In the bottom drawer lay something that aroused the curiosity of all three. These were small reels, about two inches in diameter and a quarter of an inch thick, each incased in a tight-fitting box. They resembled measuring tapes to some extent, except that the ribbons were made of marvelously thin material. Van Emmon guessed that there were a hundred yards in a roll. Smith estimated it at three hundred. They seemed to be made of a metal similar to that composing the machines. Smith pocketed them all. It was the builder who thought to look under the bench, but it was Smith who had brought a light. By its aid they discovered a very small machine, decidedly like a stock ticker, except that it had no glass dome, but possessed at one end a curious metal disk about a foot in diameter. Apparently it had been undergoing repairs; it was impossible to guess its purpose. Smith's pride was instantly aroused; he tucked it under his arm, and was impatient to get back to the cube, where he might more carefully examine his find with the tips of his fingers. It was when they were about to leave the building that they thought to inspect walls and ceiling. Not that anything worth while was to be seen; the surfaces seemed perfectly plain and bare, except for the inevitable dust. Even the uppermost corners, ten feet above their heads, showed dust to the light of Smith's electric torch. Van Emmon stopped and stared at the spot as though fascinated. The others were ready to go; they turned and looked at him curiously. For a moment or two he seemed struggling for breath. "Good Heavens!" he gasped, almost in a whisper. His face was white; the other two leaped toward him, fearful that he was suffocating. But he pushed them away roughly. "We're fools! Blind, blithering idiots--that's what we are!" He pointed toward the ceiling with a hand that trembled plainly, and went on in a voice which he tried to make fierce despite the awe which shook it. "Look at that dust again! How'd it get there?" He paused while the others, the thought finally getting to them, felt a queer chill striking at the backs of their necks. "Men--there's only one way for the dust to settle on a wall! It's got to have air to carry it! It couldn't possibly get there without air! "That dust settled long before life appeared on the Earth, even! It's been there ever since the air disappeared from Mercury!" IV THE LIBRARY "I thought you'd never get back," complained the doctor crossly, when the three entered. They had been gone just half an hour. Next moment he was studying their faces, and at once he demanded the most important fact. They told him, and before they had finished he was half-way into another suit. He was all eagerness; but somehow the three were very glad to be inside the cube again, and firmly insisted upon moving to another spot before making further explorations.