"When you've done that give him these instructions: He is never to take his eyes of us for more than a split second at a time--unless someone else takes his place. I doubt if, at this distance, this will work, but it may help us a little. If we become invisible for even the briefest of moments, he is to look at his watch and observe the exact time, even to split seconds. We shall try to follow a certain plan hereafter in quartering the stratosphere, and I shall mark our location on the navigational charts every minute until we hear from this chap, or until we decide nothing is to be accomplished by this trick. Understand?" "You're hoping that the enemy, while invisible to all eyes, yet has substance...." "Shut up!" snapped Jeter, but he was glad that Hadley had grasped the idea. It was a slim chance, but such as it was it was worth trying. If the plane were invisible for a time, then it would be proof of some opaque obstruction between the plane and the eye of the beholder on the surface of the Earth. Refraction had to be figured, perhaps. Oh, there were many arguments against it. The fliers followed the very outer edge of the area above the world they had mapped out as their limit of exploration. This circuit completed, they banked inward, shortening their circuit by about a mile of space. A mile, seen at a distance of ninety thousand feet, would be little indeed. It was almost midday when they had their first stroke of luck. The buzzer sounded at the very moment Eyer uttered an ejaculation. "The Jersey fellow says there is nothing between his lens and your plane to obstruct the view." "O.K.," retorted Jeter. "At the moment your buzzer sounded our plane suddenly jumped upward. That means an upcurrent of air indicating an obstruction under us. It must however, be invisible." He severed the connection. His brow was furrowed thoughtfully. He was remembering Sitsumi and his rumored discovery. They circled back warily. The eyes of both were fixed downward, staring into space. Their jaws were firmly set. Their eyes were narrowed. And then.... There was that uprush of air again! It appeared to rise from an angle of about sixty degrees. They got the wind against their nose and started a humming dive, feeling in the alien updraft for the obstruction which caused it. CHAPTER VII _Invisible Globe_ The buzzer of their radiophone was sounding, but so intent were they on this phenomenon they were facing, they paid it no heed. Their eyes were alight, their lips in firm straight lines of resolve, as they dived down upon the invisible obstruction--whatever it was--from whose surface the telltale updraft came. It was Eyer who made the suggestion: "Let's measure it to see what its plane extent is." "How?" asked Jeter. "Measure it by following the wind disturbance. We travel in one direction until we lose it. There is one extremity. In a few minutes we can discover exactly how big the thing is. What do you think it is?"