They urged their mounts to a faster pace. They clung to sweat-wet necks and manes as the horses flashed their hocks above the roughly humped ground. They went by one tall, metallic needle-tower after another. The towers stood like rigid fingers against the horizon as they went on and on. Grim brooded above the whipping mane of his chestnut, "If the thing that sends the change is a machine, then it would need the towers to broadcast its force. It's limited in range, itself. The towers give it a wide radius of effectiveness. Now the question is--what operates only in a limited range?" He thought of radio waves, but the ionized heaviside would throw them around the planet. He shook his head, whispering, "Some sort of wave-radio or energy." Grim lifted his eyes to the next tower and scanned it. He growled, "If I still had my disintegrator I'd try a potshot at those interlacing wires on top of it. I bet that'd stop the Change. But without that--" His mind raced as did the hooves of the chestnut. A scowl came and sat on his face. Then, after a while, his eyes brightened. He whispered, "It might work, at that--" He reined in the chestnut; called to Tlokine, and headed for the nearest tower. She stared at him as he swung down from the horses and began to climb the tower, sliding fingers and toes into the tiny slots between the interslanted metal beams. He went up and up until he reached the round ribboned globe with the tiny wires inside it. It was fastened to the tower by slotted bolts. Using his belt-buckle as a screwdriver, he unthreaded the bolts, found that the globe came away easily from the tower. It was composed of thin alumisteel ribbons and wires of spiderweb thinness. He tucked it under an arm and went backwards down the tower. Tlokine said, "What are you doing?" when he knelt and fumbled at the wiring setup. He grinned, "This thing must broadcast vibrations of some sort. In some manner, they cause the Change. If I could jam those vibrations in the same way that a radio broadcast can be jammed--maybe there wouldn't _be_ any Change to slow us down." Tlokine looked back along the path they had come. She said, "Althaya and her men will soon be here. You are wasting time, Grim Thorssen." He shook his yellow-thatched head. "Not the way I figure it. Soldiers marching in rhythmic steps across a bridge have been known to smash it. That's why they break step all the time. It changes the pulse of the vibration on the bridge. It jars--but does not destroy. "Same way here. Ultrasonic vibrations can't be heard, but they can do plenty of damage. They can kill frogs and tiny fishes. They can cause heat and put out a flame--oh, and a hundred other things. But--if the vibratory rate of those ultrasonic waves can be damped by other wave-impulses--their usefulness is destroyed."