“And in this case?”

“In this case, Mazeroux, a man like myself, who knows Neuilly and the neighbourhood of the Bois, is at once struck by those three letters, ‘B.R.W,’ and especially by the ‘W.’, a foreign letter, an English letter.  So that in my mind’s eye, instantly, as in a flash, I saw the three letters in their logical place as initials at the head of the words for which they stand.  I saw the ‘B’ of ‘boulevard,’ and the ‘R’ and the English ‘W’ of Richard-Wallace.  And so I came to the Boulevard Richard-Wallace, And that, my dear sir, explains the milk in the cocoanut.”

Mazeroux seemed a little doubtful.

“And what do you think, Chief?”

“I think nothing.  I am looking about.  I am building up a theory on the first basis that offers a probable theory.  And I say to myself ...  I say to myself ...  I say to myself, Mazeroux, that this is a devilish mysterious little hole and that this house—­Hush!  Listen—­”

He pushed Mazeroux into a dark corner.  They had heard a noise, the slamming of a door.

Footsteps crossed the courtyard in front of the house.  The lock of the outer gate grated.  Some one appeared, and the light of a street lamp fell full on his face.

“Dash it all,” muttered Mazeroux, “it’s he!”

“I believe you’re right.”

“It’s he.  Chief.  Look at the black stick and the bright handle.  And did you see the eyeglasses—­and the beard?  What a oner you are, Chief!”

“Calm yourself and let’s go after him.”

The man had crossed the Boulevard Richard-Wallace and was turning into the Boulevard Maillot.  He was walking pretty fast, with his head up, gayly twirling his stick.  He lit a cigarette.

At the end of the Boulevard Maillot, the man passed the octroi and entered Paris.  The railway station of the outer circle was close by.  He went to it and, still followed by the others, stepped into a train that took them to Auteuil.

“That’s funny,” said Mazeroux.  “He’s doing exactly what he did a fortnight ago.  This is where he was seen.”

The man now went along the fortifications.  In a quarter of an hour he reached the Boulevard Suchet and almost immediately afterward the house in which M. Fauville and his son had been murdered.

He climbed the fortifications opposite the house and stayed there for some minutes, motionless, with his face to the front of the house.  Then continuing his road he went to La Muette and plunged into the dusk of the Bois de Boulogne.

“To work and boldly!” said Don Luis, quickening his pace.

Mazeroux stopped him.

“What do you mean, Chief?”

“Well, catch him by the throat!  There are two of us; we couldn’t hope for a better moment.”

“What!  Why, it’s impossible!”

“Impossible?  Are you afraid?  Very well, I’ll do it by myself.”

“Look here, Chief, you’re not serious!”