The knowledge Jack Darrow and Mark Sampson had gained from textbooks, and much from observation and the teachings of Professor Henderson, had aided the lads in the building of the _Snowbird_. It was the first mechanism of importance that Jack and Mark had ever completed, and they had been quite confident, before the flying machine was shot from Mr. Henderson's catapult, that it was as near perfect as an untried aeroplane could be. "Hang on, Mark!" yelled Jack, as the great machine soared and pitched over the forest. Her leaps were huge, and the shock each time she descended and rose again threatened to shake the 'plane to bits. Mark swayed in his seat, clutching first one lever and then another, while Professor Henderson and Jack could only cling with both hands to the guys and stay-wires. The sensation of being so high above the earth, and in imminent danger of being dashed headlong to it, gripped Mark Sampson like a giant hand. He felt difficulty in breathing, although it was not the height that gave him that choking sensation. There was a mist before his eyes, still the sun was shining brightly. The startling gyrations of the flying machine for some time shook the lad to the core. But Jack's cheerful cry of "Hang on!" spurred Mark to a new activity--an activity of hand as well as brain. He knew that something had fouled and that this accident was the cause of the machine making such sickening bounds in the air. She was overbalanced in some way. With Jack's encouraging shout ringing in his ears, Mark came to himself. He _would_ hang on! His friends depended upon him to control the machine and to save them from destruction, and he would not be found wanting. One lever after another he gripped and tried. It was one controlling the rising power that was fouled. He learned this in a moment. He sought to move it to and fro in its socket and could not do so. He had overlooked this lever before. Again the _Snowbird_ dashed herself from a height of five hundred feet toward the earth. They still flew over the forest. The tops of the trees intervened, and Mark managed to counteract the plunge before the prow of the machine burst through the treetops. She rose again, and using both hands, Mark jerked the wheel stick into place. At once the flying machine responded to the change. She rode straight on, slightly rising as he had pointed her, and Mark dared touch the motor switch again. Instantly the machine speeded ahead. "Hurrah for Mark!" shrieked Jack. "He's pulled us through." "He has indeed," agreed the professor, and they settled into their seats and gave attention to the working of the apparatus. Mark now had the _Snowbird_ well under control. Jack changed places with his chum and managed the _Snowbird_ equally well. At his touch she darted upward at a long slant until the altimeter registered two thousand feet above the sea. And the sea was actually below them, for Jack had guided the flying machine away out from the land.