They saw Jupiter, immense, formidable, a mass of turbulent vapors, a depressingly drab scene. Suddenly Lents exclaimed, incredulous; "Look! A satellite! There is no satellite this close to Jupiter! It's mathematically impossible!" Musters laughed jovially. "It's there, isn't it? That's Jupiter's tenth satellite--The Bubble. It is less than 100,000 miles from the vapor envelope and has to travel so fast that its period is less than 8 hours. It was built by the First Race and set on its orbit so that our people would have a place where they could enjoy the sun, which is never seen from Jupiter's surface." "It _is_ a bubble!" Kass remarked, after an absorbed study of the satellite. It was racing just beneath them, at a dizzy speed, like a bubble blown before the wind. The ship followed the satellite, drawing closer, so that it grew in size and beauty. * * * * * Lents was mentally calculating the rupturing pressure exerted by the atmospheric pressure inside the crystalline ball. He stopped aghast at the thought of the tremendous strain. "That crystalline material stands the strain easily," Musters assured them. "It will resist anything but a direct hit by a very large meteorite. As you can see now, the sphere, which is about a mile in diameter, is bi-sected by a plane surface, on which the city is built. In that little area you will see reproduced the choicest conditions of Earth." He turned earnest, hungry eyes on them: "You don't know how lucky you people of Earth are!" The ship was now coming quite close to the vast curve of the crystal, and they could see glimpses of beautiful structures in fairylike colorings, of small lakes like exquisite gems, of brilliant bursts of light that they conjectured served as substitutes for the sun while it was occulted by the enormous bulk of the planet. Steadily the ship swept downward, to the level of the city, and the Earthmen became aware that the entire sphere was not transparent crystal. The part below the city level was a dull, ugly black. "That's where the machinery is," Musters answered their questions, somewhat shortly, it seemed. "Hydrogen integrators there--to generate the power. Leakage of injurious rays down there--couldn't expect the First race to work there." "Who _does_ run the machinery?" Sine asked curiously. "The labor Mugs, of course!" And Musters changed the subject. The chief left them to their own devices as he superintended the lining up of the big ship's airlocks with the lock gasket of The Bubble. This effected, he bid his guests courteous farewell, assuring them that their ship would be conveyed to the Jovian capital city of Rubio, where they would be given every facility for repairing their damaged motor.