cavity cut in the rock. They could go
no further.

"The deuce!" muttered Holmes, "nothing but bare walls. This is
provoking."

"Let us go back," said Devanne. "I have seen enough to satisfy me."

But the Englishman raised his eye and uttered a sigh of relief. There,
he saw the same mechanism and the same word as before. He had merely to
work the three letters. He did so, and a block of granite swung out of
place. On the other side, this granite block formed the tombstone of
Duke Rollo, and the word "Thibermesnil" was engraved on it in relief.
Now, they were in the little ruined chapel, and the detective said:

"The other eye leads to God; that means, to the chapel."

"It is marvelous!" exclaimed Devanne, amazed at the clairvoyance and
vivacity of the Englishman. "Can it be possible that those few words
were sufficient for you?"

"Bah!" declared Holmes, "they weren't even necessary. In the chart in
the book of the National Library, the drawing terminates at the left, as
you know, in a circle, and at the right, as you do not know, in a cross.
Now, that cross must refer to the chapel in which we now stand."

Poor Devanne could not believe his ears. It was all so new, so novel to
him. He exclaimed:

"It is incredible, miraculous, and yet of a childish simplicity! How is
it that no one has ever solved the mystery?"

"Because no one has ever united the essential elements, that is to
say, the two books and the two sentences. No one, but Arsene Lupin and
myself."

"But, Father Gelis and I knew all about those things, and, likewise--"

Holmes smiled, and said:

"Monsieur Devanne, everybody cannot solve riddles."

"I have been trying for ten years to accomplish what you did in ten
minutes."

"Bah! I am used to it."

They emerged from the chapel, and found an automobile.

"Ah! there's an auto waiting for us."

"Yes, it is mine," said Devanne.

"Yours? You said your chauffeur hadn't returned."

They approached the machine, and Mon. Devanne questioned the chauffer:

"Edouard, who gave you orders to come here?"

"Why, it was Monsieur Velmont."

"Mon. Velmont? Did you meet him?"

"Near the railway station, and he told me to come to the chapel."

"To come to the chapel! What for?"

"To wait for you, monsieur, and your friend."

Devanne and Holmes exchanged looks, and Mon. Devanne said:

"He knew the mystery would be a simple one for you. It is a delicate
compliment."

A smile of satisfaction lighted