line, and there were no marked ones beyond the fourth. He backed out of the room and studied them from the end of the corridor again.

Mara had come dizzily awake and was calling for him weakly. He replied reassuringly and turned to his task again. "First tile, then diagonally left, then straight, then diagonally right—"

0, 1, 2, 3, 3.

A hunch came. He advanced as far as the second tile, then reached as far ahead as he could and touched the square diagonally right from the fourth one. Big Joe remained motionless but began to speak. His scalp bristled at the growling voice.

"If the intruder makes an error, Big Joe will kill."

Standing tense, ready to leap back to the corridor, he touched the square again. The motionless behemoth repeated the grim warning.

Asir tried to reach the square diagonally right from the fifth, but could not without stepping up to the third. Taking a deep breath, he stepped up and extended the pike cautiously, keeping his eyes on Big Joe. The pike rapped the floor.

"If the intruder makes an error, Big Joe will kill."

But the huge figure remained in his place.

Starting from the first square, the path went left, straight, right, right, right. And after zero, the numbers went 1, 2, 3, 3, 3. Apparently he had found the key. One meant a square to the southeast; two meant south; and three southwest. Shivering, he moved up to the fifth square upon which the monster growled his first warning. He looked back at the door, then at Big Joe. The taloned hands could grab him before he could dive back into the corridor.

He hesitated. He could either turn back now, or gamble his life on the accuracy of the tentative belief. The girl was calling to him again.

"Come to the end of the corridor!" he replied.

She came hurriedly, to his surprise.

"No!" he bellowed. "Stay back of the entrance! Not on the tile! No!"

Slowly she withdrew the foot that hung poised over a trigger-tile.

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