"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. "Cheerful," he remarked. "After all, there is no danger, it's only the discomfort, and all true Scouts make light of trivial matters like this. It's another all-night business: that's my opinion." "Atherton!" shouted Green, his voice rumbling down the tube-like passage. "Hulloa?" "Our light's gone out. Is the water falling?" Atherton struck a match. "I'm sorry to say it isn't," he shouted in reply. "Then it's no use waiting there. Come back to us. It's mighty cold and we're precious hungry." "I can't feed you, Green, and if you're cold jump about a bit and flap your arms. I'll be with you soon." After giving this advice, Atherton began to walk along the now familiar tunnel. Ere he had covered a hundred paces he was surprised by the sound of a sharp detonation, followed by shouts of alarm on the part of his two comrades. "What's up?" hailed Atherton. The shouting still continued, but the anxious Scout could make neither head nor tail of what was being said. Presently a strong current of air, followed by the pungent fumes of powder, drifted down the tunnel.