When he turned back he made a sort of wild spring that I did n't understand at first. Then I saw the stone we had put over the rope rolling off the rock,—joggled off by the boat's pulling harder when a wave lifted it. The stone rolled in cornery bounces, with a dull noise, and the rope slipped after it slowly. I thought Jerry would be in time. I could n't believe that I really saw the rope floating its whole length on the water, dry at first, then darkening wetly.
"Hang on, Chris!" Jerry said. "I can get it."
I caught his hand, and he snatched after the rope. But he plunged wildly, nearly pulling me in, and scrambled up at once with one leg wet to the hip.
"There's no bottom at all," he said queerly. "I believe the thing rises straight out of the sea."
By that time the boat was ten feet away
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