"It's a man, and apparently bent on no good," said Andy Flures, at the same time starting for the hangar on a run, followed closely by the others.
But they were too late. The intruder, whoever he was and whatever his errand, had discovered them at the same time they had seen him and had made his escape in the night. They searched for fifteen minutes, but in vain. They went inside, and found everything apparently exactly as they had left it.
"Well, that's a queer one," said Big Jack, as they sat around discussing it before retiring. "The fellow might have meant no harm at all, but we're at least on our guard against any trickery."
"Yes," agreed Don, "and for one I'm in favor of going over every inch of our plane tomorrow."
And with that excellent precaution agreed upon, they went to bed.
CHAPTER III Treachery Afoot
It was perhaps four hours later, or a little after midnight, when all four of the young men were suddenly and simultaneously aroused from their peaceful slumbers by the loud clanging of a gong.
No need to ask what it was, even when coming out of a deep sleep. It was a fire bell, and pealing out in quick, insistent warning calls!