"Stop it; stop it--until you know. You see, the mercury-filled crater above is to become a great mirror for reflecting sunlight earthward. Along the resultant light beam the Selenites plan to travel in cars which are propelled in concentrated photon streams--" "_Wait_ a minute," the engineer interrupted. "The crater faces away from Earth." Peterson grinned anew. "Now it does, yes. But the moon will be turned around until it faces Earth." "Turned a-_round_!" "Just that. That's the why of last night's test. The sun inside here is to be shifted by projected forces until the center of gravity of the moon's total mass is at the proper focus. Then the shell turns over until the crater Gates called Nemesis is in the right position. By now the motor spins the mercury until centrifugal force reverses the natural convexity and the ten mile vat of mercury becomes a big concave mirror. "The reflected light beam can be narrowed down to any desired size by changing the concavity--altering the motor speed. Just by shifting Luna's inner sun." "Why," gasped Bonwitt, "if all the sun's heat over a ten mile diameter mirror were focussed on a spot say one mile in diameter on our Earth, one hundred times normal sun energy would be concentrated in this area. Anything would be instantly consumed." "You've hit the nail on the head," said Peterson. "_One_ nail. That's Gates's nail, which I intend to pull out. But the Lunarians plan only to make a plane mirror of the mercury crater, which would not overheat anything on Earth but only provide a lane through which their photon cars can pass. They believe they can effect a peaceful colonization." "What do you mean, Gates's nail?" Bonwitt's lips set grimly. "World conquest! Worse--revenge. He intends to blast all big cities to ruin, then resume the dictatorship that was once his father's." "His father's?" Memory came to the young engineer of history. Establishment of the World Government in 1975. Exiling a man who had set himself up as World Dictator. Yes, his name had been Gates. He had died in Siberia. And _this_ Gates was the son--explaining the signalling to Earth. A party of adherents waited there for a millennium or something. "But _you_ helped with his signals," Bonwitt accused. "I did," grinned the older man, "to keep this screwball's gang together where I can blast _them_ out of existence as soon as I get Gates. Gates discovered inner Luna and, the fool, told me about it. Played right into my hands." Bonwitt shuddered. Here was a double-crosser of the first water. "How do you plan to upset the beans and where do you profit yourself?" he asked.