“We shall call you, sir,” said Jack, as the boys moved away.  “And don’t worry.  I’m sure we’ll come out all right.”

CHAPTER XVI

A SOUND IN THE SKY

“Good-bye, Tom.”

“S’long, Jack.”

“Keep a watch for our signal.  We’ll call you.”

“I will that.  An’ if it’s in trouble you are, Dave an’ me’ll be ridin’ just as fast as we can to help you.  Wish you’d let me go ’long.  I’m half minded to follow you.”

“No, no.  We’ll stand our best chance alone.  They won’t suspect we’re other than a bunch of wild young fellows out for adventure.”

Tom grumbled, but the force of the reasoning was apparent to him.  They leaned from their horses for a last firm handclasp, then Jack rode on to join Bob and Frank who sat on their horses some distance ahead.

“You’re the boy to give it to ’em, Jack,” called the big ex-cowboy in a last farewell.  “Give ’em thunder.”

Jack waved a parting salute as he joined his comrades.  Frank and Bob did likewise.  Then with night settling down over the vast desert waste they rode on into old Mexico.

Beside the white stone marking the international boundary, Tom Bodine sat his horse like a statue.  Moodily he watched until they were out of sight.  It was a hard life Tom had led in his day and when he took the job at the radio plant it was with a sigh of relief at the ease ahead of him.  But now despite his fifty years, the last thirty of which had been filled with hard knocks, he felt the old call to adventure urging him on.

With drooping head, he turned his horse toward home.  But hardly had the animal started forward, than he dragged it about again.

“Let’s go,” he shouted to the empty silence, and whirling his sombrero aloft, brought it down on his horse’s flank.  Then he rode on after the three figures that had been swallowed up in the darkness.

Far ahead of him, for Tom had taken considerable time to reach his decision, rode the three companions.  The young moon shed only a wan and wraithlike radiance over the plain.  They were alone, and the parting with their last friend, combined with the solitude of the open spaces, had its effect upon them.  They rode awhile in subdued silence.  But not for long.  Frank’s lively spirits were the first to rebound.

“Race you to that rock,” he cried, pointing to a solitary outcropping of rock, about twice a man’s height, about a quarter of a mile ahead.

“You’re on,” cried Jack, spurring his horse.

“Attaboy,” yelled Bob, doing likewise.

With a shout that shattered the silence as if a band of wild Indians were hitting the trail, the three boys dashed away.

Presently they pulled up by the rock, practically neck and neck.  Their eyes were alight now with the zest of adventure.

“Gee, it’s great to be alive,” cried Frank.