"That's really impossible, absurd and ridiculous, of course," he answered. "There _must_ be something beyond. The way this stone falls proves that." He pitched a two-pound lump of granite far out into the air. It fell vertically, whirling, and vanished with the speed of a meteor. "If a whole side of the earth had split off, and what we see down below there were really sky, of course the earth's center of gravity would have shifted," he explained, "and that rock would have fallen in toward the cliff below us, not straight down." "How can you be sure it doesn't fall that way after the impulse you gave it has been lost?" "I shall have to make some close scientific tests here, lasting a day or two, before I'm positive; but my impression is that this, after all, is only a canyon--a split in the surface--rather than an actual end of the crust." "But if it were a canyon, why should blue sky show down there at an angle of forty-five degrees?" "I'll have to think that out, later," he replied. "Directly under us, you see all seems deep purple. That's another fact to consider. I tell you, Beatrice, there's more to be figured out here than can be done in half an hour. "As I see it, some vast catastrophe must have rent the earth, a thousand or fifteen hundred years ago, as a result of which everybody was killed except you and me. We're standing now on the edge of the scar left by that explosion, or whatever it was. How deep or how wide that scar is, I don't know. Everything depends on our finding out, or at least on our guessing it with some degree of accuracy." "How so?" "Because, don't you see, this chasm stands between us and Chicago and the West, and all our hopes of finding human life there. And--" "Why not coast south along the edge here, and see if we can't run across some ruined city or other where we can refill the tanks?" "I'll think it over," the engineer answered. "In the meantime we can camp down here a couple of days or so, and rest; and I can make some calculations with a pendulum and so on." "And if you decide there's probably another side to this gulf, what then?" "We cross," he said; then for a while stood silent, musing as he peered down into the bottomless abyss that stretched there hungrily beneath their narrow observation-rock. "We cross, that's all!" CHAPTER XXI LOST IN THE GREAT ABYSS For two days they camped beside the chasm, resting, planning, discussing, while Stern, with improvised transits, pendulums and other apparatus, made tests and observations to determine, if possible, the properties of the great gap. During this time they developed some theories regarding the catastrophe which had swept the world a thousand years ago.