Riding two abreast, Tom and Jack in the lead and Frank and Bob close behind, they pressed on another twenty minutes when Tom called a halt to indicate a clump of rocks close at hand which suggested in their outline a crouching camel.  Then he led the way toward the left.

“Wait, wait,” called Bob, in a tense voice that reached the ears of all, and caused them to halt.  “Keep your horses quiet and listen.  There.  I was right.”

All sat silent, and distinctly there came to their ears the hum of an approaching airplane.

CHAPTER XVII

INSIDE THE CAVE

“What is it?” whispered Tom Bodine, to whom the sound was unfamiliar.  “Sounds like machinery of some kind.”

“It’s an airplane,” Jack answered.

“Airplane? An airplane?” said Bob, low voiced.  “It’s better than that.  It’s our airplane, if I know anything.”

“Righto, Bob,” agreed Frank.  “I’d know the old baby’s voice a mile off.”

“They’ve shut off the motor,” said Jack.  “They must be going to land.  But where in the world could they land in these hills and in this darkness, too?”

Tom Bodine slapped his knee.

“That’s it,” he said emphatically.  “That must be it.”

“What?” asked Jack.

“Why, there’s a big level place just below the cave I was tellin’ you ’bout.  A plateau.  Smooth as a floor.”

The hum of the airplane had died away.  The boys and their guide never had caught sight of the machine in the darkness.

Suddenly Frank pointed in the direction whence the sound of the airplane had come, ahead and slightly to the left.

“I thought I saw a light there,” he whispered.  “It was just a faint streak of orange.  Now it’s gone.”

“Look here,” said Bob to Tom Bodine, “does that cave face this way or is it on the other side of a hill?”

“It’s on t’other side,” answered Tom, “an’ near the top.”

“Well, I’ll bet you there’s somebody in that cave.  And the light that Frank saw was some kind of a signal to the airplane.”

The big ex-cowboy scratched his head.

“Mebbe you’re right,” he said doubtfully.  “I don’t know ’bout such things.  But who’da thought that cave would be discovered.  Why, I just come on it accidental like onct when I was wanderin’ through these hills.”

“Boys, there’s only one thing to do,” said Jack in a determined voice, “and that’s to investigate.”

“Righto, Jack,” said Frank eagerly.  “Here’s our chance to get back our airplane.”

“You said it,” declared Bob.  “Let’s go.”

“Not so fast,” said Jack.  “First we must have a plan of campaign.  Tom, what’s the lay of the land?  How far away is the cave?  Would it be better to leave our horses here and approach on foot?”

“Cave’s not more’n half a mile from here,” answered Tom.  “It’s just around the shoulder o’ this hill we’re on right now and near the top.  I tole you ‘bout that big rock in front o’ the entrance an’ them three lonesome trees at the foot that give you a bee-line to the rock.  Well, we can git to them trees without bein’ noticed an’ tie our horses there an’ then sneak up afoot.”