Needless to say, I did not have to inquire as to the meaning. Only one explanation was conceivable: the metallic tube was the valve through which the X-111 had found entrance to Atlantis, the valve that admitted the ocean water and kept the Salty River supplied. The aperture at the ocean end was doubtless not very wide (I was later told that it was but twenty-five feet across); but such was the pressure at these depths that the waters burst through with the force and swiftness and tremendous volume I had observed, and had to be diverted through a long and gradually widening tube before their torrents could be controlled and safely emptied into the river channel. * * * * * As we approached the glass wall, the hoarse and resonant roaring was continuously in our ears, thudding and crashing with echoes that reverberated like the combined monody of a hundred Niagaras. But, forgetful of the tumult, I kept my eyes fastened straight ahead, where the great green dome sloped down to meet the ground in a curve modelled on that of the actual heavens. Except for the dark weird coloration, I might have fancied that I was staring toward an actual horizon on earth; and so close was the resemblance that the illusion persisted until I was almost within a stone’s throw of the barrier. Only then could I persuade myself that I actually beheld a solid mass; and, even so, the curvature was so graceful and so elusive that I could not feel that a mere wall stretched before me; but, rather, I had the sense that it was some ultimate boundary, the dividing line between reality and infinite nothingness. This impression was confirmed by the fact that the wall at close range looked opaque. Olive-green and of impenetrable thickness, it seemed impervious to the rays of light; though, remembering my experiences on the X-111, I knew that it was really transparent. All the members of our party approached the wall almost breathlessly, then held out their hands and touched it in silence--a procedure which may have had some ceremonial importance, or may have been akin to the actions of persons who, seeing the ocean for the first time, gravely dip their hands in the salt water. At any rate, I lost no time in following their example, and found that the surface of the wall was just as I had expected--smooth and polished, and of a substance that would have been apparent to a blind man. After the twenty students had duly inspected the wall, one of the tutors lifted his voice so as to be heard by the entire party.