Hanging above the mighty, flat-topped barrier, we surveyed it, stupefied. All around us was no sign of life. No sound, no movement. Only the white expanse to the north, the green one to the south, and between them, separating and defining them, the titanic wall. Lantin spoke, excitedly. "You see its purpose, Wheeler? It has been built here as a dam to hold back the glacier, to stem the tides of ice. But _how_ built? To think that men can do things like that!" I saw now that Lantin spoke aright, and that it was to dam the engulfing, southward-flowing ice that the wall had been built. And I was struck with awe at the achievement. What were the great Chinese wall and Martian canals, to this? Here in the far future, fifteen thousand years ahead of our own time, we were seeing another step in the conquest of nature by man. He had leveled mountains and turned rivers, and here, below us, had thrust forth a hand and halted the resistless glaciers. An hour we hung above the colossal barrier, fascinated, and then remembered our mission and sped again south. * * * * * As we rocketed on, we could see no sign of life below, nothing but the bleak arctic plains with here and there some sparse vegetation. Again Lantin cried out, and when I looked south, I discerned an odd flicker of light, a seeming hesitating wavering of the air. We sped down toward it, dropping down again to a scant mile above the ground. Far ahead showed expanses of bright green, and as we drew nearer, I saw that there were small patches of white against the green, oddly regular in shape. As we sped on, these white blotches changed to buildings, and the green to verdant lawns and gardens, in which they were set. Again Lantin stopped the car, while we looked down, puzzled. For in a straight line from east to west, was the boundary, the limit, of the gardens and the buildings. North of that line were the cold, wind-swept plains and stunted, arctic vegetation, while south of the same invisible line, seemingly only a few feet from the bleak tundras, began the luxuriant, tropical gardens, stretching away south as far as the eye could see. And also the elusive flicker of light seemed to begin at the same point, and to be present everywhere south of it. If you have ever seen the flicker of heated air above railway tracks or hot sand, on a warm afternoon, you will understand me. It was like that, an elusive, fleeting wavering in the air, below us. "I can't understand it," said Lantin, pointing down to the invisible line which separated arctic world from tropic. "Gardens like those, only a few feet away from the cold plain." "It's beyond me," I told him. "Another thing, Lantin, the car is as cold as ever, even with the heater functioning. Yet down there the country looks tropical."