They had studied the chart and laid the course for Aleukan without any difficulty. Now Jack strapped himself into the operator's seat and the others took their places, Washington White stowing his rooster carefully amidships as he had before. Jack started the motor and the _Snowbird_ began to quiver throughout her frame. He touched the lever by which the propellers were started. With a whir and a bound the flying machine left the earth. Never had it sprung into the air so quickly before. It shot up at a sharp incline and was over the tree-tops in a breath. The indicator registered eighty miles an hour before the plateau was behind them. Then the pointer whirled to ninety--to a hundred--to a hundred fifteen miles an hour, and both Jack, in the pilot's seat, and the others gasped for breath. Faster than when shot out of Professor Henderson's catapult the _Snowbird_ winged her way into the northwest. Jack managed to keep her on an even keel. But he had the same feeling that he would have had, had he been hanging to the bit of a runaway horse. Indeed, the _Snowbird_ was practically out of his control. CHAPTER XV A PLUNGE TO THE ICE Jack Darrow was a youth less likely to be panic-stricken than his chum; but just as Mark Sampson had lost his head for a few minutes on the occasion when the _Snowbird_ was tried out, so Jack was flustered now. The flying machine shot up at such a tangent, and so swiftly, that he was both amazed and frightened. The speed indicator showed a terrific pace within a few seconds, and when Jack first tried to reduce the speed he learned that the mechanism acted in a manner entirely different than ever before. The motor made more revolutions a minute than she was supposed to make when pressed to the very highest speed. When he had raised the bow of the flying machine at the start she had shot up almost perpendicularly into the air. He was afraid she was going to turn a back somersault. As he depressed the planes he found that it took much more depression to bring the _Snowbird_ down to even keel. And the rapidity with which they left the ground and soared upward was in itself enough to shake Jack's coolness. Suddenly (being furnished with the professor's patented ear-tabs) he heard that gentleman calling to him from below: "Get back to the five-hundred-foot level--quick!" Light as his head had become, and confused as he was, Jack realized what these words meant, and he knew enough to obey without question. He brought the _Snowbird_ down the air-ways on a long slant and at a swift pace. He realized that, as they descended, he was able to breathe more easily and his head stopped ringing. For some moments he had felt like an intoxicated person in the vastly rarified plane of the upper ether. The professor staggered to the young operator's side. "Danger! Danger above, boy!" he gasped. "We cannot cross these mountains while--while the air is so thin."