clicking
sound.

"Quite right," said Holmes. "Now, we will go to the other end of the
word Thibermesnil, try the letter I, and see if it will open like a
wicket."

With a certain degree of solemnity, Devanne seized the letter. It
opened, but Devanne fell from the ladder, for the entire section of the
bookcase, lying between the first and last letters of the words, turned
on a picot and disclosed the subterranean passage.

Sherlock Holmes said, coolly:

"You are not hurt?"

"No, no," said Devanne, as he rose to his feet, "not hurt, only
bewildered. I can't understand now.... those letters turn.... the secret
passage opens...."

"Certainly. Doesn't that agree exactly with the formula given by Sully?
Turn one eye on the bee that shakes, the other eye will lead to God."

"But Louis the sixteenth?" asked Devanne.

"Louis the sixteenth was a clever locksmith. I have read a book he wrote
about combination locks. It was a good idea on the part of the owner of
Thibermesnil to show His Majesty a clever bit of mechanism. As an aid
to his memory, the king wrote: 3-4-11, that is to say, the third, fourth
and eleventh letters of the word."

"Exactly. I understand that. It explains how Lupin got out of the room,
but it does not explain how he entered. And it is certain he came from
the outside."

Sherlock Holmes lighted his lantern, and stepped into the passage.

"Look! All the mechanism is exposed here, like the works of a clock,
and the reverse side of the letters can be reached. Lupin worked the
combination from this side--that is all."

"What proof is there of that?"

"Proof? Why, look at that puddle of oil. Lupin foresaw that the wheels
would require oiling."

"Did he know about the other entrance?"

"As well as I know it," said Holmes. "Follow me."

"Into that dark passage?"

"Are you afraid?"

"No, but are you sure you can find the way out?"

"With my eyes closed."

At first, they descended twelve steps, then twelve more, and, farther
on, two other flights of twelve steps each. Then they walked through a
long passageway, the brick walls of which showed the marks of successive
restorations, and, in spots, were dripping with water. The earth, also,
was very damp.

"We are passing under the pond," said Devanne, somewhat nervously.

At last, they came to a stairway of twelve steps, followed by three
others of twelve steps each, which they mounted with difficulty, and
then found themselves in a small