Not only did they have to carefully avoid the visible swarms of little people who hurried everywhere, but had to be on their guard as well against any who might be moving about like themselves under cover of invisibility. Nor could they use any elevator or public conveyances, but were obliged to make their way down to the concourse by heaven knew how many flights of stairs, and cross heaven knew how many teeming streets on foot, before they reached the amber court, below which the trap-door and their hope of freedom. They got there at last, however, descended, and peered down from that yawning brink upon the desert floor--to draw back with gasps of dismay. For the area still gleamed semi-molten from the stupendous blast that had wiped out Kendrick's camp. "W-what is it?" she gasped. Swiftly he told her. "But isn't there any way around it? Look, over there to the left. One edge of the crater seems to end almost underneath us." It was true that the center of the caldron was far to the right of where they stood, and that its left rim was only a little within their direct line of descent. But to land even one foot inside that inferno would be as fatal as to alight in its very midst. * * * * * Kendrick was thinking fast. "There's just a chance," he said. "It all depends upon how wide the zone of these escalator rays is, and whether we can tune in on them. At least, I can probably answer the latter question." Pushing the protrudent round button on his mysterious bracelet as he spoke, he leaned over the edge of the trap-door and awaited results. They were not long in coming. The vibration he was already under from the invisibility rays seemed to double. Alternate waves of giddiness and depression, of push and pull, swept over him. A minute of it was enough. He pressed the round button that now protruded, ending this influence, and faced Marjorie, stating: "I'm positive now that these things control descent and ascent. As nearly as I can figure, the rays work on the principle of an endless belt. If you're up here, you get carried down, and vice versa. As to how wide the belt is, and whether you can move sideways on it, remains to be seen. Anyway, I'm going to take a chance. I'll go first. If my guess is wrong, you--well, needn't follow." "No, I'm going with you!" she declared resolutely. "We've come this far together. I shan't be left alone now. Let's go!" And again her soft, warm hand was in his. Lord, what a girl! How many would be brave enough to take a gamble like that, on a fellow's mere supposition? "All right--go it is!" he said. "Push your round button, like this." He showed her the way he thought was right, pushed his own. "Ready?" "Ready!" * * * * * Their voices were grave. It was a grim prospect, stepping off into space like that, with only a guess between them and death. "Then jump!"