"'Nless it were Buttsy," remarked Washington, _sotto voce_. The professor was still asleep and the boys paid little attention to the flying object for some time. It was coming up behind the _Snowbird_, and they had no occasion to look behind. The sun arose, angry and red, while the thunder continued to roll below them, and the crackling of the electric flashes was like minute guns. The _Snowbird_ was winging its way along at about seventy-five miles per hour. Wash had gone into the covered galley to prepare breakfast. Jack was still in the operator's seat. Suddenly Andy Sudds uttered a loud shout. A huge shadow was thrown athwart the flying _Snowbird_. Some object was hovering over them and they cast their eyes upward, at Andy's cry, to see another aeroplane swooping down directly upon them. It was not the machine manned by Secret Service Agent Ford and his companion, but a much heavier and more rapid vehicle. And until its shadow fell across the _Snowbird_, the boys had had no warning of its approach. At first glance it was apparent that the strange aircraft intended mischief. It was shooting down from a higher level, its sharp bow aimed directly for the _Snowbird_. Jack pushed over the switch and raised the bow of their own ship. She leaped forward and began to slant upward, too. But instantly the course of the stranger was deflected to meet this change in the movement of the _Snowbird_. She had the advantage of the boys' craft, too. She evidently proposed to retain her overhead position, and as she shot in closer, Jack was constrained to descend again to escape collision with her. "Keep away!" he shouted through the transmitter, and at his cry, and the bustle about him, the professor was awakened. But no reply came from the strange aeroplane, although they could see several figures moving upon her. It swooped down upon them, and Jack had to deflect his planes again and slant downward toward the storm-cloud. And then he saw the other peril. He was between two great dangers. If the reckless aviator tried to ram him from above, his only escape was by plunging through the tempest which raged just below them. Down came the stranger upon the _Snowbird_ again. She surely meant them ill--she was bent on their destruction. And meanwhile the thunder roared below and the crackling of the lightning was almost incessant. Jack Darrow had to decide quickly--and he must determine which of the two risks to take. CHAPTER VI ON THE WINGS OF THE WIND Speedy as the _Snowbird_ was, she could not get out from under the shadow of the strange aeroplane. That was driven at a sharp angle down upon the boys' flying machine, and it seemed to all those in the lower 'plane that a collision was imminent. The thunder fairly deafened them all. Around them rolled the mists and the wind shrieked through the stays of the aeroplane and shook the structure like a dog worrying a bone.