n was going out Tom noticed a button dangling from the
back of his caller's coat. It hung by a thread, being one of the
pair usually sewed on the back of a cutaway garment.

"I think you had better take off that button before it falls,"
suggested Tom. "You may lose it, and perhaps it would be hard to
match."

"That's so. Thank you!" said Mr. Boylan. He tried to reach around
and get it, but he was too stout to turn easily, especially as the
coat was tight-fitting.

"I'll get it for you," offered Tom, as he pulled it off. "There is
one missing, though," he said, as he handed the button to the man.
And then Tom started as he saw the pattern of the one in his hand.

"One gone? That's too bad," murmured Mr. Boylan. "Those buttons
were imported, and I doubt if I can replace them. They are rather
odd."

"Yes," agreed Tom, gazing as if fascinated at the one he still
held. "They are rather odd."

And then, as he passed it over, like a flash it came to him where
he had seen a button like that before. He had found it in his
airship, which had been so mysteriously taken away and returned.

Tom could hardly restrain his impatience until Mr. Boylan had
gone. The young inventor had half a notion to produce the other
button, matching the one he had just pulled off his visitor's
coat, and tell where he had found it. But he held himself back. He
wanted to talk first to Ned.

And, when his chum came in, Tom cried:

"Ned, what do you think? I know who had my airship!"

"How?" asked Ned, in wonder.

"By that button clue! Yes, it's the same kind--they're as alike as
twins!" and Tom brought out the button which he had put away in
his desk. "See, Boylan had one just like this on the back of his
coat. The other was missing. Here it is--it was in the seat of my
airship, where it was probably pulled off as he moved about. Ned,
I think I've got the right clue at last."

Ned said nothing for several seconds. Then he remarked slowly:

"Well, Tom, it proves one thing; but not the other."

"What do you mean?"

"I mean that it may be perfectly true that the button came off Mr.
Boylan's coat, but that doesn't prove that he wore it. You can be
reasonably sure that the coat was having a ride in your Eagle, but
was Boylan in the coat? That's the question."

"In the coat? Of course he was in it!" cried Tom.

"You can't be sure. Someone may have borrowed his coat to take a
midnight ride in the airship."

"Mr. Boylan doesn't look to b