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 splendidly; so that they could hope to continue the search as long as their supply of gasolene held out.

“I’ll never give up hunting,” Andy declared, as he stood there, watching his chum potter with the engine and examine things in general.  “No, not as long as this fine little machine stands by us.  We can get more gasoline if necessary, for we brought a good supply aboard the boat.  When we’ve gone as far as we dare down this way we’ll make another start further on.”

“I’m with you, Chum Andy, and you don’t need to be told that,” observed Frank, quietly, while he worked on.

“As if I didn’t know that and counted on you through thick and thin,” declared the other, with a look of sincere affection.

“Well, now we’re ready to go up again,” remarked Frank; “and there’s no use asking if you feel like it.  So pile in and we’ll make a flying start from the top of this rocky plateau.”