For I saw now that that was the great barrier's nature. It was a mighty shell of perpetual vibrations in the ether itself, extending all about the universe before us, allowing light and electro-magnetic communication waves to pass through it, unchanged, but excluding and holding out the vibrations of matter, by meeting them, as I knew must be the case, with a vibration of equal frequency which opposed them, reflected them back, forming a barrier more impenetrable than any of solid matter, yet one all but invisible, extending about all this mighty universe, excluding from it for all time all matter from outside. Too, as I was later to learn, the great vibration-wall was impenetrable to the heat-vibrations, reflecting those of its dying suns that struck it back into the universe inside. It was for this purpose that the vast barrier had been erected, as the suns of the serpent-people failed, to prevent the escape of any of the precious heat-radiations of their few living suns, and also to place about all their universe a wall impenetrable to all invaders. Set in the ether about their universe eons before, the vibrations that made up the great barrier were perpetual and undying, a vast wall of defense about the serpent-universe. We were flashing close along the mighty, flickering barrier's edge, now, and the speed of our ships slackened swiftly as there loomed far ahead in space two great, dark bulks starred here and there with points of white light. Moments more and they had grown to immense size as we neared them, and now we saw that these were mighty, square-walled structures of gleaming metal, each a full five thousand feet in length along each of its four sides, and half again that much in height: two colossal metal forts that floated motionless there in space, set directly in the great wall of flickering blue vibrations, and between which there was a great opening in that wall, a clear space in which was no flickering barrier, and which I divined was the single opening in all the great wall. And flanking that opening on either side hung the massive metal structures, upheld there in the void, as I guessed, by mighty generators like those of our own ships, castles of metal whose countless deadly death-beam tubes commanded the opening between them and from whose white-lit windows the serpent-garrisons of them gazed out upon us, great space-forts hung there at the vibration-wall's one opening, guarding the gates of a universe!