Thus had twenty great caves been dug out when at last word came from the astronomers that the Sun's power could not possibly survive the week. Night and day the work continued so that thousands might be made fairly comfortable. There was much lamenting on the Earth's crust as the world watched the fading of the Sun. It was hard enough to lose the warmth of the Sun, but to have to creep into the damp cellar of the Earth was almost too much for the soft men and women who had lived luxuriously under the shadow of mighty machines that had given them all. Kuila Rei journeying in his flyer from the observatory wherein he had spent almost his whole life, since he had emerged from the City of the Children, thought of all this. The flyer had been built several thousand years before. Once it had been the very latest type of Lighter-than-Air machines that Science had produced. And it still was, for no better had been brought forth since. It sat like a bubble on the thin air of the globe, and in form it resembled a bubble. It was a perfect globe fashioned from a material that resembled glass, but its qualities were as different from that brittle, splintery material as night was unlike the day. Alu, the ancients called it, and it was as transparent as the air, yet as strong as Earth itself, malleable as clay and lighter than air. The greatest weight could not smash or crush it, the sharpest point could not even scratch its surface. Within this indestructible chamber sat Kuila Rei on a seat covered with a leathery material whose back could be let down for sleeping purposes. His feet rested on a similar pad and on a bar in front of him that extended from side to side were placed the various levers that controlled the machine. Under the seat was fitted the motor contained in a small metal box, taking up no more than a cubic foot of space. The oxygen tank twice the size of the motor stood near by. Flying thousands of feet above the ground Man had found that the thinning atmosphere was too rare and each flyer was consequently supplied with its own oxygen tank containing enough of the precious gas to sustain the pilot for one hundred hours. An open valve in the side of the globe allowed enough of the poor grade of air to flow from the outside to the tank and thence into the globe reviving the air sufficiently to the proper proportions needed for the driver. The motor, ingenious and yet of a marvelous simplicity, used for its fuel the carbon dioxide expelled by the occupant of the machine, thus also keeping the air clean and fresh. Through another vent the motor tossed off its waste.