"But we need not cross them to reach Aleukan?" suggested Jack, speaking with some difficulty himself. There was a pain in the region of his lungs and he saw that Professor Henderson was very pale. "That is a fact," panted the professor. "Descend, Jack. Make it two hundred feet. Be careful!" For as the youth depressed the planes again the ground beneath seemed to fairly leap up to meet them. "What do you know about that?" gasped the young aviator. "She--she doesn't work at all like she used to." "Less attraction," declared the scientist. "What do you mean, sir?" cried Jack. "Has the law of gravitation lost its power over us--and over the flying machine?" "There is a difference--a great difference," proclaimed Professor Henderson. "The power of attraction is lessened mightily." "What does it mean? What _can_ it mean?" murmured the disturbed youth. "I suspect--I fear--" What the professor would have said was not spoken then. Mark interrupted by shouting: "Look ahead! Look ahead! What is that--a river?" "There is no river of size in this locality," declared the professor, quickly, training his glasses on the white streak that appeared on the ground ahead. Phineas Roebach struggled forward to the operator's bench. He gasped: "This is worse than I ever thought flying could be. Do you have to go so fast? I cannot get my breath. Hullo! That's the glacier ahead. The dog trail to Aleukan follows the ice for more than fifty miles." "A glacier it is," agreed Professor Henderson. "It seems pretty smooth, Jack. You can descend still farther." That they were all suffering from the rarity of the atmosphere was plain. It seemed as though the envelope of breathable air surrounding the earth had suddenly become vastly rarified. If the atmosphere had been so changed all over the globe it would be a catastrophe unspeakable. "We certainly _can't_ cross these mountains--nor the Rockies," groaned Jack. "How are we ever going to get home again?" "If the air remains as it is now?" asked Mark. "You're right! We're imprisoned in this part of Alaska just as fast as though we were caged behind iron bars." "If we only had some of those torches we used on the moon," said Jack. "What will we do, Professor?" begged Mark. "Let us not lose hope," responded the old scientist. "First we will get to Aleukan and see if our provisions have been brought over from Coldfoot." "I'll bet they haven't been brought across the range," said the pessimistic Mark. "If the air everywhere is so rarified the men would die crossing the mountains." "Think of the people living on Mt. Washington--and other heights!" cried Jack, suddenly. "Why, they will be snuffed out like candles. It is an awful thought." "We will hope, at least, that this fearful catastrophe is local," said the professor, seriously. "Have a care, Jack! Don't dip like that. We do not want to descend here."