“Tom, I congratulate you!” declared the aged inventor.  “It is good news!”

“Yes, but we have bad news of Mr. Damon,” said Tom, and he told his father of the disappearance of the eccentric man.  Mr. Swift at once telephoned his sympathy to Mrs. Damon, and offered to do anything he could for her.

“But Tom can help you more than I can,” he said.  “You can depend on Tom.”

“I know that,” replied Mrs. Damon, over the wire.

And certainly Tom Swift had many things to do now.  He hardly knew at what to begin first, but now, since he was on the right road in regard to his photo telephone, he would work at improving it.

And to this end he devoted himself, after he had sent out a general alarm to the police of nearby towns, in regard to the disappearance of Mr. Damon.  The airship clue, he believed, as did the police, would be a good one to work on.

For several days after this nothing of moment occurred.  Mr. Damon could not be located, and Tom’s airship might still be sailing above the clouds as far as getting any trace of it was concerned.

Meanwhile the young inventor, with the help of Ned, who was given a leave of absence from the bank, worked hard to improve the photo telephone.

CHAPTER XV

THE AIRSHIP CLUE

“Now Ned, we’ll try again.  I’m going to use a still stronger current, and this is the most sensitive selenium plate I’ve turned out yet.  We’ll see if we can’t get a better likeness of you—­one that will be plainer.”

It was Tom Swift who was speaking, and he and his chum had just completed some hard work on the new photo telephone.  Though the apparatus did what Tom had claimed for it, still he was far from satisfied.  He could transmit over the wire the picture of a person talking at the telephone, but the likeness was too faint to make the apparatus commercially profitable.

“It’s like the first moving pictures,” said Tom.  “They moved, but that was about all they did.”

“I say,” remarked Ned, as he was about to take his place in the booth where the telephone and apparatus were located, “this double-strength electrical current you’re speaking of won’t shock me; will it?  I don’t want what happened to Eradicate to happen to me, Tom.”

“Don’t worry.  Nothing will happen.  The trouble with Rad was that he didn’t have the wires insulated when he turned that arc current switch by mistake—­or, rather, to play his joke.  But he’s all right now.”

“Yes, but I’m not going to take any chances,” insisted Ned.  “I want to be insulated myself.”

“I’ll see to that,” promised Tom.  “Now get to your booth.”

For the purpose of experiments Tom had strung a new line between two of his shops, They were both within sight, and the line was not very long; but, as I have said, Tom knew that if his apparatus would work over a short distance, it would also be successful over a long one, provided he could maintain the proper force of current, which he was sure could be accomplished.