"Thus they had conquered all the inconveniences that had faced them in settling upon Triton, and so upon Triton as upon Neptune were Neptunians and their cities. The civilization of both Neptune and its circling moon seemed secure and unchangeable, indeed; a civilization that existed upon our world and its moon when all the other planets of the solar system held only the lowest forms of life, if life they held at all. Easily could we of Neptune have ventured into the sun's other worlds had we wished, in our space-cylinders, but we had no desire to do so, having learned all that we wanted of those worlds by observation with our instruments, and being content to remain safe upon our great world of Neptune and smaller moon-world of Triton. And safe we remained there, for ages, yet, during all those ages, there was coming closer toward us a great crisis which we had long before foreseen, yet which we had considered so remote a peril as to give it no attention. "But now that peril had become close, and great. And it was none other than the extinction of all life on both Neptune and Triton that faced us, due to their steady cooling. For all worlds, however fiery their interior cool in time and die. And steadily, surely, the interior fires of both Neptune and its moon had been cooling and the substance solidifying. Already they had cooled so far that the surface of Neptune was much colder than ever before, and that of Triton also, and with each passing century that cold was increasing. It would be a matter of time only, it was plain, before both Neptune and its moon would lie utterly without life, a terrible frigidity reigning upon each, all life perished from them in that bitter cold. For though worlds nearer the sun might exist by means of the sun's heat, though life on them could cling to existence through the sun's warmth, Neptune and its moon were so unthinkably distant from the sun that almost no heat reached them from it, and as their interior fires cooled, they must inevitably become so cold as to annihilate all life upon them! "It was evident that some great plan must be adopted that would prevent this condition, and such a plan was quickly decided upon. This plan was to enclose both Neptune and Triton with great roofs of metal that would hold in them the heat that was being radiated out, and that would make it possible to aid the failing heat of the two worlds by artificial means. It would be a gigantic task to place those great roofs about Neptune and Triton, but we set to work upon it and for years upon years all the energies of the Neptunians were centered upon the construction of those roofs. We had established vast workshops in which the plates of metal that were to form the great roofs were turned ceaselessly forth, and these in turn were joined together to make the great roof of giant Neptune.