note. I think he did this mean trick!"

"So do I, and we'll have a hunt for him. Let's go now."




CHAPTER VI

ON THE TRACK


The two boys gazed after Professors Henderson and Roumann. The
scientists were deep in a discussion of various technical matters,
which discussion, it was evident, made them oblivious to everything
else.

"Shall we ask them?" inquired Jack in a whisper.

"No; what's the use?" queried Mark. "Let's go off by ourselves, and
perhaps we can discover something. If we could once get on the trail of
the man who wrote the note, I think we could put our hands on the
person responsible for the blowing up of the motor."

"I agree with you. We won't bother them about our plans," and he waved
his hand toward the scientists, who had, by this time, entered the
house.

"In the first place," said Mark, as he and his chum turned from the
yard, and walked along a quiet country road, "I think our best plan