er yonder toward the mountains; but I guess it
must be a big bird hovering high up, a condor perhaps," Frank replied.

"Well, there isn't any sign of the biplane, that's sure," Andy went on
in a relieved voice. "Perhaps they didn't have as good luck in landing
as we did, and had a nasty spill. Don't I hope they busted some of the
planes, or part of the little old Gnome engine, so we won't have to be
bothered with 'em again?"

Frank made no remark. While as a rule he refused to let anything like
bitterness dwell in his heart, still, this was a case where everything
was at stake; and if the bothersome revolutionists kept chasing them in
the biplane they were apt to give a great deal of trouble. And secretly
he could echo Andy's wish that the biplane might be temporarily
crippled, so as to be unfit for flying.

"Now, what's the programme?" asked Andy, when they had covered several
miles.

"We've just got to head for the mountains yonder," replied his
chum. "You know, he declared it was a valley that lay among the
mountains; and it must be, to be surrounded by high cliffs. Once we get
among the hills, we'll sail back and forth, combing the whole region,
and hoping sooner or later to discover his queer prison."

Andy lapsed into a state of silence; but he kept watching ahead as they
drew gradually nearer the uplifts. Doubtless but one thought held
dominion in his mind, and this was that somewhere amidst those same
mountains the father whom he loved so dearly was waiting, and hoping for
an answer to his appeals for aid.

CHAPTER XXI.

THE LAST LITTLE HOT AIR BALLOON.

"Did you ever see a wilder region?" asked Frank, about the middle of the
morning, when they had alighted on a broad, level plateau, so as to
allow him to look over some little matters connected with the engine,
that he believed needed attention.

Andy had been using the binoculars pretty much all the time they were
aloft, but without any success. Many times be began to think he had
sighted something that looked like cliffs rising up, and a wild hope had
seized upon his devoted heart; but upon Frank bringing the airship in
that quarter, in answer to his frantic appeals, it had proven to be a
false clue.

Cliffs they saw in plenty, but as yet none enclosing a valley so as to
imprison an unfortunate aeronaut, whose runaway balloon had dropped with
him into its depths.

Still, the day was not nearly half over. And the monoplane behaved
splendi