“For a long time I’ve been suspicious that the rock on which the abbey was built was honeycombed with passages. My father told me something about it, and I’ve seen an old plan that shows an elaborate system of corridors, though the family has always thought this was largely imaginative on the part of the artist.”

“Have you the plan now?” asked Gilder.

Dick shook his head.

“Harry took everything of that nature away with him the night he left the house.”

“It is not amongst the books you found in the underground room?” said Puttler, and a search was made of the library, but without success.

They were on their way to the ruins when Puttler saw the aeroplane in the sky. It circled twice and then began to dip steeply.

“I believe that fellow is coming here,” he said.

And so it proved. The machine roared its progress for a hundred yards or more, and then dropped. Presently they saw a man get down. Though he wore an airman’s helmet, Dick recognised him. It was Arthur Gine.

He met Gilder’s scowl with a little laugh.

“I’ve got some money of yours, Gilder,” he said, and dragged with some difficulty a huge packet from the pocket of his leather coat. “That is more or less the amount I owe you, unless the franc has depreciated in value since I left Paris. And now you can do your damnedest!”

Gilder took the packet without a word and Arthur turned to Dick Alford.

“I read about Leslie in the French papers,” he said simply, “and so I came back. Has she been found?”

Dick shook his head.

“Have you any idea where she is?”

Dick told him all that had happened that afternoon, and Arthur Gine listened in silence. When Dick came to speak of his plan, he shook his head.

“I had my early training as an engineer before I went into the law,” he said, “and I tell you, from my elementary knowledge of the science, that you’re likely to blow in the whole well, and if there’s anybody on the other side, God help them!”

He accompanied them to the lower room and was swung down on the derrick to make an inspection. When he returned to the surface his report was not very promising.

“So far as I can see,” he said, “whilst you may enlarge the opening of any of these air-holes, you may also bring about a fall of the rock inside. You’re dealing with surfaces which have been exposed to the chemical action of the air.”