For all its careful designing, his present Wind Bird could not steadily ride this high, cold aerial river. There were certain necessary points of construction that needs must be considered in flying craft for the high, thin air strata. Hal Dane's heart leaped to the thrill of what he was learning. Those great aerial currents had half battered the life out of his body, but from his terrible contact with them, he had wrested secrets to carry back to the builders of airplanes.