“How long, boy?” he cried hoarsely in desperation, gripping his voice as he realized that he could not balance the _Sphere_ accurately on its course more than a few seconds longer. He felt his control slipping. Too bad--too bad. The boy had had the idea, and he had failed--failed. He felt suddenly broken, as a very old man. His gray head nodded wearily. Too bad! “Professor--professor,” someone was whispering huskily. He recovered from his lapse of semi-consciousness as he felt a hand placed nervously upon his shoulder. He turned to see Robert’s eager young face behind him, smiling! That might mean--but he hesitated even to hope, stifling its ray of comfort almost before it came to him. He waited dully for Robert to go on. “We are holding our course now,” went on Robert, controlling his voice with an effort. “See?” he pointed to the glittering heavens visible through the observation windows. Instead of the dizzily swerving canopy of lights with which they were already too familiar, the stars hung stationary. “How did you do it, Robert?” gasped the professor. As he spoke he was suddenly aware that the gyrostats had stopped! “You see, it came to me like a flash,” explained Robert, “that it all hinged on velocity. If the disk was suddenly shut off--covered--the _Sphere_ would at once cease to be pulled around in various directions. Instead it would then rush ahead only in the direction in which it was last moving when the disk’s power was shut off. Beyond the forces of gravitation and with nothing but void on all sides, we would shoot forward forever until stopped by nearing some planet.” “Of course, of course,” murmured the professor. “Why didn’t I think of that before? Dunce that I am!” “That is why I asked you to try to hold the _Sphere_ in its course for a little bit--long enough to maintain its momentum toward Mars, when I would stop the wavering interference of the disk. First, I stopped the gyrostats. Then, as I clicked the shutters to cover the disk’s face, the _Sphere_ became simply a dead weight already launched with terrific velocity toward our goal. With the influence of the gyrostats removed, the heaviest or bottom side of the _Sphere_ became the head of our velocity. Result--the eccentric revolutions of the _Sphere_ ceased at once. We have established a temporary stability of our own--velocity.” “Robert,” said the professor, after a pause, “we owe our lives to your keenness of mind. You thought of what it was my business to have known in the first place. With the _Sphere’s_ course automatically maintained now, it remains but to wait until we are near enough Mars to establish stability based on _its_ attraction. Then we can again control the _Sphere_ at will. In the meantime we conserve all our power.” “Just as simple as A-B-C,” broke in Taggert, who had been roused from his sleep by their excited talking, and had been listening interestedly for some time, unobserved.