From long practice, Hal's hands mechanically shifted gear in the speeding plane, to hold to an even flight, while his mind wrestled continually with the many problems that even his brief taste of stratosphere flying had opened up to him. On future "high flyers", the sealed cabin would necessitate the working out of some method of absorbing the gases given out by the breath. Then, too, the ordinary systems of controls could not be used, as they would necessitate holes in the "sealed cabin." Ah, he had it!--electric controls could be devised to govern rudder, elevators, ailerons. This great power could be used to fight the terrible high air cold, also, and electrical heating would keep the deathly dangerous ice sheath from coating wings and--