By the time the five-mile height was reached Earth's surface had changed definitely from concave to convex. The plane was ascending by then in a somewhat wider spiral, but its climb was as steady and sure as ever. Frost begin to form quickly on the cabin's windows, creeping out from the edges. Norman spoke a word over the motor's muffled thunder, and Hackett snicked on the electrical radiators. The frost crept back as their warm, clean heat flooded the cabin. Ten miles--fifteen--they had reached already altitudes impossible but a few years before, though it was nothing to the X-types. As they passed the ten-mile mark, Hackett set the compact oxygen-generator going. A clean, tangy odor filled the cabin as it began functioning. Twenty miles--twenty-two-- * * * * * After a time Norman pointed mutely to the clock on the instrument board, and Hackett nodded. They were well within their time schedule, having calculated to reach the forty-mile height at ten, the hour when, by its computed orbit, the second satellite should be passing overhead. "--26--27--28--" Hackett muttered the altimeter figures to himself as the needle crept over them. Glancing obliquely down through the window he saw that Earth was now a huge gray ball beneath them, white cloud-oceans obscuring the drab details of its surface here and there. "--31--32--" The plane was climbing more slowly, and at a lesser angle. Even the X-type had to struggle to rise in the attenuated air now about them. Only the super-light, super-powered plane could ever have reached the terrific height. It was at the thirty-four mile level that the real battle for altitude began. Norman kept the plane curving steadily upward, handling it with surpassing skill in the rarefied air. Frost was on its windows now despite the heating mechanism. Slowly the altimeter needle crept to the forty mark. Norman kept the ship circling, its wings tilted slightly, but not climbing, Earth a great gray misty ball beneath. "Can't keep this height long," he jerked. "If our second satellite doesn't show up in minutes we've had a trip for nothing." "All seems mighty different up here," was Hackett's shouted comment. "Easy enough to talk down there about hopping onto the thing, but up here--hell, there's nothing but air and mighty little of that!" Norman grinned. "There'll be more. If I'm right about this thing we won't need to hop it--its own atmosphere will pick us up." Both looked anxious as the motor sputtered briefly. But in a moment it was again roaring steadily. Norman shook his head. "Maybe a fool's errand after all. No--I'm still sure we're right! But it seems that we don't prove it this time." "Going down?" asked Hackett. "We'll have to, in minutes. Even with its own air-feed the motor can't stand this height for--" * * * * *