As the afternoon wore on, Hal twice again mounted high into thin air and rode the winds for a brief space, each time coming down before exhaustion could quite claim him. Risky as these performances were, they were undertaken in no mere daredeviltry of spirit. Instead, it was for experiment's sake that Hal Dane repeatedly dared the weird dangers of the stratosphere's violence of wind and cold. He wanted to prove to himself that his newfound, splendidly dangerous river of the wind flowed always into the west. For already he was dreaming of a plane built to more efficiently take advantage of this great aerial current.