For one thing, there were moving to and fro above it a number of great cylinders like that in which we were, and in the compartments of the darkened side moved, too, a few Neptunians here and there. And the great globes of metal that dotted this compartment-city of Triton, even as that of Neptune, were here glowing with radiant light; glowing, I knew, with radiant heat. For this was the secret of the Neptunians' existence on Triton, this heat that glowed from the numberless giant globes set in compartments here and there. Those great glowing globes kept the air beneath Triton's great roof warm and comfortable, the great roof itself preventing that warmer air from escaping into the moon's colder outer atmosphere. As we shot on over the darkened side of Triton, the side turned away from the sun, I could not but think that the remnants of the Neptunian race must be few indeed, so few of them moved in the shadowy compartments beneath. At last, as Marlin and I gazed ahead, we could make out a brighter crescent of light at the edge of the strange moon-world, and as we shot on we saw that we were approaching the edge of the sunward or sunlit side. A moment more and we could see it clearly and as we did so Marlin and I gasped in utter amazement. For that part of the great compartment-city that lay on Triton's sunward side, in the pale sunlight, was swarming with incalculable millions upon tens of millions of Neptunians! Crowding, seething, pressing together, they were pouring to and fro through the compartments in the pale light of day, busy with the mechanisms that scarce had room in those compartments, so great were their crowds! And over this sunward side hundreds upon hundreds of cylinders swarmed, rushing to and fro! "Neptunians! Neptunians in countless millions here on the sunward side of Triton! But why then are there so few upon the dark side?" Marlin shook his head at my exclamation. "I can't guess," he said. "And I never dreamed that----" Before he could finish the sentence there came an amazing interruption from beneath. As we gazed downward from our speeding cylinder we saw a giant band of intensely brilliant white light spring suddenly into being at the very line that marked where dark side and sunlit side of Triton met. A mile in width, that great brilliant band of light seemed to extend clear from Triton's north pole to its south, as far north and south of us as our eyes could reach. And then, even as we stared, astounded at it, that brilliant and immense band of light was moving around Triton's surface over the dark side! Swiftly it moved, like a great wave of brilliant light sweeping around Triton's surface, and in a moment had disappeared from view far around the horizon from us on the dark side!