"You know," he said slowly, "I read about those sarkonivals once. They always fly in groups of an even twenty, save when some atmospheric disturbance causes them to alter their formation."
He pointed upward. "They are all in groups of twenty except over that block over there. Above that they seem to be in confusion."
Jimmy followed his gaze and frowned thoughtfully. He paced forward to the block in question, stood there watching the movements of the sarkonivals.
Suddenly he turned to Linda. "Look. See how their flying formation is always the same? They're twenty of them up there all right, and they start to circle the block in a compact mass. But as soon as they strike a point directly above it, they separate. First five, then three, then two, six, one, and three. Always the same order. Do you suppose that might be the combination? A magnetic disturbance issuing from the block in such a way as to prevent the usual twenty-formation and break it up in that fashion?"
"Jimmy!" Her eyes lighted. "I think you've got it!"
He seized the ancient combination wheel, put his strength to it. Slowly, a fraction of an inch at a time, it began to turn. Jimmy hesitated.
He seized the ancient combination wheel....