"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.