gins the line, represents the
Demoiselles, that is to say, the cave in which you have to begin by
taking up your position, and the single letter F, placed in the middle
of the line, represents Frefosse, that is to say, the probable entrance
to the underground passage.

Between these various signs, are two more: first, a sort of irregular
rectangle, marked with a stripe in the left bottom corner, and, next,
the figure 19, signs which obviously indicate to those inside the cave
the means of penetrating beneath the fort.

The shape of this rectangle puzzled Isidore. Was there around him, on
the walls of the cave, or at any rate within reach of his eyes, an
inscription, anything whatever, affecting a rectangular shape?

He looked for a long time and was on the point of abandoning that
particular scent when his eyes fell upon the little opening, pierced in
the rock, that acted as a window to the chamber.

Now the edges of this opening just formed a rectangle: corrugated,
uneven, clumsy, but still a rectangle; and Beautrelet at once saw that,
by placing his two feet on the D and the F carved in the stone
floor--and this explained the stroke that surmounted the two letters in
the document--he found himself at the exact height of the window!

He took up his position in this place and gazed out. The window looking
landward, as we know, he saw, first, the path that connected the cave
with the land, a path hung between two precipices; and, next, he caught
sight of the foot of the hillock on which the fort stood. To try and
see the fort, Beautrelet leaned over to the left and it was then that
he understood the meaning of the curved stripe, the comma that marked
the left bottom corner in the document: at the bottom on the left-hand
side of the window, a piece of flint projected and the end of it was
curved like a claw. It suggested a regular shooter's mark. And, when a
man applied his eye to this mark, he saw cut out, on the slope of the
mound facing him, a restricted surface of land occupied almost entirely
by an old brick wall, a remnant of the original Fort Frefosse or of the
old Roman oppidum built on this spot.

Beautrelet ran to this piece of wall, which was, perhaps, ten yards
long. It was covered with grass and plants. There was no indication of
any kind visible. And yet that figure 19?

He returned to the cave, took from his pocket a ball of string and a
tape-measure, tied the string to the flint corner, fastened a p