiplying for Tom Swift. He admitted as much himself after the failure to capture the man who had telephoned to Mrs. Damon. He had hoped that his plan of sending detectives to the location of the telephones would succeed. Since it had not the youth must try other means.

"Now, Ned," he said to his chum, when they were on their way from Mrs. Damon's, it being impossible to do anything further there. "Now, Ned, we've got to think this thing out together."

"I'm willing, Tom. I'll do what I can."

"I know you will. Now the thing to do is to go at this thing systematically. Otherwise we'll be working around in a circle, and won't get anywhere. In the first place, let's set down what we do know. Then we'll put down what we don't know, and go after that."

"Put down what you don't know?" exclaimed Ned. "How are you going to put down a thing when you don't know it?"

"I mean we can put a question mark after it, so to speak. For instance we don't know where Mr. Damon is, but we want to find out."

"Oh, I see. Well, let's start off with the things we do know."

The two friends were at Tom's house by now, having come from Waterford in Tom's airship. After thinking over all the exciting happenings of the past few days, Tom remarked: "Now, Ned, for the things we do know. In the first place Mr. Damon is missing, and his fortune is about gone. There is considerable left to Mrs. Damon, however, but those scoundrels may get that away from her, if we don't watch out. Secondly, my airship was taken and brought back, with a button more than it had when it went away. Said button exactly matched one off Mr. Boylan's coat."

"Thirdly, Mr. Damon was either taken away or went away, in an airship--either in mine or someone else's. Fourthly, Mrs. Damon has received telephonic communications from the man, or men, who have her husband. Fifthly, Mr. Peters, either legally or illegally, is responsible for the loss of Mr. Damon's fortune. Now: there you are--for the things we do know."

"Now for the things we don't know. We don't know who has taken Mr. Damon away, nor where he is, to begin with the most important."

"Hold on, Tom, I think you're wrong," broke in Ned.

"In what way?"

"About not knowing who is responsible for the taking away of Mr. Damon. I think it's as plain as the nose on your face that Peters is responsible."

"I can't see it that way," said Tom, quickly. "I will admit that it looks as though Boylan had been in my airship, but as for Peters taking Mr. Damon away--why, Peters is around town all the while, and if he had a hand in the disappearance of Mr. Damon, do you think he'd stay here, when he knows we are working on the case? And would he send Boylan to see me if Boylan had been one of those who had a hand in it? They wouldn't dare, especially as they know I'm working on the case."

"Peters is a bad lot. I'll grant you, though, he was fair enough to pay for my motor boat. I don't believe he had anything to do with taking Mr. Damon away."

"Do you think he was the person who was talking to Mrs. Damon about the papers?"

"No, Ned. I don't. I listened to that fellow's voice carefully. It wasn't like Peters's. I'm going to put it in the phonograph, too, and let you listen to it. Then see what you say."

Tom did this, a little later. The record of the voice, as it came over the wire, was listened to from the wax cylinder, and Ned had to admit that it was not much like that of the promoter.

"Well, what's next to be done?" asked the young banker.

"I'm going to set a trap," replied Tom, with a grin.

"Set a trap?"

"Yes, a sort of mouse-trap. I'm glad my photo telephone is now perfected, Ned."

"What has that got to do with it?"

"That's going to be my trap, Ned. Here is my game. You know this fellow--this strange unknown--is going to call up Mrs. Damon to-morrow. Well, I'll be ready for him. I'm going to put in the booth where he will telephone from, one of my photo telephones--that is, the sending apparatus. In Mrs. Damon's house, attached to her telephone, will be the receiving plate, as well as the phonograph cylinder."

"When this fellow starts to talk he'll be sending us his picture, though he won't know it, and we'll be getting a record of his voice. Then we'll have him just where we want him."

"Good!" cried Ned. "But, Tom, there's a weak spot in your mouse- trap."

"What is it?"

"How are you going to know which telephone the unknown will call up from? He may go to any of a hundred, more or less."

