Down toward that mighty roof our eleven cylinders were rushing. When they seemed just above it, they halted their drop and raced along above it, around Triton's vastly curving surface. As they did so, Marlin and I, gazing downward, saw that the vast roof that shielded Triton appeared to be, from above, of the same dark metal as that which protected Neptune, and that it extended away without break or seam as far as we could see over the big moon's surface. And as our cylinders flashed above it, around the world's surface, we were aware that Triton had an atmosphere even as had its great parent-world, since from outside was coming the dull roar of air against the speeding cylinder. It was a fact startling enough, but at that moment it was driven from our minds by a thing more startling still. For as we flashed thus around Triton's sunward side, Marlin suddenly uttered a hoarse exclamation, pointed ahead and to the left. I gazed in that direction instantly and for a moment saw nothing unusual, but then as the cylinders flashed on I saw that in that direction was what seemed a great round opening in the smooth, dark metal, a titanic circular opening that must be miles across. Up out of that opening was rising what seemed at first glance a vast cylinder of pale light that sprang straight up and outward from the gigantic opening, and that was only visible for a short distance above that opening, fading swiftly into invisibility as it shot out into the gulf of space from Triton. Instinctively my eyes followed the fading length of that mighty beam outward, and then as they did so I felt sudden, awed understanding descending upon me and stared with Marlin toward the giant pit and its great ray in stunned silence. For that giant ray was pointing straight into the great gulf of space, toward the tiny, fiery disk of the distant sun! "The giant force-ray!" Marlin whispered. "The great force-ray that's turning the sun ever faster--and that we came out here to find!" "And pointing straight toward the sun!" I exclaimed. "Pointing through Neptune's upper atmosphere toward the sun!" For we could see now that the giant ray, visible only there at its source, must indeed be cleaving through the upper limits of great Neptune's atmosphere as it reached across the great gulf toward the sun. For since Triton was on the other side of Neptune from the sun, was on its outward side, the great green sphere of Neptune lay almost between the sun and Triton, the big moon being high enough above the great planet, though, due to the inclination of its orbital plane, to make the sun visible to it through the upper reaches of Neptune's vast atmosphere. Through that atmosphere, therefore, we knew, the giant force-ray must be driving on its path across the solar system toward the sun, hurtling across the gulf to strike against that spinning sun's edge with terrific pressure and to spin it ever faster toward that day of division and doom that was marching relentlessly upon it!