Then a counter-move, wherein the French had advices from some source unknown of the coming flight of a Zeppelin out of the Black Forest, and three French aëroplanes were ready to charge at the big dirigible, which, after a continuous exchange of fire lasting forty minutes, made narrow escape to the north, just when the lighter craft had succeeded in getting above it for a finishing stroke. As it came about, and in a queer way, too, the boys were the first to blunder upon a cunning ruse being resorted to by a smooth worker in getting away information under the very nose of the astute Roque. Billy and Henri, indulging their liking for high places, and having a little leisure to look around, found a favorite perch in one of the famous towers of Strassburg. They were interested, as airmen, in watching the daily flying exhibit of the pigeons 'round about. "Have you noticed, Henri, the streak of feathers every once in a while that don't stop to associate with this housekeeping bunch? I've seen two of these birds already this morning; they act just like an aëroplane, circle about, and then break away like a bullet. There's one now. Look!" Henri followed the aim of Billy's finger, and, sure enough, a long-tailed flyer was cutting the air like greased lightning in a straight line west, without the slightest notice of the many of its kind pluming themselves on neighboring towers and housetops. "They make long visits," commented Billy; "I've watched, but never see any of these air hustlers come back." "That's funny," observed Henri, "let's borrow a glass this afternoon and find out, if we can, where they start from. Why, this is good sport; we'll be wearing badges next as pigeon detectives." The boys had small notion then that they were butting into a real business proposition, but one that did not advertise! They were just curious to find out from where came the busy birds that would not take time to visit with their brothers and sisters. The most that the tower observers could discover, even with the field glasses, borrowed without leave from Roque's traveling outfit, was that the next bird comer took its bearings over a red-roofed building, rising out of a circle of tall trees, a full mile to the east. Had it so happened that Roque was in a social mood, and the boys making him a confidant of their bird study diversion, there would, without doubt, have been no delay in striking at the heart of the problem--and everything else under that red roof. Carrier pigeons were not beneath the notice of the big man with the delicate touch! But Roque was not inclined at the time to indulge in fireside fancies. He was hooked up to a procession of events that needed constant attention, and as it was all ground work for the present, he had no use for aviators. So he missed the first bang at the very musser-up of his plans whom he was, day and night, seeking to locate.