Worse than the terrible cold was the lack of oxygen he would have to combat up in the heights of rarefied air. But a marvelous artificial aid had been prepared for this also. In the Wind Bird was installed a special oxygen apparatus that could furnish him a strong flow of life-giving gas through a tube adjusted to the mouthpiece of his helmet. With minute care, Hal examined every section of his two separate systems of gaseous oxygen, the main system, and the emergency system. He wanted to be very sure that nothing was left to mere luck. Other men before him had ridden high. But today, he must ride the highest stratosphere if he were to really explore the vast speeding wind river that his other searchings had merely tapped.