"Well, this gets me!" exclaimed Tom.

"It sure is strange," agreed Ned. "How did she come here?"

"She didn't come alone--that's sure," went on Tom. "Someone brought her here, made a landing, and got away before we could get out."

The two chums were standing near the Eagle, which had come back so mysteriously.

"Just a couple of seconds sooner and we'd have seen who brought her here," went on Tom. "But they must have shut off the motor some distance up, and then they volplaned down. That's why we didn't hear them."

Ned went over and put his hand on the motor.

"Ouch!" he cried, jumping back. "It's hot!"

"Showing that she's been running up to within a few minutes ago," said Tom. "Well, as I said before, this sure does get me. First these mysterious men take my airship, and then they bring her back again, without so much as thanking me for the use of her."

"Who in the world can they be?" asked Ned.

"I haven't the least idea. But I'm going to find out, if it's at all possible. We'll look the machine over in the morning, and see if we can get any clues. No use in doing that now. Come on, we'll put her back in the hangar."

"Say!" exclaimed Ned, as a sudden idea came to him. "It couldn't be Mr. Damon who had your airship; could it, Tom?"

"I don't know. Why do you ask that?"

"Well, he might have wanted to get away from his enemies for a while, and he might have taken your Eagle, and--"

"Mr. Damon wouldn't trail along with a crowd like the one that took away my airship," said Tom, decidedly. "You've got another guess coming, Ned. Mr. Damon had nothing to do with this."

"And yet the night he disappeared an airship was heard near his house."

"That's so. Well, I give up. This is sure a mystery. We'll have a look at it in the morning. One thing I'll do, though, I'll telephone over to Mr. Damon's house and see if his wife has heard any news. I've been doing that quite often of late, so she won't think anything of it. In that way we can find out if he had anything to do with my airship. But let's run her into the shed first."

This was done, and Koku, the giant, was sent to sleep in the hangar to guard against another theft. But it was not likely that the mysterious men, once having brought the airship back, would come for it again.

Tom called up Mrs. Damon on the telephone, but there was no news of the missing man. He expressed his sympathy, and said he would come and see her soon. He told Mrs. Damon not to get discouraged, adding that he, and others, were doing all that was possible. But, in spite of this, Mrs. Damon, naturally, did worry.

The next morning the two chums inspected the airship, so mysteriously returned to them. Part after part they went over, and found nothing wrong. The motor ran perfectly, and there was not so much as a bent spoke in the landing wheels. For all that could be told by an inspection of the craft she might never have been out of the hangar.

"Hello, here's something!" cried Tom, as he got up from the operator's seat, where he had taken his place to test the various controls.

"What is it?" asked Ned.

"A button. A queer sort of a button. I never had any like that on my clothes, and I'm sure you didn't. Look!" and Tom held out a large, metal button of curious design.

"It must have come off the coat of one of the men who had your airship, Tom," said his chum. "Save it. You may find that it's a clue."

"I will. No telling what it may lead to. Well, I guess that's all we can find."

And it was. But Tom little realized what a clue the button was going to be. Nothing more could be learned by staring at the returned airship, so he and Ned went back to the house.

Tom Swift had many things to do, but his chief concern was for the photo telephone. Now that he was near the goal of success he worked harder than ever. The idea Ned had given him of being able to take the picture of a person at the instrument--without the knowledge of that person--appealed strongly to Tom.

"That's going to be a valuable invention!" he declared, but little he knew how valuable it would prove to him and to others.

It was about a week later when Tom was ready to try the new apparatus. Meanwhile he had prepared different plates, and had changed his wiring system. In the days that had passed nothing new had been learned concerning the whereabouts of Mr. Damon, nor of the men who had so mysteriously taken away Tom's airship.

All was in readiness for the trial. Tom sent Ned to the booth that he had constructed in the airship hangar, some distance away from the house. The other booth Tom had placed in his library, an entirely new system of wires being used.

"Now Ned," explained Tom, "the idea is this! You go into that booth, just as if it were a public one, and ring me up in the regular way. Of course we haven't a central here, but that doesn't matter. Now while I'm talking to you I want to see you. You don't know that, of course."

"The point is to see if I can get your picture while you're talking to me, and not let you know a thing about it."

"Think you can do it, Tom?"

"I'm going to try. We'll soon know. Go ahead."

A little later Ned was calling up his chum, as casually as he could, under the circumstances.

"All right!" called Tom to his chum. "Start in and talk. Say anything you like--it doesn't matter. I want to see if I can get your picture. Is the light burning in your booth?"

"Yes, Tom."

"All right then. Go ahead."

Ned talked of the weather--of anything. Meanwhile Tom was busy. Concealed in the booth occupied by Ned was a sending plate. It could not be seen unless one knew just where to look for it. In Tom's booth was a receiving plate.

The experiment did not take long. Presently Tom called to Ned that he need stay there no longer.

"Come on to the house," invited the young inventor, "and we'll develope this plate." For in this system it was necessary to develope the receiving plate, as is done with an ordinary photographic one. Tom wanted a permanent record.

Eagerly the chums in the dark room looked down into the tray containing the plate and the developing solution.

"Something's coming out!" cried Ned, eagerly.

"Yes! And it's you!" exclaimed Tom. "See, Ned, I got your picture over the telephone. Success! I've struck it! This is the best yet!"

At that moment, as the picture came out more and more plainly, someone knocked on the door of the dark room.

"Who is it?" asked Tom.

"Gen'man t' see you," said Eradicate. "He say he come from Mistah Peters!"

"Mr. Peters--that rascally promoter!" whispered Tom to his chum. "What does this mean?"