As he spoke, a sound broke unexpectedly on the silence of their prison. A dull thud seemed to make itself faintly heard from beyond the thick wall of sand that cut them off from the daylight. Cyril stared with surprise. It was a noise like a pick-axe. Stooping hastily down, he laid his ear against the rail beside the shattered carriage. "They're digging!" he cried earnestly, finding words in his joy. "They're digging to reach us! I can hear them! I can hear them!" Elma glanced up at him with a certain tinge of half-incredulous surprise. "Yes, they're digging, of course," she said quickly. "I knew they'd dig for us, naturally, as soon as they missed us. But how far off are they yet? That's the real question. Will they reach us in time? Are they near or distant?" Cyril knelt down on the ground as before, in an agony of suspense, and struck the rail three times distinctly with his walking-stick. Then he put his ear to it and listened, and waited. In less than half a minute three answering knocks rang, dim but unmistakable, along the buried rail. He could even feel the vibration on the iron with his face. "They hear us! They hear us!" he cried once more, in a tremor of excitement. "I don't think they're far off. They're coming rapidly towards us." At the words Elma rose from her seat, still paler than ever, but strangely resolute, and took the stick from his hand with a gesture of despair. She was almost stifled. But she raised it with method. Knocking the rail twice, she bent down her head and listened in turn. Once more two answering knocks rang sharp along the connecting line of metal. Elma shook her head ominously. "No, no, they're a very long way off still," she murmured, in a faltering tone. "I can hear it quite well. They can never reach us!" She seated herself on a fragment of the broken carriage, and buried her face in her hands once more in silence. Her heart was full. Her head was very heavy. She gasped and struggled. Then a sudden intuition seized her, after her kind. If the rail could carry the sound of a tap, surely it might carry the human voice as well. Inspired with the idea, she rose again and leant forward. A second time she knocked two quick little taps, ringing sharp on the rail, as if to bespeak attention; then, putting her mouth close to the metals, she shouted aloud along them with all the voice that was left her-- "Hallo, there, do you hear? Come soon, come fast. We're alive, but choking!" Quick as lightning an answer rang back as if by magic, along the conducting line of the rail--a strange unexpected answer. "Break the pipe of the wires," it said, and then subsided instantly. Cyril, who was leaning down at her side at the moment with his ear to the rail, couldn't make out one word of it. But Elma's sharp senses, now quickened by the crisis, were acute as an Oriental's and keen as a beagle's.