must start from the Chambre des Demoiselles, pass
under Fort Frefosse, descend perpendicularly the three hundred feet of
cliff and, by means of a tunnel contrived under the rocks of the sea,
end at the Hollow Needle.

Which was the entrance to the underground passage? Did not the two
letters D and F, so plainly cut, point to it and admit to it, with the
aid, perhaps, of some ingenious piece of mechanism?

The whole of the next morning, Isidore strolled about Etretat and
chatted with everybody he met, in order to try and pick up useful
information. At last, in the afternoon, he went up the cliff. Disguised
as a sailor, he had made himself still younger and, in a pair of
trousers too short for him and a fishing jersey, he looked a mere
scape-grace of twelve or thirteen.

As soon as he entered the cave, he knelt down before the letters. Here
a disappointment awaited him. It was no use his striking them, pushing
them, manipulating them in every way: they refused to move. And it was
not long, in fact, before he became aware that they were really unable
to move and that, therefore, they controlled no mechanism.

And yet--and yet they must mean something! Inquiries which he had made
in the village went to show that no one had ever been able to explain
their existence and that the Abbe Cochet, in his valuable little book
on Etretat,[8] had also tried in vain to solve this little puzzle. But
Isidore knew what the learned Norman archaeologist did not know,
namely, that the same two letters figured in the document, on the line
containing the indications. Was it a chance coincidence: Impossible.
Well, then--?

[8] Les Origines d'Etretat. The Abbe Cochet seems to conclude,
in the end, that the two letters are the initials of a passer-by. The
revelations now made prove the fallacy of the theory.

An idea suddenly occurred to him, an idea so reasonable, so simple that
he did not doubt its correctness for a second. Were not that D and that
F the initials of the two most important words in the document, the
words that represented--together with the Needle--the essential
stations on the road to be followed: the Chambre des Demoiselles and
Fort Frefosse: D for Demoiselles, F for Frefosse: the connection was
too remarkable to be a mere accidental fact.

In that case, the problem stood thus: the two letters D F represent the
relation that exists between the Chambre des Demoiselles and Fort
Frefosse, the single letter D, which be