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.
