When they returned to the depot they found the professor busy over
some scientific book, sitting as undisturbed in the station, filled as
it was with shifting crowds, as if he was in his quiet study at the
museum.

"The train will be here in about fifteen minutes," he informed the
boys. "Better sit down and wait."

The three chums were rather tired, and were glad enough to take their
places on the comfortable benches.

"Chicago is a great place," announced Bob. "That restaurant, where we
had dinner--"

"Can't you say something that hasn't got any eating in it?" asked Ned.
"You're the limit, you are."

"Well," said Bob, "they certainly had fine pie in that place. I
wish--"

He stopped suddenly, as Jerry help up his hand to indicate silence.

"What's the matter?" asked Ned in a whisper, as he leaned forward.
"See some new kind of a bug for the professor?"

"I overheard that man back of us speaking," replied Jerry in a low
tone, nodding his head to indicate where he meant. The benches were
arranged so that travelers occupying them sat back to back. "His voice
sounded like one I've heard before, but I can't place it. I thought