Free The Human Edge by Gordon R. Dickson

and later in the day, Harry saw the indifferent treads of one of the prospecting machines bury it in the trampled earth in passing.
    That evening, Harry brought several captive, small animals bound with grass up to the wand line and thrust them through, one by one at different spots. All died.
    The next night he tried pushing a captive through a small trench scooped out so that the creature passed the killing line below ground level. But this one died also. For several days he was baffled. Then he tried running behind a slow-moving machine as it returned and tying a small animal to it with grass.
    For a moment as the front of the machine passed through, he thought the little animal would live. But then, as the back of the machine passed the line, it, too, died.
    Snarling, Harry paced around outside the circle in the brush until the sun set and stars filled the moonless sky.
    * * *
    In the days that followed, he probed every gap in the wand-fence, but found no safe way through it. Finally, he came to concentrate on the two points at which the diverted stream entered and left the circle to flow beneath the storklike machine.
    He studied this without really knowing what he was seeking. He did not even put his studying into words. Vaguely, he knew that the water went in and the water came out again unchanged; and he also wished to enter and come out safely. Then, one day, studying the stream and the machine, he noticed that a small creature plucked from the water by the storklike neck's mass of tentacles was still wriggling.
    That evening, at twilight, while there was still light to see, he waded up the two-foot depth of the stream to the point where the killing line cut across its watery surface and pushed some more of his little animals toward the line underwater.
    Two of the three surfaced immediately, twitched, and floated on limply, to be plucked from the water and cast aside on the ground by the storklike machine. But the third swam on several strokes before surfacing and came up living to scramble ashore, race for the forest, and be killed by wands further around the circle.
    Harry investigated the channel below the killing line. There was water there up to his midthigh, plenty to cover him completely. He crouched down in the water and took a deep breath.
    Ducking below the surface, he pulled himself along with his fingertips, holding himself close to the bottom. He moved in as far as the tentacled ends. These grabbed at him, but could not reach far enough back to touch him. He saw that they came within a few inches of the gravel bottom.
    He began to need air. He backed carefully out and rose above the water, gasping. After a while his hard breathing stopped, and he sat staring at the water for a long while. When it was dark, he left.
    The next day he came and crept underwater to the grabbing area of the storklike machine again. He scooped out several handfuls of the gravel from under the place where the arms grabbed, before he felt a desperate need for air and had to withdraw. But that day began his labors.
     
     
     
IV
     
    Four days later the bottom under the grasping tentacles was scooped out to an additional two feet of depth. And the fifth twilight after that, he pulled himself, dripping and triumphant, up out of the bend of the diverted stream inside the circle of the killing wands.
    He rested and then went to the pyramid, approaching it cautiously and sidelong like a suspicious animal. There was a door in the side he approached through which he had seen the prospecting machines trundle in and out. In the dimness he could not see it; and when he touched the metallic side of the structure, his fingers, grimed and toughened from scrabbling in the dirt, told him little. But his nose, beast-sensitive now, located and traced the outline of the almost invisible crack around the door panel by its reek of earth and lubricant.
    He settled down to wait. An hour later, one of the machines came back. He jumped up, ready to