We heard a stir of excitement in the streets below as that mighty air-city came closer to us; then saw it slowing down until at last it had come smoothly to rest out to the south of Berlin, hanging there in mid-air a half-dozen miles away. It was London! Even as I had recognized it, Hilliard had done so also. London! The great air-city that held all southern England for the European Federation, could be clearly recognized, not only by its size but by the somewhat different architectural design of its metal towers and plazas. We could make out clearly now the surface of the other city, its huge batteries of heat-guns, and its great towers surmounted by a central pinnacle. And now, as we scanned the horizon away to the north, we could see another dark disk, another mighty air-city, rushing swiftly toward us! "They're gathering!" Hilliard's voice was agonized. "Gathering--all the air-cities of the European Federation! It's the beginning of the end." "Gathering for their great attack," I said. "God, if Connell and Macklin could have escaped!" Hilliard's cry burst from his tortured soul. "If our own air-cities had only the speed and the power to resist this attack!" "Steady, Hilliard," I told him, my hand on his shoulder. "It's the end, I think--the end for our Federation as well as ourselves--but we must face it." Now the air-city from the north was rushing closer, was hanging northward of Berlin, and we saw that it was Stockholm. And, even as it came to rest out there beside us, two other air-cities were rushing up from the south; looming larger swiftly and identifying themselves, when they too shot up to hang near our central city, as Geneva and Rome. And then from the west were coming others, Paris and Brussels and Amsterdam; while down from the cold east were speeding Moscow and Helsingfors and Leningrad. City after city was rushing from all quarters of the compass, from every part of the European Federation, until they filled the sky. Through the hours of that afternoon we watched their numbers grow until they numbered over a hundred. They had come from every part of the earth, over which the European Federation held sway. From the bleak eastern steppes, from the jagged peaks and green valleys of the Pyrenees, from the great ice-locked fiords of the north and from the blue plains of southern Africa, they were rushing at colossal speed to gather here in a great circle about their capital city--Berlin! Great air-cities, each of which flashed through the air at the same tremendous speed, each of which bore upon it great batteries of those giant heat-guns that nothing else in the air could sustain, each of which held upon it a soaring electrostatic tower and thousands of other clustered pinnacles. As in Berlin, the crowding, seething millions of its peoples had been left on the ground quarters prepared for them. The gathering of the cities! At last, with the coming of sunset, all but the last few of the Federation's mighty