"He might--yes. But that's a chance we've got to take. It isn't so much of a chance, though when you stop to think that he will probably go to some public telephone in an isolated spot, and, unless I'm much mistaken he will go to a telephone near where he was to-day. He knows that was safe, since we didn't capture him, and he's very likely to come back."

"But to make the thing as sure as possible, I'm going to attach my apparatus to a number of public telephones in the vicinity of the one near the sawmill. So if the fellow doesn't get caught in one, he will in another. I admit it's taking a chance; but what else can we do?"

"I suppose you're right, Tom. It's like setting a number of traps."

"Exactly. A trapper can't be sure where he is going to get his catch, so he picks out the place, or run-way, where the game has been in the habit of coming. He hides his traps about that place, and trusts to luck that the animal will blunder into one of them."

"Criminals, to my way of thinking, are a good bit like animals. They seem to come back to their old haunts. Nearly any police story proves this. And it's that on which I am counting to capture this criminal. So I'm going to fit up as many telephones with my photo and phonograph outfit, as I can in the time we have. You'll have to help me. Luckily I've got plenty of selenium plates for the sending end. I'll only need one at the receiving end. Now we'll have to go and have a talk with the telephone manager, after which we'll get busy."

"You've overlooked one thing, Tom."

"What's that, Ned?"

"Why, if you know about which telephone this fellow is going to use, why can't you have police stationed near it to capture him as soon as he begins to talk?"

"Well, I did think of that, Ned; but it won't work."

"Why not?"

"Because, in the first place this man, or some of his friends, will be on the watch. When he goes into the place to telephone there'll be a look-out, I'm sure, and he'd either put off talking to Mrs. Damon, or he'd escape before we had any evidence against him."

"You see I've got to get evidence that will stand in the courts to convict this fellow, and if he's scared off before we get that, the game will be up."

"That's what my photo telephone will do--it will get the evidence, just as a dictaphone does. In fact, I'm thinking of working it out on those lines, after I clear up this business."

"Just suppose we had detectives stationed at all the telephones near the sawmill, where this fellow would be likely to go. In the first place no one has seen him, as far as we know, so there's no telling what sort of a chap he is. And you can't go up to a perfect stranger and arrest him because you think he is the man who has spirited away Mr. Damon."

"Another thing. Until this fellow has talked, and made his offer to Mrs. Damon, to restore her husband, in exchange for certain papers, we have no hold over him."

"But he has done that, Tom. You heard him, and you have his voice down on the wax cylinder."

"Yes, but I haven't had a glimpse of his face. That's what I want, and what I'm going to get. Suppose he does go into the telephone booth, and tell Mrs. Damon an address where she is to send the papers. Even if a detective was near at hand he might not catch what was said. Or, if he did, on what ground could he arrest a man who, very likely, would be a perfect stranger to him? The detective couldn't say: 'I take you into custody for telephoning an address to Mrs. Damon.' That, in itself, is no crime."

"No, I suppose not," admitted Ned. "You've got this all thought out, Tom."

"I hope I have. You see it takes quite a combination to get evidence against a criminal--evidence that will convict him. That's why I have to be so careful in setting my trap."

"I see, Tom. Well, it's about time for us to get busy; isn't it?"

"It sure is. There's lots to do. First we'll go see the telephone people."

Tom explained to the 'phone manager the necessity for what he was about to do. The manager at once agreed to let the young inventor have a free hand. He was much interested in the photo telephone, and Tom promised to give his company a chance to use it on their lines, later.

The telephone near the sawmill was easily located. It was in a general store, and the instrument was in a booth. To this instrument Tom attached his sending plate, and he also substituted for the ordinary incandescent light, a powerful tungsten one, that would give illumination enough to cause the likeness to be transmitted over the wire.

The same thing was done to a number of the public telephones in that vicinity, each one being fitted up so that the picture of whoever talked would be transmitted over the wire when Tom turned the switch. To help the plan further the telephone manager marked a number of other 'phones, "Out of Order," for the time being.

"Now, I think we're done!" exclaimed the young inventor, with a sigh, late that night. He and Ned and the line manager had worked hard.

"Yes," answered the young banker, "the traps are set. The question is: Will our rat be caught?"