“This is a volume of Shakespeare, Monsieur le Prefet, Volume VIII.  You will see that, contrary to the other volumes, the inside is empty and the binding forms a secret receptacle for hiding documents.”

“Yes.  What sort of documents?”

“Here they are:  sheets of paper, blank sheets, all but three.  One of them gives a list of the dates on which the mysterious letters were to appear.”

“Oho!” said M. Desmalions.  “That’s a crushing piece of evidence against Florence Levasseur.  And also it tells us where Don Luis got his list from.”

Perenna listened with surprise:  he had utterly forgotten this particular; and Gaston Sauverand had made no reference to it in his narrative.  And yet it was a strange and serious detail.  From whom had Florence received that list of dates?

“And what’s on the other two sheets?” asked M. Desmalions.

Don Luis pricked up his ears.  Those two other sheets had escaped his attention on the day of his interview with Florence in this room.

“Here is one of them,” said Weber.

M. Desmalions took the paper and read: 

“Bear in mind that the explosion is independent of the letters, and that it will take place at three o’clock in the morning.”

“Yes,” he said, “the famous explosion which Don Luis foretold and which is to accompany the fifth letter, as announced on the list of dates.  Tush!  We have plenty of time, as there have been only three letters and the fourth is due to-night.  Besides, blowing up that house on the Boulevard Suchet would be no easy job, by Jove!  Is that all?”

“Monsieur le Prefet,” said Weber, producing the third sheet, “would you mind looking at these lines drawn in pencil and enclosed in a large square containing some other smaller squares and rectangles of all sizes?  Wouldn’t you say that it was the plan of a house?”

“Yes, I should.”

“It is the plan of the house in which we are,” declared Weber solemnly.  “Here you see the front courtyard, the main building, the porter’s lodge, and, over there, Mlle. Levasseur’s lodge.  From this lodge, a dotted line, in red pencil, starts zigzagging toward the main building.  The commencement of this line is marked by a little red cross which stands for the room in which we are, or, to be more correct, the alcove.  You will see here something like the design of a chimney, or, rather, a cupboard—­a cupboard recessed behind the bed and probably hidden by the curtains.”

“But, in that case, Weber,” said M. Desmalions, “this dotted line must represent a passage leading from this lodge to the main building.  Look, there is also a little red cross at the other end of the line.”

“Yes, Monsieur le Prefet, there is another cross.  We shall discover later for certain what position it marks.  But, meanwhile, and acting on a mere guess, I have posted some men in a small room on the second floor where the last secret meeting between Don Luis, Florence Levasseur, and Gaston Sauverand was held yesterday.  And, meanwhile, at any rate, we hold one end of the line and, through that very fact, we know Don Luis Perenna’s retreat.”