A broad yellow line marked the zone beyond which it was death to stray. It was the first time Norman had been close to the force wall. He looked at it curiously.
A ribbon of some unfamiliar silver metal wound like the track of a mono rail around the base of the hill where the Dohlmites had their houses. There were no visible rays arising from the ribbon, no distortion of the atmosphere, nothing. It looked utterly harmless.
"I wonder what would happen if you broke the circuit," speculated Norman.
"It's impossible," replied Koal. "The zone of force protects the ribbon. Look." He threw a pebble toward the silver track of metal. While still a yard from the ribbon, the stone exploded like a hand grenade. It was as if the force radiating from the track had touched off the atoms of the pebble. Norman blinked his eyes involuntarily.
"How do the Dohlmites pass through?"
"There's a gate only a short distance from here where they can shut off a segment of the wall."
The buildings of the human colony, Norman noticed, were set well back from the yellow warning line, leaving a broad road which paralleled the silver track. There was no one in sight. It seemed to be a very unpopular neighborhood.
"Get him out," grunted Acpsahme. They tossed the body of the disease-infected Dohlmite to the road.
"The Trojan horse," thought Norman, remembering a tale from the dawn of history. He glanced back once as they sped away.