"That won't do for me, lads," said Atherton. "I'll go. You keep what's left of the candle. When I find the level is sinking I'll shout and let you know. This tunnel is like a giant voice-tube: you'll hear me plainly enough."
"Oh, I'll go if you want," said Mayne, somewhat shamefacedly.
"Or I will," added Green.
"You'll jolly well stop here. Take half of these matches. Keep on trying, and perhaps you'll find the secret of the opening after all."
So saying, Atherton felt his way down the steps, and began his solitary progress along the tunnel. On and on he went, feeling the rough wall with his hand and methodically counting the number of paces he took.
At the five hundred and twentieth step his foot splashed into the water. The Scout halted, struck a match and examined the rock close to the surface of the pool. It was fairly dry.
"That means the water is still rising," thought the Leader. "I'll draw a line in the dust, and look again in five minutes' time, just to make sure."
Taking a piece of twine from his pocket, Atherton measured off as near as he could guess a length slightly exceeding a yard. To one end he attached his knife. Holding the other end in his hand, the Scout allowed the weighted string to swing.
"A pendulum thirty-nine inches in length swings one every second," he said to himself. "This ought to be near enough for my purpose."
He waited till the knife had swung three hundred times—it seemed more like an hour than five minutes,—then, striking another match, he examined the mark he had made on the ground. It was already on the point of being covered. The water was still rising.