Smithers went over to the gesticulating group of draftsmen. He settled down to explain by drawings, since he had not a word of their language. In a few minutes a group went rushing away with the sketch tablets held jealously to their breasts, bound for workshops. Other men appeared to present new problems. A wave of sheer enthusiasm was in being. A new idea which would lessen the demands of the machines was a godsend to these folk. Then Denham blew a smoke ring and said meditatively: "I think I've got something too, Tommy. Ultra-sonic vibrations. Sound waves at two to three hundred thousand per second. Air won't carry them. Liquids will. They use 'em to sterilize milk, killing the germs by sound waves carried through the fluid. I think we can start some ultra-sonic generators out there that will go through the wet soil and kill all vegetation within a given range. We might clear away the jungle for half a mile or so and then use ultra-sonic beams to help it clear while new food-plants are tried out." Tommy's eyes glowed. "You've given yourself a job! We'll turn this planet upside down." "We'll have to," said Denham drily. "This city may believe in you, but there are others, and these folk are a little too clever. There's no reason why some other city shouldn't attack Earth, if they seriously attack the problem of building a Tube." Tommy ground his teeth, frowning. Then he started up. There was a new noise down in the city. A sudden flare of intolerable illumination broke out. There was an explosion, many screams, then the yelling tumult of men in deadly battle. * * * * * Every man on the tower terrace was facing toward the noise, staring. The white-bearded man gave an order, deliberately. Men rushed. But as they swarmed toward an exit, a green beam of light appeared near the uproar. It streaked upward, wavering from side to side and making the golden walls visible in a ghostly fashion. It shivered in a hasty rhythm. Aten groaned, almost sobbed. There was another flash of that unbearable actinic flame. A thermit-thrower was in action. Then a third flash. This was farther away. The tumult died suddenly, but the green light-beam continued its motion. Tommy was snapping questions. Aten spoke, and choked upon his words. Tommy swore in a sudden raging passion and then turned a chalky face toward the other two men from Earth.