He felt a sharp shock as the rifle tried to kick out of his hands, but he had bolstered it well. He saw a chunk of rock split from the cliffside just over the German's head. Von Borck ducked as the dust began to fall upon him in its slow Lunar fashion, then the German moved back into the break. Robin again aimed the rifle, this time directly at the dark center of the break in the cliff. Again he fired. This time the figure of the space-suited man backed out of sight entirely. "What now?" asked Robin. "Shall we wait for him to come back or shall we try to follow him?" "Better take the chance and go after him," said his brother's voice. "Must follow up every advantage." "Then let's go," said Robin and leaped to his feet. Peter jumped up with him and they both started to sprint for the entrance in the cliff. They ran for it in low, swift leaps, and this time Robin saw what ease and fun running on the Moon's surface could be if you had the proper outfit for it. It was so light and easy, like running in a dream, gliding rapidly over the faintly lighted eerie moonscape in a world of absolute silence and motionlessness. For an instant, as they closed in on the cliff, Robin saw Von Borck's figure appear, there was another flash of red and then the man vanished again. But the boys did not halt. Together they charged the entrance. In a matter of seconds, they reached it, blocked it. There was no sign of the German. They shone their flashlamps into the channel behind the opening. There was nothing. Robin could feel the faint rustling movement of the rushing air current, but he could see nothing in motion. Again he was struck by the weirdness of the phenomenon. "Where'd he go?" he whispered, even though his voice could not be heard outside of their helmets. "He's probably hiding somewhere. We'll have to follow him. Get ready and then remember to throw yourself hard across that air blast. It's strong." Peter checked the nylon that tied them together. "Shall I untie this or shall we jump together?" "Let's go together," said Robin. They held hands, and, backing up, took a running start and threw themselves into the darkness of the break. There was again the buffeting of a powerful wind, and Robin felt himself being caught off his feet by the force of a hurricane. Before he could be swept away, a jerk at the cord around his waist threw him down, and he rolled over on the windless far side of the tunnel, safe with Peter. He became aware of outside noises. He followed Peter's example and opened the plate of his helmet. For an instant he gasped for air, then adjusted to the thin atmosphere. Both brothers listened. But they heard nothing. "He must have headed back for the cleft," said Peter. "We'll have to follow him."