This was the point that especially puzzled Roque.
A farmer boy, listening, open-mouthed, to the questioning, offered a solution.
"You see, Monsieur," he bashfully explained, "it was a ghostly noise that was making between the big noises, like the wind blowing through the neck of a bottle stuck in a knot hole. I heard it in the road, a long way."
It occurred to the boys that this distress signal must have been given before they got away from the roar of the fire, or while they were probing the smoke in the gallery to reach Anglin.
"They were flying mighty close down and could probably hear a howl like that, if they were listening for it and knew what it meant." This opinion was advanced by Billy.
"I don't much believe they could hear a call from the ground, unless it came from the business end of a gun." Henri was the doubter.
"It is no use to argue," said Roque. "The fact remains that the air fellow had his bearings, and he got the lead from somewhere. I am not giving him credit for being a mind reader."
"That reminds me, Mr. Roque," remarked Billy, "that we might test this bearing business by a little air trip somewhere and soon."
"I have just such a thing in thought," grimly advised Roque, "and I will warrant that you will hear a few ground sounds before the quitting minute. We are going to take a down look at Belfort."
Now Belfort is a French fortress, where the soldiers in red and blue had been finding security every time they were rolled back from the plains of upper Alsace.