Bennie held the cylinder for Atterbury, while the latter attempted to pour it in through a funnel, but, in spite of all his care, some of the liquid fell upon the floor with a hiss like that of water dropping upon a red-hot stove. "What makes it smoke like that?" asked Rhoda. "Of course, I know it isn't _hot_!" "Condensed moisture," explained Bennie. "We never could have made this trip without it!" With the greatest caution, they finally succeeded in filling all the cylinders, and Burke and Atterbury started to don their vacuum armor. Bennie was about to do the same, when he noticed an expression of disappointment on Rhoda's face. "_You_ go!" he said. "I've got to fix up something inside. Go out along with the others and look around. I'll take my turn when you come back. You won't want to stay long, I guess!" "Oh, thanks!" she cried. "I _do_ want to see what the moon is like!" The men had by this time got into their strange costumes, but Rhoda found the arrangement of her skirts more or less complicated and was forced to retire to the galley, where she finally adjusted her attire to lunar requirements. Then, all four of them rolled the huge cylinder of uranium into the air-lock, and Atterbury closed and fastened the inner air-tight door behind them. They stood crowded together for a moment in that confined space, like divers in a divingbell, unable to speak to each other, and fully mindful of the fact that they were about to essay an experiment in physics never before attempted or even conceived of--the entry of a human being into a perfect vacuum. Atterbury made a gesture of inquiry, and the others nodded their helmets. He raised one hand in warning and placed the other upon a valve in the outer door and pressed it quickly down. With a shriek, the air in the lock rushed through the valve into space, and their suits swelled perceptibly from the pressure of the contained air, as if pulled outward from their bodies by some invisible force. They stood motionless for several minutes to accustom themselves to their strange environment, making futile grimaces at one another through the glass of their helmets. Then Rhoda was startled by a curious fluttering or palpitation just above the top of her head--a sort of metallic twitter like that which might be expected to emanate from a mechanical bird--and she turned a startled face toward Burke, who only grinned in response and pointed to the escape-valve upon his own helmet. Then she remembered that he had previously explained to her how the vitiated air inside the helmets must needs escape in order to give place to the new fresh air liberated by the supply-tanks. But, in spite of her knowledge that this fluttering was due simply to a necessary device, she never heard it without a momentary tremor of fear--a sudden conviction that her soul was unexpectedly starting upon the Great Adventure.