"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. A tremendous amount of gunpowder had been burned on the flat ground in front of this stronghold, and our boys were in for a smell of it--something that would recall perilous travel with Colonel Bainbridge and Sergeant Scott in previous campaigns. A wire to Friedrichshaven had started on the way the makes of biplanes that Billy called "Roque's best bet" since the day of the famous race over Lake Constance. "Business will soon be looking up," joked Henri, when he heard of the order for the shipment of "No. 3's." The presence of Ardelle in this region, extreme southwestern Germany, had raised suspicion in the mind of Roque that some special demonstration was brewing, and the lurid performance of the French airmen in blowing the roof from over his head served to further elevate the confirmed idea that trouble and the French agent always traveled together. Roque was not here to mix in the actual military operations--that was not his business, but he was ever open-eyed on the trail of the boss gamester on the other side. He had expected this time to put his rival on the safe side of the ground, but spades did not prove to be trumps. Somewhere in the gap of Belfort, as the valley south of the Vosges mountains is popularly known, Ardelle was, no doubt, preparing for another comeback, and Roque was scheming to meet him halfway. There was no chance to get under the guns of the frowning fortress beyond the frontier, so the only way to size up the situation was to go over them. Here was where flying experts jumped to the front. CHAPTER VIII. ALONG THE BATTLE LINE.