It reached her at last, battered, its glass broken. She put the little instrument on the floor.

“Put it where I can see it,” he said. “Have you a light?”

She flashed the lamp upon it.

“Where is the north? Just touch the place with your finger. Wait, I will send for field-glasses.”

Ten minutes passed, and then he said again: “Now show me.” And when she had indicated the north, he asked her where the cavern was.

“Exactly west,” she said with tremulous triumph. “Will it be a long time before you reach us?”

He made no answer to this.

“Tell me how many paces you are from the compass,” and when she had paced it off and had told him, he groaned.

By this time the consulting engineer to whom he had telephoned in the afternoon was on the spot.

“The cavern is exactly under the bed of the river,” said that official.

“Could we enlarge this hole?” asked Dick.

The surveyor shook his head.

“Impossible. It would take you the best part of a month to blast a way down. There’s a long fault in the rock here which accounts for the river’s course,” he added. “Both banks are solid; I can assure you on that point, because my predecessor bored for water for your respected father.”

Dick groaned. He could keep the girl alive for a month, but the strain of it would kill her. Then there flashed simultaneously to two minds a solution.

“Why not break the dam of the Ravensrill?” he said, and Puttler, who had the words on his lips, nodded.

“That’s the idea,” he said. “Undo the work of your ancestor! Turn the course of the river to the Long Meadow – there’s a natural bed for it!”

Ten minutes later the telephone at Fossaway Manor was busy, and here Mary Wenner was a heaven-sent helper. Every great contractor within twenty miles had his instructions, and within an hour charabancs, motorcars, omnibuses, crowded with horny-handed workmen, were lumbering up the drive. Car succeeded car, and disgorged the fustian-clad navvies. They had been taken from ale-houses, from their homes, from workmen’s clubs, drawn even from the cinemas of distant Brighton, and every hour the number swelled, until there were a thousand men working by the light of naphtha flares on the great dump behind Fossaway Manor.