course the 'plane now followed carried it at times much farther from the
mountain side than it had been when first the strange light was noticed.
That light followed the _Snowbird_ up and up in similar spirals, and the
boys were soon convinced that Professor Henderson's discovery was a
fact. The lamp was in the bow of another air craft.

"But why should we keep over them?" asked Jack. "There is no danger; is
there?"

"We do not know who they are," said the professor, shortly. "The craft
came right out of a fastness in the mountain-side--a place difficult
to reach, and which would not seem to attract aviators of the ordinary
class." "I know what he is thinking of," cried Mark, suddenly. "I read
in the paper that the Department of Justice officers are after some
big smugglers and that it is believed the criminals, in going back and
forth into Canada, use some kind of an aerial craft. Isn't that so,
Professor Henderson?"

"I had the fact in mind. The flying machine is being put already to
uses that are not commendable, to say the least. The Maine and Canadian
border has for years been used by bands of smugglers, and if one of
these gangs have purchased and can use a flying craft, they may make
the revenue men a deal of trouble."

"You're right, sir. And I read likewise that the government officers
proposed using an aeroplane themselves to track the smugglers. Perhaps
the villains, if that is their ship below us, may take us for secret
service men."

As he spoke the lamp so far below them darted up at a sudden and sharp