The tests for great altitude, however, thrilled Hal Dane more than anything else. Through all this present labor and study, the call of the river of the winds still lured him. And to ride the currents of the air rivers at their swiftest, one must be able to withstand the velocities and the pressure of the air heights. The fearful cold of high altitudes unbalanced ships with heavy coatings of ice, clogged instruments and air indicator tubes with snow. Wiljohn men were pushing experiments for such flying. Already a spread of emulsions on the wings had been found to reduce the ice danger. But for real success, aviators must learn to wholly master the air heights.