On Triton alone, indeed, or on a world of similar size and gravitational power, was such a feat possible, for only thus could one leap with such new impetus each time from wall to wall, and twist in mid-air to strike braced for another leap. And as I leapt up in that criss-cross fashion from wall to wall, my heart beating rapidly, putting all my strength into each great leap, I could see Marlin on the cell's floor below gazing up tensely through the dusk, knew what depended upon our escape, and so struggled upward with a superhuman strength. Up--up--back and across--across and back--in leap after slanting leap upward I progressed, until with a half-dozen more leaps the cell's open top lay close above me. By then, though, the energy which I had summoned for this superhuman feat seemed fast waning, and as I shot from wall to wall I realized that I was gaining less and less toward the top with each leap! Another leap--another--and as I shot back across the cell's width from wall to wall I was aware of the wall's top but a few yards above me, yet felt at the same time the exhaustion that had gained upon me, now almost near to overcoming me. Another leap--with agonized muscles I propelled myself back to the opposite wall, with the top of that wall but a few feet above me. One more up-slanting leap would take me back up and across to the opposite wall's top, I knew, but in that tortured moment I felt that I could never make it, and knew that if I missed it I must inevitably fall downward. So, as I struck that wall feet-foremost, I put the last of my strength into a great effort and shot floatingly across the cell's width for the last time. And this time, with hands outstretched, I struck the top edge of that opposite wall, fumbled with it for an agonizing moment, and then had grasped it and had drawn myself up on the thick wall's top! For a moment I lay across its top, oblivious to all else in the exhaustion that possessed me, inhaling and exhaling great panting breaths. Then as I drew myself up a little I peered about me. Far away on all sides of me stretched the walls of the compartment-city that covered all of Triton, those walls' tops intersecting like a great checkerboard, and all level with the thick wall's top on which I lay. Twilight lay over a broad band of that compartment-city about me, the twilight band between the dark and sunward sides, the brighter day of the sunward side stretching away to one side, humming with activity and with many cylinders moving to and fro above it, while to the other side stretched the silent, sleeping dark side, beneath its unchanging night. Now I gazed down through the dusk toward the cell's floor far beneath, and saw Marlin gazing up toward me anxiously, gestured silently to him. And in a moment more he was coming up toward me by the same great zig-zag leaps from wall to wall that I had used.